CMG-6TD Digital Output Broadband Seismometer Operators Guide · CMG-6TD Digital Output Broadband Seismometer Operators Guide V1.00 ISSUE ECN DATE COMMENTS A 11.07.2002 First Issue

CMG-6TD

Digital Output Broadband Seismometer

Operators Guide

V1.00

ISSUE ECN DATE COMMENTS A 11.07.2002 First Issue

DESIGNED AND MANUFACTURED BY:

GÜRALP SYSTEMS LIMITED 3 MIDAS HOUSE CALLEVA PARK ALDERMASTON

READING BERKS, RG7 8EA

ENGLAND

Telephone: +44 (0) 118 9819056 Fax: +44 (0) 118 9819943

Guralp Systems Limited CMG-6TD Operators Guide ____________________________________________________________________________

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Contents

CONTENTS 3

HOW TO USE THIS MANUAL 5

1 FIRST ENCOUNTERS 6

1.1 Introduction to the CMG-6TD digital seismometer 6

1.2 Unpacking 6

1.3 Quick Start 7

2 INSTALLATION 11

2.1 What you will need to install the system 11

2.2 Choosing a location 11 2.2.1 Reducing background noise 11 2.2.2 Temperature stability 11 2.2.3 Additional considerations 12

2.3 Installing the sensor in a vault or basement 12

2.4 Pit Installations and recovery for rapid deployment experiments. 12 2.4.1 Installation. 12 2.4.2 Sensor recovery 16

2.5 Assembling the system 17 2.5.1 For stand alone operation. 17 2.5.2 For continuous data collection via PC. 17

2.6 Installing the GPS time-code antenna unit 18

2.7 Data acquisition 18

2.8 Data collection procedure 19

3 POWERING-UP AND TESTING THE SYSTEM 20

3.1 Testing the power supply 21

3.2 Testing the mass position 21

4 CMG-6TD CONFIGURATION AND CONTROL MODE 22

4.1 CMG-6TD Configuration:- Using ‘SCREAM’ 22

4.2 Digitiser Configuration: - Using Terminal 31

5 STATUS INFORMATION 39

6 DIGITISER DATA ACQUISITION & TELEMETRY 44

6 CARE AND MAINTENANCE 46

7 TROUBLESHOOTING AND ASSISTANCE 47

7.1 Frequently-asked questions 47

7.2 Getting Help 48 7.2.1 Limited Warranty 48


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7.2.2 Technical support 48

8 ADVANCED TECHNICAL PROCEDURES 49

8.1 Calibration 49

8.2 System ID and Serial Number 51

9 TECHNICAL DATA 53

9.1 CMG6TD system specifications 53

9.2 Sensor calibration data 53

9.3 Güralp Compressed Format (GCF) 54


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How to Use This Manual This manual describes the installation and operation of the CMG-6TD seismometer, and assumes no prior experience with the operation of seismometers. Users unfamiliar with the CMG-6TD should read Sections 1-4 of the manual, in order, following the instructions in step-by-step fashion. The rest of the manual (Sections 5-8) contains troubleshooting information plus additional technical and reference data that may be useful after the CMG-6TD has been installed.


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1 First Encounters

1.1 Introduction to the CMG-6TD digital seismometer The Güralp CMG-6TD is an ultra lightweight portable broadband three component seismometer with a direct RS-232 digital output. When used with a personal computer running the appropriate data acquisition software, (e.g. SCREAM!) the CMG-6TD becomes a complete high-quality seismic data recording system, suitable for a wide range of applications in local, regional, and teleseismic studies. CMG-6TD can be used as a self-contained recording station as the sensor can have up to 8 G byte of non-volatile flash memory. The internal memory is accessible either through the RS-232 link or through the IEEE 1349 fast serial link. The use of IEEE 1349, which is also known as ‘Fire Wire’ reduces the transfer of data time substantially. The data transfer of 2 G byte on average takes no longer than 6 minutes. The data can be transferred to commercially available ‘fire wire’ disc drives or GSL manufactured hard disk drive units, which also have SCSI interface. CMG-6TD is delivered with a GPS unit and breakout box for easy installation. Its rugged design simplifies packing and transport and allows for rapid deployment in the field for aftershock studies and other special projects. CMG-6TD sensor has been designed to be easily deployed for rapid installation. All parts of CMG-6TD is water proof

1.2 Unpacking CMG-6TD is delivered either as a single three-component sensor or in a pack of 5 sets of CMG-6TD sensor. In both, the package should contain the following items:

1 CMG-6TD seismometer 1 CMG-6TD Breakout box 1 Power supply cable 1 Data output serial cable 1 Operator's Manual 1 Test and Calibration Data booklet 1 GPS receiver unit (with mounting rod) 1 15 meter GPS unit extension cable

1 Waterproof IEEE1394 cable.

Carefully unpack the shipping box. Be sure to save all packing materials, as these will be useful later on should you need to transport the sensor.


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Five sets of CMG-6TD with the break out boxes and GPS units are shown in the packing case. The packing boxes have been designed such that the sensors can be huddle tested within the same packing case and the data can be monitored and analysed using SCREAM. The interface to the PC is via USB and the cable for the USB is accessible through the front cover of the packing case. Within the packing case the power is distributed via power distribution box. The high speed data transfer from the sensors can be done through the IEEE1349 hub which is also packed with in the transport package.

Caution: Although the CMG-6TD sensor is extremely rugged, it should always be handled with care. Never kick or drop the sensor or otherwise subject it to strong accelerations, as excessive force may damage the mechanical pivots in the sensor. Using the accompanying illustration as a guide, please take a moment to familiarise yourself with each component. If any item is missing from the package or appears to be damaged, contact Güralp Systems, Ltd. immediately [email protected] "see Getting help".

1.3 Quick Start The quick start section allows the user to familiarise them selves with the operation of the CMG-6TD system. It is advised that the user does not change any of the default setting of the sensor while using this section until they have become familiar with later sections of this manual. The aim here is to operate the system so that the user can check that the equipment is functioning on arrival and to give a basic understanding of its operation.


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1. Connect the breakout box to the sensor via the integral cable and ‘blue’ connector.

2. Connect the GPS cable ( the one with green connectors either end) to the ‘green’ connector on the breakout box.

3. Connect the other end of the cable to the GPS and position the GPS such that it has a good view of the sky. See section 2.6 for details of GPS installation. The GPS is not absolutely necessary for the sensor to function.

4. Connect the data cable ( the one with a white connector one end and a 9 way D-type at the other) to the white connector on the breakout box. The D-type connector should be connected to the P.C’s serial port ( for example COM1).

5. Connect the power cable to the power source ( the supplied cable will have bare ends so it may be necessary at this stage to attach an appropriate connector). Connect the ‘red connector of the power cable to the ‘red’ connector on the breakout box.

6. Install SCREAM software by inserting the scream disc into drive A: of your

computer and run the set-up program. START. RUN. A:SETUP 7. Follow the set-up instructions to install SCREAM. 8. Launch scream from the start menu. It can be found in

START.PROGRAMS.GURALP SYSTEMS.SCREAMxx 9. Follow the instructions in the help to acquire data. If data does not appear then

check all connections and that the power supply is providing the correct voltage and current.

10. In the Available Streams window the identifier of the digitiser will appear in the

left hand frame (which appears similar to the tree type format of Windows Explorer) under:- Network >

Local > Com1’ (if Com1 is used)

11. The data streams will appear in the right-hand frame.


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12. The Stream ID’s are six character strings uniquely identifying each instrument,

component and sample rate. (There may be up-to four different sample rates per channel) The stream ending in ‘00’ contains status information from the digitiser. Depending upon the selected sample rate, then the streams with the higher sample rates will appear in the display sooner than the slower sample rates.

13. If a digitiser module is running then the format will most likely be 16 or 32 bit format as indicated in the Available Streams window, due to the seismic data.

