Design of a data logger and instrument-mounting platform for seabed sediment-transport research

Continental Shelf Research, Vol. 12, No. 5/6, pp. 543-562, i992. 02784343/92 $5.00 + 0.00 Printed in Great Britain. Pergamon Press Ltd

Design of a data logger and instrument-mounting platform for seabed sediment-transport research

M. O. GREEN,* N. D. PEARSON,~ M. R. THOMAS,t C. D. REES,t J. M. REESt and T. R. E. OWEN*

(Received 12 December 1990; in revised form 17 April 1991; accepted 4 September 1991)

Abstract--A microprocessor-based data logger and an instrument-mounting platform used in shelf sediment-transport and boundary-layer research are described. The data logger is based on a master-slave concept and can support, in principle, any number of sensors, which all operate on the same real-time base. Considerable flexibility in experimental design results, which enables the optimum use of data-storage capacity and power supplies, as well as the implementation of complex sampling schemes. The instrument-mounting platform provides the physical support for current meters and optical and acoustic devices for measuring suspended-sediment concentration. The guiding design principle has been that the structure should disturb the flow only in ~vays that do not compromise the application of typical methods of data analysis.

1. I N T R O D U C T I O N

THE USE of flee-standing structures as instrument platforms in shelf boundary-layer and sediment-transport research was pioneered by STERNBERG et al. (1973), who mounted a pressure transducer, camera and single rotor on a tripod, and by MCCAVE (1973), who used a stack of rotors mounted on a tetrapod to study the dynamics of sediment-laden tidal currents. CACCHIONE and DRAKE (1979) mounted a vertical array of electromagnetic current meters on a tripod (the "GEOPROBE") together with sensors for measuring pressure, light scattering, light transmission and temperature. The sensors were interfaced to a SEA-DATA tape-based data logger and the package provided some of the first detailed measurements of sediment resuspension on the continental shelf during storms (CAccHIONE and DRAKE, 1982). STERNBERG et al. (1986) extended the utility of GEOPROBE by mounting on the frame a combination of miniature nephelometers and suspended-sediment samplers. BUTMAN et al. (1979) used a self-contained instrument package to obtain long time series of near-bed currents and sediment movement from the Mid-Atlantic Bight continental shelf. The same instrumentation, which is described in BUTMAN and FOLGER (1979), was used in support of a long-term programme of measurements sponsored by the U.S. Geological Survey and which LYNE et al. (1990) recently used to describe the spatial and temporal variation of bed shear stress along the U.S. east-coast continental shelf. Shorter experiments focused on the dynamics of single events have also

*Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, U.K. tMAFF Fisheries Laboratory, Pakefield Road, Lowestoft NR33 0HT, U.K.

543

544 M.O. GREEN et al.

been conducted; for example, WRIGHT et al. (1986) measured the bed response to a single storm in 8-m depth off the North Carolina coast with a high-resolution digital sonar altimeter interfaced to a logger on a nearby instrumented tripod. In the U.K., the STABLE rig (HUMPHREY, 1987) has been used in support of detailed studies of sand resuspension by turbulent flows and the dynamics of combined-flow boundary layers (e. g. SOULSBY and HUMPHREY, 1989). In the deep sea, acoustic current meters mounted on a tripod (WILLIAMS, 1985) and deployed at the HEBBLE site yielded measurements of the seabed stress and low-frequency forcing of the boundary-layer flow (GRANT e t al., 1985). For a later deployment at the HEBBLE site, a sophisticated onboard-processing scheme was used which rendered in situ calculations of kinetic energy and Reynolds stress (GRoss et al., 1986).

All realizations of the original concept share common elements: an open pyramidal framework with weighted feet or legs to provide ballast, and a self-contained data logger/ power supply residing in one or more pressure housings and connected to various types of instruments that are attached to the framework. The primary advantage of the self- contained instrument package over the package that must be tethered to shore or to a vessel is that the self-contained package may be left unattended and thus data may be obtained over longer periods. This increases the chance that a high-energy event will be observed, and also improves the resolution of large-scale processes.