14. To view data coming from the

sensor. From the main ‘Available Streams’ window, Click on the ‘Windows’ button, Choose ‘New WaveView Window’ to create a WaveView window for displaying the data. 15. Select the data streams in the right side of the window and drag them into a

‘Waveview’ window. 16. SCREAM will now display digitised data in the ‘Waveview’ window

Above is shown a basic WaveView window showing one 3 component instrument. 17. To see status information coming from the digitiser, right click on the status

stream, from the pop-up menu select ‘View’. A new window, ‘Status’ should


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open containing text. The first blocks will give the boot message from the CMG-6TD, including its software revision and the data streams selected for down-loading and triggering ( this data would have been lost if the sensor was running before SCREAM to see this information power cycle the sensor once and wait). Later blocks give information on the expected GPS satellites, the location of the GPS antenna, time synchronization status and transmit and receive baud rates for each channel and the data link.

2 Installation

2.1 What you will need to install the system Installation of the CMG-6TD system requires the following:

1 12 volt, 1.2 watt regulated DC power supply,

2 PC with GCF-compatible data acquisition software (Scream! software for Windows recommended) or a palm top with ( Shout software )

3 Installation equipment such as shovels, plastic bags ect.

2.2 Choosing a location

2.2.1 Reducing background noise The CMG-6TD is a sensitive instrument designed to respond accurately to extremely small movements of the ground. The motion of the ground at any site represents the sum of the motions from many possible causes: distant earthquakes, local earthquakes, microseisms, heavy surf on nearby beaches, and man-made sources such as quarry blasts, heavy machinery, and vehicular traffic. It is therefore important to give careful thought to choosing a site that is appropriate for your particular application and that will minimise the effects of unwanted noise. For earthquake studies, the ideal site for a seismometer is in an underground vault, with the sensor installed on a concrete pier that is itself in direct contact with bedrock. Below-ground installation tends to reduce the effects of natural microseisms, temperature fluctuations, and man-made surface noise, while good contact with bedrock tends to minimise the signal complexities and noise that are often introduced as seismic waves travel through layers of soil and sediment underneath a station. The pier should be decoupled from the foundation of the surrounding structure so that vibrations within the structure itself are not transmitted to the sensor. Understandably, this ideal installation may well lie beyond the reach of many CMG-6TD users. Fortunately, however, excellent results can be obtained by installing the sensor on the cement floor of the basement or sub-basement of an existing building, preferably in a quiet corner away from foot-traffic and heavy machinery (air-conditioners, heating systems, elevators, etc.). Installation on upper floors is not recommended.

2.2.2 Temperature stability Although the sensor operates over a wide temperature range (-10 to +75 deg. C), the sensor mass can be quite sensitive to rapid fluctuations in ambient temperature, thereby affecting the sensor's long-period velocity response. Additionally, sunlight or other bright lights striking the sensor or the floor and walls nearby can induce small mechanical stresses that may be detected by the sensor. These undesired effects can be minimised by installing the sensor in a dark, protected corner of a basement or by enclosing the sensor in an insulated box.


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2.2.3 Additional considerations

Routine monitoring of data produced by the CMG-6TD is carried out remotely on a data acquisition computer that is connected to the breakout box via a serial cable. As long as the serial cable is kept to a reasonable length (<50m) there is no particular restriction on the location of the data acquisition computer. It is helpful, however, to have the computer located nearby during the initial set-up and testing procedures. The GPS time-code antenna unit should be located above ground level with a good view of the sky. Neither the sensor nor its cable should be situated near other electrical cables (power, telephone, data, etc.) as stray radiation from these sources may interfere with the sensitive electronics in the sensor. The surface upon which the sensor is installed should be free of cracks. Small cracks tend to open and close slightly with changes in temperature and humidity, causing small movements in the floor, which can affect the seismometer.

2.3 Installing the sensor in a vault or basement

Choose a spot on the floor that is smooth, level, and free of cracks. Prepare the surface by cleaning it thoroughly of any loose particles of dirt and dust. The sensor should be placed vertically, with the three metal feet making good contact with the floor. The sensor does not require precise levelling, as long as it is oriented to within approximately +/-3 degrees of the vertical. Proper levelling of the sensor will be discussed in Section 3.2. The sensor system should be assembled as in section 2.5. It is important to cover the sensor with a thermally insulated box. Care should be taken that the box will not interfere with the sensor cables or the sensor package its self.

2.4 Pit Installations and recovery for rapid deployment experiments.

2.4.1 Installation. Pit installation could be used when seismometers are put down to test volcanic explosions or the activity of a volcano that is very active, etc.

1) Dig a hole between 60-90cm deep, as shown,


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Figure 1

2) Clean the hole down to the bottom, and remove any loose material around the mouth of the hole, ensure that the bottom of the hole is relatively flat, as shown,

3) Set up the instruments to make sure you know how they fit together. See section 1.3

4) Prepare the instrument in a package to keep the water out, as shown,


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1) Place the seismometer into the hole, inside the plastic bag, with the two wires coming out of the top. It may be necessary to place some lose sand or soil in the bottom of the hole so that the sensor can be levelled, make sure if this is the case that the sensor is pressed down firmly (NOT TAPPED or HIT in any way) so that it beds itself into the soft sand. You should also ensure that the sensor is level at this stage by checking the bubble level on top.

2) Pack soil or sand around the seismometer, to hold it steady, make sure that the soil or sand is firmly compacted and not all loose. Check now that the sensor is still level using the bubble level on top of the sensor. If it is not it may be possible to adjust the sensor in the hole and then repack the soil or sand around the sensor. If this is not successful the sensor must be removed, the hole cleared of debris and the operation should be restarted from step 2.

3) Once the soil or sand is packed to the top of the sensor and it is level, place the breakout box and any excess cable on top of the sensor inside the plastic bag. Group the cables coming from the breakout box together for about a meter ( use some insulating tape to ensure that they stay together). Tie the top of the plastic bag and fold it over so that the water cannot get in. Excess cable should be left in the plastic bag to minimise any possibility of damage during the recovery of the sensor.


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4) Then cover it with the soil or sand until you cannot see it. Make sure that the

soil is compressed and the ground is level.

5) Attach the GPS and position it so that it has a good view of the sky. Bury the

cable so that it is not visible. If possible place the GPS near the sensor pit so that the location on the sensor can be found more easily. The 9 way D-Type of the data cable and the connector on the IEEE1394 cable should be sealed inside a plastic bag to protect them from moisture, and this should be buried next to the GPS, if possible, so that data can be collected in the future.

6) Dig a hole large enough for the battery to fit into. Attach the sensor power cable to battery and wrap up in a waterproof packaging (plastic bag). Tie up the bag to make the battery as water proof as possible. It is not necessary to


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make this hole as deep as the sensor it only needs to be as deep as the battery plus 10cm.

7) Place the battery into the hole burry the power cable between the battery hole and the sensor.Compact soil or sand around the battery and fill in the hole, just as you did with the sensor.

2.4.2 Sensor recovery ***The only thing you have to be aware of is that you aren’t just digging normally; if you are not care-full you could break a very expensive piece of equipment. It is advisable that you should wear some clothes that you don’t mind getting dirty.

If you have installed the sensor as indicated above the first thing to do is to locate the GPS. The cable from the GPS to the sensor now allows you to find the location of the sensor. Once you have found the sensor you should carefully remove earth until you find the power and data cables. Using the power cable as a guide find the battery and carefully dig away the earth to reveal it. Don’t forget that the battery is only in a shallow hole. Once you have found the battery remove the power to the system by disconnecting the power cable from the battery. This will reduce the possibility of damaging the sensor electrically if the power cable is cut during instrument recovery.

If you have used the depths that were mentioned earlier. Between 60 and 90 cm deep, then it should be all right, to dig about the depth of a spades head, without hitting the instrument. Take great care not to damage the wires coming out, if the instrument has been installed as described above all cables should be together.

Using a small hand shovel carefully continue to remove soil/sand from the top, following the group of cables until you start to uncover the plastic bag. This operation should be carried out carefully as it would be quite easy to damage the sensor cabling that is buried under ground. Now using the small shovel to gently dig out more earth enlarging the hole until you can see the entire plastic bag. Once you can see it the best way to proceed is to use your hands, that way you can’t harm the instrument and you can get the earth out easily, but if you are someone that doesn’t like getting their hands dirty you can either put on some gloves or carry on using the shovel but be extremely careful.

Once you have cleared the earth to approximately half height of the instrument you should be able to lift it out. There are two ways to do this. If the hole is relatively


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dry open the plastic bag, remove the cables and breakout box, then lift the sensor out by its handle (DO NOT PULL IT OUT WITH THE CABLES), finally remove the empty plastic bag. Alternatively remove the sensor breakout box and cables in one go by lifting the entire bag out. This is only an option if the bag is strong and capable of with standing the load involved. Again it should be stressed that the cables should not used to pull the sensor out as this will result in damage which may not be visible but will make future installations unreliable.

2.5 Assembling the system

2.5.1 For stand alone operation. All cables to the sensor should be laid such that they do not place any force on the sensor. They should always lay on the floor near the sensor before being routed elsewhere. Picture of Cable laying.

2.5.1.1 Connect the breakout box to the sensor via the integral cable and ‘blue’ connector.

2.5.1.2 Connect the GPS cable ( the one with green connectors either end) to the ‘green’ connector on the breakout box.

2.5.1.3 Connect the other end of the cable to the GPS and position the GPS such that it has a good view of the sky. See Section 2.6 for details of GPS installation.

2.5.1.4 Connect the data cable ( the one with a white connector one end and a 9 way D-type at the other) to the white connector on the breakout box. The D-type connector should be connected to an appropriate monitoring device, a P.C. or Palm top computer with appropriate software.

2.5.1.5 Connect the power cable to the power source ( the supplied cable will have bare ends so it may be necessary at this stage to attach an appropriate connector). Connect the ‘red connector of the power cable to the ‘red’ connector on the breakout box.

2.5.1.6 Check that the sensor is operating and if the unit is operating in stand alone mode remove the P.C or Palm computer.

2.5.1.7 Connect the IEEE1394 download cable to the sensor. Make sure that it is screwed down tightly so that it is water tight. The other end of this cable should be placed along with the data connector in a continent location for use when data retrieval is require.

2.5.2 For continuous data collection via PC. The system assembly is as previously explained in section 2.4.1. It is not necessary to connect the IEEE1394 cable unless the internal data is to be collected as well as stored on the monitoring computer. The computer can be located up to 50 meters away as long as baud rates of 19200 or less are used.


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For longer distances short haul modems or RS422 could be used. The computer should be running an appropriate piece of acquisition software, for example SCREAM. Note: To avoid electrical interference with the sensor, do not run any other cables near the sensor or the sensor cable.

2.6 Installing the GPS time-code antenna unit

Although the internal clock in the CMG-6TD system is quite stable (typically less than 30 ms drift per day), best results will be obtained when using the GPS time-code antenna units. These units keep the internal clock synchronised with satellite-based UTC time signals The Global Positioning System (GPS) is a network of satellites in 12-hour earth orbit, broadcasting radio signals that, when suitably decoded, provide accurate position and time information to ground-based receivers almost anywhere on earth. The Güralp GPS unit can track up to eight satellites simultaneously, providing a time base for the CMG-6TD system that is accurate to within 1 microsecond. The GPS unit should be mounted outdoors where there is a clear view of the sky, with the top face pointed skywards. The satellites in the GPS constellation do not cover either the north or south pole well so it is important that the GPS be positioned with a good view to the south in the northern hemisphere and with a good view to the north in the southern hemisphere.

Caution: The GPS unit operates normally over a temperature range of -30 to +80 deg. C (-20 to +175 deg. F). It is therefore not recommended for use in extremely cold climates, or in hot climates where it may be exposed to direct sunlight for prolonged periods of time.

2.7 Data acquisition The CMG-6TD is designed to operate in two different modes. Firstly in a mode where all data is acquired internally into the units FLASH memory. Secondly in conjunction with a personal computer which is running an appropriate acquisition software package. The computer serves the crucial role of acquiring and monitoring the digital seismic data transmitted by the CMG-6TD, as well as configuring some of the sensor's internal settings. The Güralp Systems, Ltd. program Scream! (Seismometer Configuration, REal time Acquisition and Monitoring) is designed to serve these multiple roles and is fully compatible with all aspects of operating the CMG-6TD. Although any software that is compatible with GCF format data may be used to acquire data from the CMG-6TD, Scream! is recommended to ensure full compatibility. (See Section 7.3, "Güralp Compressed Format," for details about the GCF format.)


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It is also possible to use a PALM top to monitor the sensor outputs as diagnostics tool and also to configure the sensor parameters. The PALM top should have RS232 interface so that it can be interfaced to the sensor and you should load the software SHOUT in to the palm top. ( refer to the SHOUT manual for details of its operation).

2.8 BUFFERING – FLASH DATA STORAGE There are 5 modes available to the user with CMG-6TDs that have flash memory fitted. These include direct, adaptive, fifo, dual, and filing. To change between these modes, open the terminal connection to the digitiser and type either direct , adaptive, fifo, dual or filing ; the unit does not need to be re-booted. Direct Mode is where the digitiser ignores the flash file installed and continues operation as normal transmitting all data to the external recording device. Adaptive Mode will store all data that the sensor has not received an acknowledge for, in the internal flash memory. This situation would occur for example if the communication link to the central station failed. When the connection is restored, the real-time data transmission is resumed and recorded data are transmitted using any spare available bandwidth in the communication link. So past data during the fault period can be reconstructed. FIFO Mode uses the flash as a large output buffer when the sensor is disconnected from the external computer the data is stored in the flash memory. Once the connection is remade the oldest data from the flash is transmitted and data is then transmitted in sequence. Dual Mode uses the flash to store only triggered data while real time data is continuously transmitted. Filing Mode causes the digitiser to record all the data to the flash in a circular buffer. When in this mode, the unit transmits only ‘heartbeats’ which are just status messages informing the user that it is recording data. When in this mode, and it is required that all the recorded data be transmitted, right click on the digitiser icon and click download.

2.9 Data collection procedure The normal process for data collection is very simple:

1) Connect a firewire disk to the IEEE1394 port of the instrument using a suitable cable. The CMG-6TD is supplied with a waterproof IEEE1394 cable.

2) Ensure that the firewire disk is powered up and the cable is securly connected to the disc. In the case of the DFD unit ensure that both Disc and Firewire switches are in the on position and that a terminator is connected to the SCSI port connector.


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3) Connect a PC or palm computing device to the instrument using a suitable RS232 cable

4) Open a terminal session, and send the command ‘flush’ (pressing ^S if necessary to get the instrument into command mode)

5) Watch the messages showing progress of the flush operation. . flush Last Flush : 00200056 2002 12 17 09:22:22 00200056 Starting Transfer 00200056 CHIP - 00200056 00200812 00201012 00201812 00202012 00202812 00203012 00203812 Transfer Finished 6) When complete, ( this is indicated by the transfer finished message) send the

command ‘GO’ to resume data transmission. 7) Close the terminal session, and disconnect the IEEE1394 cable. 8) If necessary, disconnect the RS232 cable (only if realtime monitoring is not required)

The 6TD keeps track of ‘FLUSH’ operations and, under normal circumstances, will not transmit the same data twice (ie the FLUSH command only transfers data that has been recorded since the last FLUSH or FLUSHALL command). In the event that the FLUSHed data is lost (e.g. a disk failure in the field), all data from a 6TD can be recovered using the FLUSHALL command. This is used in the same way as FLUSH above, but will transfer all valid data in the flash. Should something go wrong during a FLUSH operation (e.g. loss of power), the instrument assumes that none of the data has been transferred successfully, and will not update its record of transferred data. If a terminal device (e.g. PC or palm) is not available, or connection to the instrument is not possible (e.g. damaged cable, or non-functional RS232 port due to lightning damage), data can still be flushed to disk. At power-on (not a software reset), the 6TD will check for the presence of a firewire disk. If found, it will automatically initiate a FLUSH. So, to recover data when a terminal session is not available:

1) Connect and power up a firewire disk to the 6TD. 2) Power-cycle the 6TD instrument 3) Watch the firewire disk for activity. Using a disk with an activity or access

LED is recommended. 4) When disk activity stops, the firewire disk can be unplugged.

3 Powering-up and Testing the System


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3.1 Testing the power supply Turn on the power supply and measure the DC power supply current to see that it is within the range 100-170 mA.(with the GPS connected) If the current lies outside these limits, turn off the power supply immediately, and contact Güralp Systems, Ltd. for assistance (see Section 7.2, "Getting help").