Experimental design has been limited in the past by the capabilities of the data logger. Often, only a fixed number of burst-sampling modes have been available, and there has been little or no provision for conditional or mixed-mode sampling. Data storage is typically sequential (i.e. tape-based), which complicates adaptive use of precious data- storage capacity. This also makes quick access to the data difficult, which can provide an important check on individual sensors and on the progress of the experiment. However, with the advent of low-power, high-density and reliable solid-state devices, the traditional switch-selected tape-based data logger is being phased out. It is important that the new technology does not simply function as a larger, faster and easier-accessed data store, but that it be used to solve some of the fundamental restrictions imposed by the old technology on the collection of high-quality and useful data.

In this paper, we revisit the basic concept of data logging, and describe a microprocessor-based data logger which has been developed to meet the long-term needs of a boundary-layer and sediment-transport research programme. The device described herein represents the continued evolution of the self-contained instrument package. We describe the philosophy of the design and how this has been guided by the scientific requirements; the technical details have been published elsewhere (PEARSON and THOMAS, 1991). We also describe in this paper the rationale behind the design of the platform itself.

The data logger and instrument platform have been developed in support of a collaborative effort amongst workers from the Ministry of Agriculture, Fisheries and Food (MAFF) Fisheries Laboratory, the University of Cambridge, the University of East Anglia, and the Proudman Oceanographic Laboratory, investigating sediment dynamics in the coastal waters of the U.K. to water depths of 50 m.

2. OVERVIEW OF THE INSTRUMENT-MOUNTING PLATFORM

Figure 1 is a photograph of the instrument platform: four galvanized-steel legs with weighted feet (a "tetrapod') support a stainless-steel octagonal framework within which

Design of a data logger 545

pressure vessels containing electronics and power supplies are mounted. The base of each leg is 2{}5 cm from the central axis of the tetrapod, and the outside diameter of each leg at the base is 6 cm; we comment in a later section on the choice of these dimensions. The octagonal frame, the base of which stands 2.35 m above the ground and is 1.25 m in diameter, is enclosed within, and thus protected by, upper and lower stainless steel crash rings (Fig. 1). The whole structure, fully rigged, weighs approximately 400 kg in air and 320 kg submerged; 160 kg of the total weight is due to lead plates attached to the feet in order to provide ballast (Fig. 1).

The letrapod is lowered to the seabed on a buoyant line which is jettisoned from the ship and marked with surface floats; the same line is used for recovery. In the event the surface line becomes detached there is a backup recovery device: a rope drum wound with 12-mm diameter kevlar line with a buoy attached at one end. The drum is locked with a pin that is withdrawn upon receipt of a coded acoustic signal, thus freeing the buoy and attached line which are subsequently picked up at the surface and used to recover the tetrapod.

A two-axis tiltmeter is used to monitor the attitude of the tetrapod. If the tilt exceeds a set value then an acoustic warning is issued, and the rig can be lifted and repositioned.

All of the pressure vessels are constructed of class D or E acrylonitrile butadiene styrene plastic, which is commonly used for water pipes. The plastic housings, which are rated to 100-m depth, are cheap, lightweight, unaffected by corrosion and easily manufactured and maintained. It is a simple matter to polish or remachine "O"-ring seats should they become damaged during the normal abuse that pressure vessels tend to suffer.

Four legs were chosen instead of the more typical three; this enabled the use of less obtrusive, thinner legs and permits favourable orientation of the structure relative to rectilinear tidal flows. The arrangement of spherical-head Marsh-McBirney electromagnetic current meters shown in Fig. i is only one of the arrangements we have used; we discuss in some detail in a later section flow disturbance by the tetrapod, the mounting of the current meters and how the orientation of the rig is monitored.

We employ a range of sensors (described fully in PEARSON and THOMAS, 1991) in addition to the electromagnetic current meters visible in Fig. 1. A Paroscientific "Digiquartz" pressure sensor, which has become the de facto industry standard, is used to measure pressure fluctuations due to surface gravity waves and tides. A downward- looking three-frequency acoustic backscatter sensor (ABS) mounted approximately 120 cm above the bed provides high-frequency and time-averaged vertical profiles of suspended-sediment concentration (PEARSON and THOMAS, 1990), as does a vertical array of miniature-head optical backscatter sensors (MOBS), which have been developed at the University of Cambridge from a design by DOWNINC (1983). Five-centimetre and 25-cm pathlength Seatech transmissometers are used to measure water clarity at two elevations above the bed; these data provide a check on the acoustic and optical backscatter data. A camera mounted on one leg of the tetrapod provides information on the bed roughness, which is critical to the interpretation of vertical profiles of time-averaged velocity.