Note: To measure the power supply current, either of two methods may be used. If the power supply has a built-in current meter, the current may be read directly from that. Alternatively, you may insert a 1-ohm, 5-watt resistor in series with the POSITIVE lead from the power supply, and measure the voltage drop across the resistor. The meter reading, in volts, will be equivalent to the current, in amps, through the circuit. Remove the resistor from the circuit after performing this test.

3.2 Testing the mass position In order for the sensor to operate properly the sensor mass must be able to move freely along the vertical axis, which requires that the sensor be installed in a nearly vertical position. Precise levelling of the sensor is not required, however, as long as the sensor is oriented to within +/-3 degrees of the vertical. To verify that the sensor is indeed positioned correctly, the orientation of the sensor mass may be inferred by measuring the MASS POSITION by examining the mass position output stream. Alternatively the Mass Position can be checked instantaneously by opening a terminal from within scream ( See scream manual for details) and typing masses? cr This will display 3 numbers which represent the mass position (Z,N,E) in terms of counts. For best results from the sensor these values should be less than 1,000,000 but the sensor will still perform adequately above this range. The unit features a bubble level on its lid to clearly indicate the position of the unit. Levelling can be achieved by adjusting the two adjustable feet.

4 CMG-6TD Configuration and Control Mode A CMG-6TD may be reconfigured using the SCREAM configuration set-up interface or by the use of terminal commands. Using SCREAM, for any given CMG-6TD, the configuration interface module may be accessed by double-clicking with the left mouse button on the CMG-6TD’s icon in the Available Streams window. If you single-click on the CMG-6TD’s icon with the right mouse button, you must select Configure from the pop-up menu. Using this module of SCREAM, you may interactively set the CMG-6TD’s system characteristics, control the output of streams at different digitisation rates, turn on or off the output from the low digitisation rate multiplex channels (Mux Channels), set output baud rates and CMG-6TD buffering parameters, as well as invoke seismometer calibration. Using any standard terminal program such as Hyperterm or Kermit, these parameters may also be changed by sending text commands to the CMG-6TD. This mode may also be invoked from SCREAM by single-clicking on the CMG-6TD’s icon with the right mouse button and selecting Terminal from the pop-up menu. When using standard terminal programs, you must initiate command mode by typing Control-S when in the text mode. This is done automatically by SCREAM when a terminal window is opened to a CMG-6TD. If you use the SCREAM configuration set-up interface, data collection will continue while you are setting CMG-6TD parameters. If you use SCREAM’s terminal mode or another standard terminal program, data collection will be interrupted until you exit terminal mode by issuing a re-boot command. Parameters from most of the commands are stored to the battery-backed CMOS and only take effect when the CMG-6TD is rebooted. Some seismometer control commands, such as calibration take place immediately. When you click the Download button from the CMG-6TD configuration set-up interface, the parameters you have chosen are transferred to the CMG-6TD and it is automatically rebooted. You will notice a data gap in the Waveview window corresponding to the CMG-6TD you have rebooted. This occurs because the reboot automatically clears the data buffer and resets the output block counter.

4.1 CMG-6TD Configuration:- Using ‘SCREAM’ To access the CMG-6TD configuration set-up from SCREAM, double-click with the left mouse button on the CMG-6TDs icon in the Available Streams window ( NOT the Local or COM port icons). Alternatively, you can single-click on the CMG-6TDs icon with the right mouse button, then select Configure from the pop-up menu.


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System Identifier and Serial Number: The CMG-6TD type is identified by its

system identifier and serial number. These two parameters are stored as the first two 32-bit fields in the header of each data and status block generated by the CMG-6TD to indicate the blocks origin. Each of these parameters consists of 6 alphanumeric encoded as base 36 numbers. On delivery from the factory, the system identifier and the serial number are, respectively, set to the GSL works order number and the CMG-6TD serial number. The System-ID can be reset to any convenient combination of letters and numbers, such as an abbreviation of your institution.

Sensor Type: The sensor type in the case of the CMG-6TD is fixed and can not be

changed. If an attempt is made to change the sensor type it will be ignored. GPS Type: The CMG-6TD will only function with a Motorola GPS and so this parameter is fixed and cannot be changed. CMG-6TD OUTPUT CONTROL PROGRAMMING The screen shot below shows the Output Control window for a CMG-6TD.


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Sampling rate: The raw data of the CMG-6TD is sampled at 2000 Hz. These data are

filtered and reduced to lower rates using a digital signal processor (DSP). The DSP has 4 cascaded filter/decimation stages each of which can be programmed for decimation factors of 2, 4, 5, 8 or 10. The output of each stage is called a “tap”. Each filter stage may be configured for a different decimation factor.

The four windows on the left of the Output Control screen (shown above) allow you to select the sampling rates for the four filter taps. The upper window corresponds to tap 0 and can be set to the highest sample rate required. Each of the other taps may have a sampling rate lower than its predecessor above, if the rate can be achieved by decimation by 2, 4, 5, 8 or 10. Clicking on the window shows a list of the rates that are permitted, given the sampling rate in the window above it.

If some of the outputs are not required then leave the buttons ‘unchecked’ to save communications capacity.

Stream selection: The CMG-6TD has three channels or streams. These are depicted

by the three columns of small windows labelled Z, N and E in the Output Control window shown above.

A tick in a box will give an output for the corresponding channel (column) at the corresponding sample rate (row). For each sample rate there are two possible rows to tick. The upper row for each sample rate will give a continuous output at that sample rate; the lower row, shown diagrammatically as passing through a switch, will only output data when its trigger criteria are met (see below).


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The Stream IDs displayed in the main Available Streams window has six-character ID’s. The first four characters identify the CMG-6TD, the last two characters identify the stream from the CMG-6TD. The first of these two characters identify the channel, the second defines the ‘tap’, or filter output. For example; for the Output Control configuration shown above, at the beginning of this sub-section, there will be three data streams, Z, N and E, outputting data at 100sps, 20sps and 2sps. This is shown below, where the digitiser ‘1123’ has the following streams:-

• Z0,N0,E0 are input channels Z, N, E output through the second tap ‘0’; • Z2, N2, E2 are input channels Z, N, E output through the second tap ‘2’; • Z4, N4, E4 are input channels Z, N, E output through the third tap ‘4’, • Z6, N6, E6 are input channels Z, N, E output through the fourth tap ‘6’, • 00 is the digitiser status stream (notice no sample rate), • M8, M9, MA are sensor mass positions for Z, N, E channels • MB, ME, MF are three of the optional eight 16bit channels available

Triggered output stream selection: For each tap there are two rows of boxes where

the user can tick either triggered or continuous data outputs. The CMG-6TD applies a simple short term average (STA) - long term average (LTA) algorithm to a selected stream or set of streams to determine whether the trigger condition is met. These streams may be bandpass filtered before evaluation using standard bandpass parameters. The data transmitted due to the trigger may be from different streams than those used to determine the trigger.

For this to function properly, triggering streams must be selected and trigger criteria must be set by clicking on the Trigger button. When at least one stream is selected for triggered output, selection of triggering streams and

System Identifier

Serial Number

Channel & tap number


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trigger criteria are enabled. It is possible to trigger off of one tap but record data from one or several different taps. (TRIGGERED)

Triggering streams selection: The triggering tap is selected by marking the circle

next to the tap. This tap need not be the tap from which streams are transmitted when a trigger occurs. If you set triggering, you must also set the parameters for the trigger criteria. (TRIGGERS)

PROGRAMMING TRIGGER PARAMETERS

In general, it is not advisable to trigger from broadband data. The button near the bottom of the Configuration Setup window therefore allows the user to select from a set of standard band pass filters from a pull-down menu. The chosen filter will be applied to the streams from the triggering components before they are tested for the trigger condition. The corner frequencies of the pass band of the filter are determined by the Nyquist frequency, which is

Ratio of STA/LTA needed to declare a trigger

1 1 1 1

1 1 1 1


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given by the sampling rate of the triggering data. The three filter options have pass bands between 10 % and 90 %, between 20 % and 90 % and between 50% and 90% of the data’s Nyquist frequency, respectively.

Trigger criteria: Trigger criteria for the simple short term average/long term average

(STA/LTA) trigger function may be set in the Trigger Setup window, accessed by clicking on the Trigger button near the bottom of the Output Control window.

The three tick boxes down the left side of the Trigger Setup window (Z, N, E) allows the user to choose the channels which will be tested for a trigger condition. The second column of boxes, the user must set the averaging intervals for the short and long term averages (STA and LTA). Typically, the time interval for the short term average should be about as long as the signals you want to trigger on, while the long term average should be taken over a much longer interval. Both the STA and LTA values are recalculated continually, even during a trigger. The third column of boxes are for defining the STA/LTA ratio which will constitute a trigger for each of the components selected for triggering. The system declares a trigger when any one of the triggering components exceeds this value. The trigger ratio is continuously recalculated for all components and the system will detrigger when all the components selected for triggering have fallen below their respective ratio values. The final column of two boxes on the right-hand side allows the user to select the pre-trigger and post-trigger data intervals. These values determine the minimum length of data that will be saved prior to the trigger condition, and how much data will be saved after the trigger condition has lapsed. Regardless of the intervals chosen, the data for the triggered streams will begin on an even second. If the box Common Values for parameters is ticked, a trigger parameter entered for one component will be used for all selected components. (BANDPASS, STA, LTA, RATIO, PRE-TRIG, POST-TRIG)

SETTING CMG-6TD COMMUNICATON PARAMETERS If the CMG-6TD has a duplex link for handshake communications with an acquisition computer, its COM ports must be configured to match the communications parameters of the attached device or computer. If the digitiser is connected directly to a computer running SCREAM, SCREAM can auto-detect the communication baud rate under most conditions. If data from the digitiser must pass through a telemetry link or into a Guralp Storage and Acquisition Module (SAM), it is very important that the baud rate and other COM port parameters for the digitiser match that of the telemetry device, whether it be a modem, a radio or the SAM.


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Data Port Baud Rate. The Baud Rates window of the CMG-6TDs configuration

set-up allows you to program the baud rate and stop bits for the data port. The baud rate you choose must satisfy two conditions. It must be high enough to allow all the transmission of all data generated by the digitiser at the sampling rates you have chosen. For three streams of data at 100 samples per second, for example, 9600 baud will usually be sufficient. If you wish to transmit 200 samples per second data, however, the baud rate must be at least 19200. The second condition for the baud rate, is that it matches that of the telemetry equipment. While modern modems often offer transfer rates up to 56 kbaud, the telephone or transmission lines may not support these rates. The same holds true for radio telemetry.

Stop Bits In most circumstances this can be left set at 1. The Stop Bit option gives

the user the choice of setting the serial transmitted data stop bits to 2 if required over ‘difficult’ transmission lines. Whilst it can be an aid with say, a radio link, it will add an overhead to the data of 10

1 .


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SENSOR CALIBRATION

The CMG-6TD can generate either sinewave or step signals to calibrate the attached seismometer. Sinewave frequency can be from 10 Hz to 0.1 Hz (0.1 to 10 second period) and is applied to all components simultaneously. The step (squarewave) calibration is specified in minutes between changes in state. The Z component must be specified to activate the calibration circuit and the frequency or period by Hz or Seconds respectively. Note that only integers can be specified for frequency/period, so to generate a 0.5 Hz signal use ‘2 Seconds’ or for a 0.25 second period use ‘4 Hz’ .e.g. Z 4 Hz SineWave will generate this signal in the calibration coil of all components. The calibration signal will be automatically disconnected after the specified time. This avoids the system being inadvertently left in the calibration mode. If the signal is required for longer this can be specified, e.g. ‘5 Minutes’ will set the timer so that the calibration is disconnected after 5 minutes - this timer is then reset to its default value of 2 minutes. NOTE: Sinewave calibration signal starts and stops on the zero crossing. Step calibration is specified by the Squarewave button, which generates a positive step on the start of the next minute (of the internal clock) and a negative step the specified number of minutes later (default 2 minutes). This calibration is also disconnected after the same (specified) number of minutes after the negative edge.


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SENSOR MASS CONTROL

The button Centre will re-centre the mass for operation when installed. This actually switches the sensor into 1 second mode for 10 seconds and the back to broad band. The system can be set to perform this function if the mass position exceeds a predefined percentage of full scale. Enable this by checking the box in this window. AUXILIARY (MUX) CHANNELS The CMG-6TDhas as standard, five 24bit, 1 samples per second channels. Three of these report the sensor mass positions, one is for inputting calibration signals and the final one for reporting the internal temperature of the digitiser. In addition to these five channels, there are a further three channels available at the time of manufacture for user specified options (pressure monitor, borehole orientation, etc). The additional eight Auxiliary channels are not implemented on the CMG-6TD.


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Collection and transmission of the environmental channels is controlled using the Mux Channels window. If a tick is placed in the box next to a channel, its data will be collected and transmitted as a data stream in GCF format, just as with the normal data channels. To indicate that the data comes from a Mux channel, the Stream ID will be in the format ‘****Mn’, where M stands for Mux and n is a hexadecimal integer. The integers between 0 and 15 are indicated by 0 - 9 and A - F, for 10 through 15. Mux channels M8, M9 and MA will produce mass position for the vertical, north/south and east/west components respectively. Channel MB monitors the calibration signal and channel ME monitors the internal temperature of the digitiser.

4.2 Digitiser Configuration: - Using Terminal These commands are not case sensitive, but in order to identify commands, all commands have been written as uppercase.

GO

N Resumes data transmission, in the case of Scream this command will also cause the terminal window to close. If it is required that the terminal session is to be terminated and the data transmission is to be resumed, type ‘GO’

OK-1 N Enables ‘protected’ mode commands indicated as a ‘Y’ in this table. In order not to access all commands all the time, some of the commands are locked until the OK-1 is typed from the terminal.

[SEAL] Y Returns to ‘user’ mode (opposite of ‘OK-1’) RE-BOOT N Reboots the instrument (using the new configuration if

changed). This will also cause Scream to close the terminal


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window. SET-CONFIG N The ‘registers’ are set-up by simply defining an 8 digit ‘hex’

code corresponding to the required contents, the most significant byte (2 hex digits) corresponding to the first tap (#0) and the least significant byte the last tap (#3). When setting the configuration all 8 digits should be entered with the , [comma] as shown. For example, to select all 3 components of a standard CMG-DM24 output at 100s/s the code is 0007,0000

For tap1: 07 in hex = 00000111 in binary = channels 0,1,2 The environmental channels can be selected (when the hardware

is fitted) in a similar fashion, but in this case only 4 hex digits are required to set the appropriate bits - msbit=channel#15, lsbit=channel#0. For example: Therefore the whole SET-CONFIG example will look like this:- Enter set-config cr Disp. Hex code to select DSP#1 'taps' 0070,0000 Enter 0007,0000 cr Disp. Hex code to select DSP#1 'taps'0070,00000007,0000 Hex code to select mux channels 00ff Enter 0307 cr Disp. Hex code to select DSP#1 'taps' 0070,00000007,0000 Hex code to select mux channels 00ff0307 ok Factory default is 0700,0000 for DSP#1 and 0000 for mux channels.

SET-ID N Sets the system ID and stream ID. Interactive command.

0007,0000Tap 0

Tap 1

Tap 3

Channel

Channel 2

Channel 1

Channel 0

0307 in hex = 0000 0011 0000 0111 in binary

Current settings


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System ID Stream ID Sensor Type

Enter set-id cr Disp. System Identifier ( GSLTST ) ? Enter GURALP cr Disp. System Identifier ( GSLTST ) ? GURALP cr Serial # (620600) 6206 ok Text in brackets indicate the current setting if nothing is entered then the current setting is retained. Factory default is GURALP 6TD.

.IDS Y Display the System ID and Stream ID which id used to label all GCF data packets. Enter .ids cr Disp. GURALP 620600 CMG-6 ok

p n BAUD N Sets the baud rate to ‘n’ for port ‘p’.6TD only has one port so the only valid parameter for p is ‘0’ all other values have no effect. Valid parameters for ‘n’ are 4800, 7200, 9600, 14400, 19200, 28800, 38400, 57600, 115200. System responds with ok prompt. A RE-BOOT command must be executed for the new baud rate to take effect. Factory default is 19200 baud.