3. THE DATA LOGGER

At the heart of the instrument package is the microprocessor-controlled data logger. We required a system that could cope with the addition of extra sensors without encountering any fundamental hardware limits, such as a fixed number of analogue channels. Provisions had to be made to sample each sensor at a unique rate if desired (maximum 10 Hz), and all

546 M . O . GREEr~ et al.

SLAVE 1

SLAVE 2 Q~II~ ~ ASTER

Fig. 2. Schematic of the master-s lave concept. Each slave is connected to the master by two wires plus a common: addressed and unaddressed commands are issued to the slaves by the master over the wires indicated by the solid lines. An unaddressed command triggers every slave to take a sample, addressed commands issued in sequence request datawords from slaves, which are re turned over the wires indicated by the broken lines. Extra slaves are simply grafted to the master.

sensors had to operate on the same real-time base. Any sensor not required for a particular experiment had to be turned off in order to conserve power. To satisfy these requirements, a "master-slave" principle (e.g. HAZEN et al., 1987) was adopted.

The master unit, which contains only digital circuitry, communicates with any number of slave units and with plug-in RAM cards which act as the data store (Fig. 2). The RAM cards are battery backed, and could be replaced by floppy-, hard- or optical-disk devices. Each slave is microprocessor-controlled and can have up to 16 input channels, of which up to six can be analogue. All analogue-to-digital conversion is done by slaves.

Prior to an experiment, a "mission description record" (MDR) is downloaded from a PC into the master unit. This record, which is generated by the user on the PC (Fig. 3), is used by the master to identify the sensors to be used in the deployment and contains the details of how each sensor is to be logged. The user may select from several standard configurations, or a complete experiment may be designed from scratch. Data from any sensor may be logged in either or both of two forms: in a burst mode, or as a time-averaged value which is stored in a header that is attached to each burst output. Status parameters are also logged in headers: roll, pitch and orientation of the rig and power consumption of each slave, for instance. Following downloading of the MDR, the user informs the master of the start time, header length, burst length and interval between bursts, as well as parameters governing conditional sampling schemes (described in a later section). At any stage prior to deployment, the master unit can be switched into a "calibrate mode", in which the output from any chosen sensors can be viewed on the PC in real-time, or into a "test mode" , which can be used to generate a complete burst of data for verification without writing to the data store.

To achieve synchronous sampling, all active slaves are commanded by the master to read each attached sensor every 100 ms, and the master then requests data from the slaves (Fig. 2) in accordance with the mission description record. Note that each channel of each slave is recognized by the master, and thus the sensors of any one slave can be logged at different rates and for different durations if necessary. In this way only data that are to be logged are transferred to the master, and thus traffic on the data-links is minimized. For programming convenience, all data collected during a burst are first saved into 400 kbytes


power unit 1 00,i,0 01,i,0

unit 2 00,i,0 01,0,1

unit 3 02,1,0 05,1,0

header unit 1 00,10,1 01,i0,i

unit 2 00,10,2 01,10,2

MISSION DESCRIPTION INPUT

(Text file generated by any text editor)

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*record slave 1 in the header *channel 0, take data every 10 ticks, beginning tick 1 *channel I, take data every i0 ticks, beginning tick 1

burst unit 1 *record slave 1 in the burst 00,2,1 *channel 0, take data every 2 ticks, beginning tick 1 01,2,1 *channel l, take data every 2 ticks, beginning tick 1

unit 2 * 00,2,2 * 01,2,2 *

unit 3 * 02,10,1 * 05,2,2 *

report 25,600 *generate run statistics and full simulation for

*header length 25 s and burst length 600 s

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1 DOWNLOAD TO

MASTER

RUN STATISTICS ) FULL SIMULATION

Fig. 3. The sequence of operations involved in the generat ion on a PC of the mission description record, which is downloaded into the master unit.


of working RAM in the master unit and then transferred to the main storage RAM. This large working memory allows for 200 000 readings per burst. If a slave should fail to reply to a data request, the master will replace the expected data value with an arbitrary value of

- 1 and proceed to poll the next slave, thus not affecting the timing or the data format in the storage unit. If the master detects a slave drawing too much current, the slave is shut down to prevent damage or excessive battery drain.