SET-RTC N Sets the Real Time Clock. This is used when the systems is initaily powered up until the GPS is detected. Enter 2002 12 13 17 10 00 00 set-rtc cr Year Month Day Hour Minute Second 1/100ths Sec 1/100ths Sec should always be 00. System responds with the ok prompt.

TIME? N Display the internal time read from the systems real time clock. Enter time? cr Disp. time? 2002 12 16 08:38:57 ok Year Month Day Hour Minute Second

.FIX N Display the current state of GPS timing fix. The command is

only valid when the GPS is ON so if checking the GPS state use the XGPS command to switch on the GPS first. Enter .fix cr Disp. .fix 2002 12 16 09:40:09 =>> Auto 3-D SV#'s 18 29 28 5 7 9 ( 6 ) ok Date and time Fix mode current satalliets no. visable Fix mode shows the type of fix. These will be 2-D (no altittude


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can be resolved) and 3-D. The system will not carry out clock trimming unless there is a 3-D fix.

.POSITION Y Display the reported GPS position in degrees and minutes Enter .position cr Disp. .position Lat 51'21.6488N Long 001'09.8149W ok

n XGPS Switches GPS relay on/off n=0 for off, n=1 for on. The GPS relay is used for switching the power to the GPS on/off in order to save power consumption during normal operations when the digitiser is locked on to the GPS. This command can be used to override the GPS power cycling. If left on the system will check the clock and then switch off for the period set by the HR-CYCLE command. System responds with the ok prompt.

n HR-CYCLE Sets the interval between power-on operations on the GPS relay. Power-off is automatically performed when the internal clock is sufficiently closely trimmed to the GPS reference. The parameter n can be any integer from 0 – 24. When the parameter of 0 is used the GPS will be on continuously. System responds with the ok prompt.

n HEARTBEAT N Filing mode not yet implemented n TRIGGERS N Triggering not yet implemented n1 n2 TRIGGERED N Triggering not yet implemented n1 n2 CONTINUOUS N Triggering not yet implemented n BANDPASS N Triggering not yet implemented STA N Triggering not yet implemented LTA N Triggering not yet implemented RATIOS N Triggering not yet implemented PRE-TRIG N Triggering not yet implemented POST-TRIG N Triggering not yet implemented CENTRE N Performs a centring operation. On the 6TD this means switching

into 1 second mode and back. The system stays in One second mode for 2 seconds before switching back to broad band operation. Once data transmision is resumed the centring operation can be seen on the traces in scream.

n %AUTO-CENTRE N Set the threshold signal level that triggers auto-centring, as a percentage of full-scale Mass position. See also CENTRE. To switch Auto-centre off set % to zero.

n RESP N Set the instrument response (n= 0,1,2,3) Refer to calibration data for details of instrument response options. System responds with the ok prompt.

MASSES? N Display an instantaneous reading of the sensor mass positions in counts. Range is +/-8000000. Enter masses? cr Disp. masses? -203916 -2143452 2205093 ok Z Mass Position N/S Mass Position E/W Mass Position


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The following commands refer to the calibtration output control and these commands are used in combination. See additional details below for their operation. AMPLITUDE N Reads current AMPLITUDE setting MINUTES N Reads current MINUTE setting n %AMPLITUDE N Set the calibration amplitude as a percentage of full scale signal.

Valid parameters are 0 to 100 % n MINUTE N Set the duration of a calibration signal. Valid parameter is 0 to

255 minutes. Also used to set the length of step input for SQUAREWAVE command.

n HZ N Set the frequency of a calibration signal (cf SECOND). Vailid range is 0.1 to 10 Hz

n SECOND N Set the period of a calibration signal (cf HZ) Vailid range is 0.1 to 10 seconds

Z N Sets cal enable line. SINEWAVE N Starts generation of a sine wave calibration signal using the

parameters previously set by MINUTE, HZ / SECOND and %AMPLITUDE. See details below

SQUAREWAVE N As SINEWAVE, but generates a square wave (step) signal (HZ / SECOND value not used) The step (squarewave) calibration is specified in minutes between changes in state using the MINUTE command. See example below.

A sinewave or step signals can be generated to calibrate the seismometer. The component to be calibrated is specified by Z, N/S, orE/W in the case of the 6TD only the Z component is valid but this will cause the calibration to be injected into all components at the same time and the frequency or period by HZ or SECOND respectively. Note that only integers can be specified for frequency/period, so to generate a 0.5 Hz signal use ‘2 SECOND’ or for a 0.25 second period use ‘4 HZ’ . e.g. Z 4 HZ SINEWAVE will generate a 4 Hz signal in the calibration coils of each component. The calibration signal will be automatically disconnected after the specified time. This avoids the system being inadvertently left in the calibration mode. If the signal is required for longer this can be specified by using the MINUTE command. e.g. ‘5 MINUTE’ will set the timer so that the calibration is disconnected after 5 minutes. NOTE: Sinewave calibration signal start and stops on the zero crossing. Step calibration is specified by the command SQUAREWAVE , which generates a positive step on the start of the next minute


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(of the internal clock) and a negative step the specified number of minutes later (default 2 minutes). This calibration is also disconnected after the same (specified) number of minutes after the negative edge. e.g. Z 10 MINUTE SQUAREWAVE would be used to generate a long period step function suitable for calibrating a 360 second response instrument. Once the calibration command has been executed at the terminal the terminal can be closed thus resuming data transmision and the effect of the calibration command can be seen on the data within SCREAM.

SHOW-FLASH N Displays the current state of the flash file. It shows the size of

the flash file in Kilo bytes and the pointers relating to the current data and the last data that has been flushed. Enter show-flash cr Disp. show-flash FILESTORE C 0000FFFF00000000 FILESTORE K 1048160 Last Flush : CHIP - -1 0000FFFF Last write : CHIP - 35 00000FF8 ok

RESET-FLASH N Not yet implemented DIR N Display a directory list of disk contents

Enter dir cr Disp. dir N00000002 V31333934 Logon0000C000 00000000 @L00000000,00100000,0000FFC1 FW INIT DISKSIZE K 60051600 STREAM | Start | finish | length 620600 18 2002 12 13 15:08:50 2096304 2001 01 C5 04:43:24 2096288 Diskfree (sectors) 118006880 Diskfree (MB) 57620 ok The directory shows from left to right, stream ID of the sensor, the starting cluster number, the start date and time, the end cluster number, the end date and time and the length of data. If a disc is not connected or there is a fault with the cable the following mesage will be displayed. dir N00000001 FW IErr ok

FLUSH N Initiate a transfer of data from Flash memory to firewire disk, from the last point of data has been copied to the disk(where the previous FLUSH stopped) to the current write pointer.

Number of devices connected to the bus

Disc sucessfully mounted

Disc size in K Bytes


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Eg. Enter flush cr Disp. flush Last Flush : 00200056 2002 12 17 09:22:22 00200056 Starting Transfer 00200056 CHIP - 00200056 00200812 00201012 00201812 00202012 00202812 00203012 00203812 Transfer Finished If a flush is attempted imidiatly after a previouse flush the following mesage will be displayed. flush Last Flush : 00230FF8 2002 12 16 16:54:51 No data to save If when a flush command is executed the sensor cannot find a vailid disc on the IEEE1394 bus the following measage will be displayed. flush Last Flush : 00200037 2002 12 17 09:10:26 No disk found

FLUSHALL N Initiate a transfer of data from Flash memory to firewire disk, including all data. If the flash memory has wraped around then this will save the entire memory from the oldest dat to the newest. Operation is the same as for flush.

FILESAVE Y Makes a copy of the entire file store, ignoring the read and write pointers, the operation of this command is the same as the flush cammand above. In the case of this command however a repeate of the command will repeat the saving of the data in flash.

RESET_DISK RESET_DISC

N Initialises the FAT on the firewire disk to be an empty disk. It is essential that the firewire disk should be connected to the sensor. This command should only be used if the user wishes to overwrite the data on the disc. Enter reset-disc cr Disp. reset-disc Are you sure you want to re-use disc? y/n y N00000002 V31333934 Logon0000C000 00000000 @L00000000,00100000,0000FFC1 FW INIT DISKSIZE K 60051600 Disc is reset ok If no disc is connected or there is a fault with the cable the sensor will respond with the following. Enter reset-disc cr Disp. reset-disc Are you sure you want to re-use disc? y/n y N00000001 FW IErr ok

Sector that is being itt


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LOAD Y Load new firmware See subsuction on updating firmware for

details of this command. This command should not be used unless the user is familiar with its operation. Contact with the factory is advised before this operation.

5 STATUS INFORMATION Various status information is output from the CMG-6TD to report the system operation such as GPS and time synchronisation status. This status information is in plain ASCII text packaged in the same block structure as the channel data. There are usually 12 lines of information in a block. To access a Status window right click on the Stream ID ‘****00’, (where **** is the digitiser). In the example below this is 102600

Notice this is the only stream with ‘0’samples per second During boot-up the units report their model type, firmware revision number, the system-ID and serial number. This information is followed by the count of resets that have occurred and the time of this re-boot from the internal back-up clock. The following lines report the size and status of the internal flash data file as well as the date and time of the last flaush. Typical digitiser re-boot status message:

GPS StatusFirmware Version

System type

System ID & Serial number

Unit ID

Internal Time

File store chip mask

File store size in Kilo bytes

Current file store pointer

Time and date of last flush Status of IEEE1394 port


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The system will produce this status message whenever it is powered up. If this status is reported at other times it indicates that the system has been reset by the built-in ‘watchdog’ monitor. This will occur if the system has suffered a corruption due to external noise or power dips. If the CMG-6TD does not have a GPS unit connected for time synchronisation no further status reports are produced. When a GPS unit is fitted its operational status is reported and the behaviour of the time synchronisation software will also be shown. From a ‘cold’ start GPS will initially report the raw NMEA messages. All messages from the GPS that involve a change of its status are automatically reported. The raw NMEA status is reported once a minute. ( refer to NMEA status reference for details) If GPS is having difficulty in acquiring satellites there can be a delay of several minutes before a new message is displayed, but normally if the system has not been moved from its previous location it should report acquisition of 1 or more satellites and GPS time in a very short time. The report will also show the satellite numbers and their corresponding signal strengths.

The internal time synchronisation and control software will wait for the GPS unit to report a good position fix (requires 3 satellites) before starting operation. The system actually waits for 6 consecutive minutes of good messages If GPS maintains a good fix from the satellites available, the system will then switch on the control process and set the internal clock as shown by the status messages over the page.

Type of fix

Current satellites in view Number of satellites in view

Raw NMEA Messages


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The system jam-sets its internal clock at this point to be synchronised to GPS time and will also re-synchronise its sampling so that the data is accurately time-stamped to this new reference. The data transmitted up-to this point will be stamped with the time from the internal back-up ‘Real-Time Clock’ ; this is also now reset to this accurate time. Re-synchronisation will also result in the received data showing a discontinuity. The control process will now attempt to keep the internal time-base synchronised to the GPS 1 pulse per second output by adjusting the voltage controlled crystal oscillator. The control algorithm has two stages - initially it compares its internal 1 Hz time-base with the GPS 1pps and adjusts the voltage control to minimise the error. Once this has been achieved it then controls the crystal to minimise both the ‘phase error’ (offset between its internal 1 Hz and GPS) and the drift (frequency error) relative to GPS. During the control process the system reports the measured errors and the control signal applied, as a ‘pwm’ value - Pulse Width Modulation - digital to analogue conversion. If the system is operating in a similar environment to that when the system was last powered (i.e. same temperature) the saved control parameters will be appropriate and the system should rapidly adjust the drift. The system reports its control status and parameters each minute as shown below:

Internal clock synchronised to GPS. Notice the date and time are repeated. The first is the internal clock, the second is the GPS time the clock will change to


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The offset and drift figures are the total accumulated error measurements during the previous minute in time-base units (nominally 0.5 µsec). To convert the figures to time, divide by 120 (60 * 2) to give micro-seconds. In a stable temperature environment the system should soon settle down showing an offset error of only a few thousand (average error < 100 µsec) and a drift rate under 100 counts (< 1 in 10 -6). The screen shot below shows, from the top graph down, the offset, drift and pwm of a CMG-6TD’s internal clock tracking and homing-in on a GPS clock pulse over approximately a twelve hour period.

Synchronisation control parameters.(zero ‘drift’ in the first instance during coarse adjustment. ‘pwm’ is a saved parameter)


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The above graphical image was printed from a Guralp plot module to demonstrate the effectiveness of digitiser clock synchronisation and subsequently time stamped data.

Offset or phase error between digitiser internal 1Hz and GPS 1pps.Closes in to less than 5µs

Drift or frequency error between digitiser internal clock and GPS

PWM – Pulse WidthModulation control signal applied to a DtoA to control internal clock/GPS error

6 DIGITISER DATA ACQUISITION & TELEMETRY Guralp Systems provides complete broadband seismological data acquisition solutions. All of the equipment is modular in connectivity and the CMG-6TD will integrate into the system seamlessly. Other equipment that can be included in the system comprises of broadband sensors with a choice of digitisers, SAM units for data Storage and Acquisition and the CRM for data Combining and Repeating. Due to the modular nature of the equipment and the power offered to the user from Guralp Systems’ SCREAM! Software, complete system set-ups are highly flexible, allowing rapid deployment of equipment tailored to the customers requirements. Over the page is a diagram of some of the combinations possible of data acquisition, transmission over a desired medium and collection for subsequent analysis. Shown below are two examples of typical arrangements. The CMG-6TD can be used in any location indicated as a sensor/DM combination. Example 1:

Two 3 channel analogue sensors could be connected to a DM24, then via a direct cable link to an acquisition computer. This could then retransmit via telephone modems or the internet.

Example 2: Up-to seven sensor arrays could be connected to a Guralp SAM (Storage and

Acquisition Module) or CRM(Combiner/Repeater Module) unit. These units can combine several serial input channels into one serial stream for transmission via a radio modem, cable or, using a serial server, directly over the internet. A GPS can be used on the CRM or SAM, instead of each digitiser, to synchronise and time stamp all the data streams.

The CRM is designed specifically as a telemetry repeater station. Incoming RS232 data streams from Guralp digitisers will be at 9600, 19200 or 38400 baud depending upon sample rates, number of channels and compression factor. The CRM combines the asynchronous incoming data streams and outputs at an appropriate baud rate (up to 115200 baud output). For example four incoming streams at 9600 baud will be output at 38400 (assuming the transmission medium can sustain this baud rate; i.e. some radio modems may have lower baud capacity). The SAM unit functions in a similar manner to the CRM, except it has a SCSI hard drive to store data. If, for instance, the transmission link is less than 100% effective or in danger of being disrupted, the SAM continuously records all the data in a ring buffer. Should the transmission link then be lost, this ensures no data has been recorded. The main design purpose of the SAM though, is where there is no means of transmitting data from a remote or difficult site, where it can store data over long periods, depending upon the quantity of data and disc size (2.1Gb standard, or 4.3Gb option).


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6 Care and Maintenance Once the system is installed in a suitable location the unit should not require any maintenance, and they can be left unattended for long periods of time. It is prudent; however, to inspect the site at least monthly for signs of insect or animal infestation, water seepage through the floor, or any other signs of structural deterioration that may threaten the operation of the CMG-6TD later on. The GPS unit should be checked from time to time to see that it is clear of leaves or other debris and obstructions.


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7 Troubleshooting and Assistance

7.1 Frequently-asked questions

• Question: The observed signal from the CMG-6TD contains substantial noise

at about 1 Hz and above. What can I do? Possible cause: Man-made noise. Solution: Keep sensor away from noise sources such as machinery (elevators, air conditioning, heating systems, pumps, etc.) and traffic (pedestrian, motor vehicles, railroad). Mount sensor below ground level. Enclose the sensor in an insulated box to reduce the effects of drafts and temperature fluctuations. If none of these solutions is practical, then you may be able to remove some of the noise later during signal processing through the judicious use of filters.

• Question: The observed signal from the CMG-6TD contains substantial noise

at about 0.2 Hz. What can I do? Possible cause: This is the natural microseismic background noise. Solution: The bane of earthquake seismology, microseisms are an inescapable fact of life in many locations. If possible, install the sensor directly on bedrock, or underground in a seismic vault.

• Question: The observed signal from the CMG-6TD contains a significant

long-period drift. What can I do? Possible cause: Temperature fluctuations in the sensor, installation problems. CMG-6TD is a very light instrument and the thermal time constant of the sensor is substantially smaller than similar broadband sensors. Small thermal time constant is required for quick settling of the sensor outputs for quick deployments, however once the sensor reaches thermal equilibrium with the medium, it is also necessary to keep the sensor temperature stable at least with 20 times the time constant of the sensor long period corner frequency. Solution: Isolate the sensor from temperature changes and drafts by enclosing it in an insulated box. Keep the sensor away from strong sources of light and heat (e.g., direct sunlight, incandescent lights, etc.). Keep a record of the times of day when this drift is observed; such a record can help you track down its origin.

7.2 Getting Help

7.2.1 Limited Warranty Your CMG-6TD system was thoroughly tested and calibrated before it left the factory and should provide years of continuous problem-free operation. It is guaranteed to be free of all defects in workmanship and materials for a period of 24 months from the purchase date shown on the purchase invoice. (This warranty does not apply in the event of misuse or abuse of the product or as a result of unauthorised alterations or repairs.) If you should require service, either during or after the warranty period please contact Güralp Systems, Ltd. for further information.

7.2.2 Technical support If you have any questions about the installation or operation of any aspect of the CMG-6TD system, contact Güralp Systems, Ltd. Technical Support by e-mail ([email protected]) or by telephone at +44 (0) 118 9819 056 (9am-5pm GMT). Updates to this manual, as well as other technical information about this and other products from Güralp Systems, Ltd., may be found on the Internet at our web site: http://www.guralp.com/.


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8 Advanced Technical Procedures

8.1 Calibration Your CMG-6TD system was thoroughly tested and calibrated before it left the factory. The results of those calibration tests may be found in the Test and Calibration Data booklet that accompanied the sensor. This section describes a method by which the user may determine the gain of the CMG-6TD. It is assumed that the calibration monitor channel MB is enabled during this test if not it should be before proceeding. The method used here is to inject a fixed sinusoidal calibration signal into the calibration coil of the instrument. This is achieved by opening a configuration window of the sensor you wish to calibrate and selecting the calibration page. Set the parameters on the page as indicated in the screen shoot bellow.

Wait for a new block of data from the Calibration monitor channel. This could take several minutes. Once the data is available open a wave view window to display the data. Using the amplitude cursors measure the peak to peak amplitude in counts. By taking the peak value over several cycles of the calibration signal any aliasing errors will be reduced.


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Repeat this process with the velocity output data.


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You now have two values the peak to peak input and output amplitude in counts. We now have to convert the input counts into volts so that we can find the input current. This is achieved by multiplying the measured counts by the volts per bit, which can be found in the calibration document. Now with the calibration resistor which is nominally a 51 KΩ resistor which is installed in series with the calibration transducer. We can find the in put current. Suppose, from the example data above we have injected 1,000,000 count (peak to peak) sinusoidal calibration signal. The bit weight of the calibration monitor channel is 0.58uV/count so the input voltage is 0.58V. This will drive a current of 0.58/51K Amps (11.4 A), and provide a calibration acceleration of (11.4*10-6) / K m/s2, peak to peak, where K is the feedback coil constant (supplied on the calibration sheet which in this case is 0.00293). The calibration acceleration is an analogue of the ground acceleration. Before the velocity output sensitivity is calculated, the sinusoidal acceleration function needs to be converted to a velocity function by dividing by the angular frequency (2ƒ) of the calibration signal. Thus velocity @ ƒ = 11.4 * 10-6 = 0.00619 m/s (2ƒ) * 0.00293 By measuring the peak to peak velocity output (in counts) which we have done above, and calculating back to the equivalent input velocity using the expression given the velocity sensitivity can be calculated as shown bellow. The sensitivity, S, at a given frequency is defined as the ratio of the velocity, v, to the digital amplitude, A (number of counts):

S = v / A From our example the we get : S= 0.00619m/s/26,000,000 = 0.238*10-9 m/s/count By repeating this at a range of spot frequencies a frequency response function may be obtained.

8.2 System ID and Serial Number The System ID and Serial Number are alphanumeric strings that appear in the header of each data block transmitted by the CMG-6TD. These strings function as the


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"name" of the sensor and, in a multi-sensor station, serve to identify the source of each data stream. The System ID is a six-character string. In a seismographic station containing a number of different sensors in various orientations, the System ID might include the name of the station and the component of the sensor (e.g., PPNYCZ for the vertical component of station PPNYC, or BOS3 for sensor #3 at station BOS, etc.). The Serial Number is a four-digit string that constitutes part of the Stream ID, the GCF parameter that identifies the device, component of motion, and sample rate (see Section 0, "Güralp Compressed Format," for details). The Serial Number may also be modified to suit the user's particular requirements. (The actual serial number of the system is printed both on the outer jacket of the sensor and on the processor box, so nothing is lost by reprogramming this parameter internally.) To display the current System ID and Serial Number, type the following:

.ids <return> To change the System ID or Serial Number, type:

set-id <return> The system responds with:

System Identifier? (e.g. ALPHA,) To which you reply by typing the new System ID (up to six characters), followed by a comma. For example:

PPBOS1, <return> The system responds with:

Serial #? (e.g. 1234,00) Next, type in the new four-digit serial number, followed by a comma and two zeros:

7015,00 <return> The system responds by reporting the new System ID and Serial Number:

PPBOS1 701500 ok If you make a mistake, simply press <return> and repeat these steps. Your changes will take effect and be saved when the system is re-booted.


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9 Technical Data

9.1 CMG6TD system specifications Outputs and Response Sensor Sensitivity 2000 V/m/s Sensor output Sensitivity 2.0 *10-9 m/s/count (nominal) Standard output format 24 bits Noise-free resolution, NPR, @ 20s/s >22bits (>132dB) rms, Distortion TBD Standard frequency Band 0.033Hz (30 seconds) to 100Hz Digital Signal Processor TMS3200, 144MHz Hardware sampling rate 20KHz Output Rate User selectable RS-232 Baud Rate User selectable Physical Lowest spurious resonance: 450 Hz Operating temperature range -10 to +75 deg. C Sensor Base plate Hard anodised Aluminium Sensor jacket Hard anodised Aluminium Base diameter: 154 mm Sensor height 242 mm (including handle) Sensor weight 3.0 Kg Power Standard power supply +12 Vdc, with internal DC/DC converter (Can operate over 10 to 36 Volts) Maximum current at 12 V 165mA (with GPS connected)

9.2 Sensor calibration data See the separate Test and Calibration Data booklet that was shipped with your CMG-6TD.


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9.3 Güralp Compressed Format (GCF) All data blocks used by Güralp digital devices are in GCF format. This is a format for the efficient transmission of digitally sampled data streams where multiple sources may share a common transmission medium. Each GCF block consists of two parts: the header, and the body. For data blocks, the body contains differentially compressed sample values, and for status blocks, the body contains text status information. The header is 16 bytes long, split into four 4 byte fields. The rest of the block contains the data fields. The first field contains an absolute 32 bit sample. The difference samples then follow. Note: since the absolute value is the first sample value, the first difference will be zero. Finally, as a check, the ending absolute sample value is added. This should match the last decompressed sample value. It is NOT another sample. For additional details on the GCF format refer to the SCREAM manual.

Note: Since samples are 24 bit values, the top byte of each 32 bit field is not required, therefore not included in the data block. Therefore, for compression code 1 the data fields are 3 bytes in size. This only applies for serial transmission. Disk recording expands this field to the full four bytes.

Note: 24 bit values have a 16 million range (8 million). The difference values must be able to indicate 16 million, which is 25 bits. Since the compression code discards the 25th bit, the sign of a 24 bit difference must be determined during decompression. A simple method is to check whether the addition of the 24 bit difference would place the sample value outside the range of a 24 bit number. If this occurs, then the sign of the difference should be inverted (i.e., the difference should be subtracted).

Documents

CMG-6TD Digital Output Broadband Seismometer Operators Guide · CMG-6TD Digital Output Broadband Seismometer Operators Guide V1.00 ISSUE ECN DATE COMMENTS A 11.07.2002 First Issue