The basis of the master-slave concept arose from the requirement that every sensor should share a common real-time base. This requirement is fundamental to most experiments. For instance, the measurement of turbulent or wave-driven fluxes of suspended sediment, which arise from cross-correlations in the time domain of fluctuating velocities and suspended-sediment concentrations (e.g. GREEN and VINCENT, 1990), requires exact synchronization of current meters with the optical and acoustic backscatter sensors. Similarly, co-spectra of pressure and velocity, which can be used to filter wave energy from the spectrum of turbulent kinetic energy given certain assumptions about the wave directional spread at each frequency (THORNTON, 1979), can only be computed when the pressure and velocity series are properly aligned.

Since the mission description record assigns each sensor its own sampling parameters, mixed-mode sampling can be employed. For instance, we typically sample the 5-cm pathlength transmissometer in a burst-mode at the same rate as the optical backscatter devices, and use the data for cross-checks between the two different types of instruments. For the 25-cm pathlength device, we conserve data-storage space by storing in the burst header a single value that represents an average over, typically, 25 s.

Mixed-mode sampling can also be used to great effect in the estimation of bed shear stresses using boundary-layer velocity data. Different properties of the near-bed velocity field can be used to estimate bed shear stress. The application of the log-profile method to measurements of time-averaged velocity shear (e.g. GREEN et al. , 1990) is well established, but not without its difficulties. Some of these difficulties include the effects on the measured velocity profiles of current-meter zero drift and offset, selection of an appropriate averaging interval, and specification of the mean level at which the bed shear stress appears to act. Given these uncertainties, it is desirable to have redundant estimates of bed shear stress. Two other methods of estimating bed shear stress are the inertial dissipation method (e.g. HUNTLEY, 1988), which makes use of the properties of the inertial subrange of the horizontal or vertical turbulent-velocity energy spectrum, and the cross-correlation method (e.g. GROSS et al. , 1986), in which Reynolds stresses are computed directly from the covariance of the horizontal and vertical turbulent-velocity fluctuations. The latter method suffers greatly from small, often unavoidable, errors in misalignment of the sensors (HUNTLEY, 1988), and is hardly tractable in the marine environment. The former method requires fast sampling (typically greater than 4 Hz) in order to extend the measured spectrum into the inertial subrange, and therefore generates great quantities of data. A useful compromise, and one which we have accommodated with mixed-mode sampling, is to sample just one or two of the five current meters in the vertical array at high frequency, and apply the dissipation technique at those levels. In this way, data-storage capacity is conserved while redundant estimates of bed shear stress are obtained.

Although we have not fully implemented it, onboard processing of data prior to storage is possible. The rationale is to conserve data-storage capacity by writing to memory not the raw data, but computed statistics such as means, variances, smoothed spectra and co-spectra. Either the onboard processing can be accomplished by the master unit, or the


workload can be distributed by assigning the computation to processors located in each of the slave units; each slave unit has 32 kbytes of program space and 32 kbytes of working memory. The former approach would be used to process data between bursts, thus not disturbing the communications with the slaves. The latter approach might be used, for instance, to process a data stream from a digital instrument interfaced to a slave before passing the required words to the master.

The central difficulty in such a scheme is the protection of data quality (e.g. GROSS et al., 1986). For example, although the current-meter data are usually of very high quality, sophisticated de-spiking routines must be applied prior to any analysis. The automatic implementation of such routines is not a trivial matter.

We have implemented some simple conditional sampling schemes. One such scheme is designed to enable sampling upon the occurrence of a storm. Two mission description records are downloaded: a "background" and a "foreground" task. The background task specifies a minimal number of logged sensors, and the foreground task specifies high- frequency sampling of all appropriate sensors. During execution of the background task, the pressure signal is being monitored by the master unit, and when the variance or peak- to-peak variation of the pressure data exceeds some pre-set minimum the foreground task is commenced. The foreground task continues for either 24 h or until the variation of the pressure signal falls to a pre-determined value (whichever is the longer). Prior to deployment the master unit is informed of the expected duration of the experiment so the foreground task can be run towards the end of the experiment to pad out the data store. If the storm is judged to be of enough importance (i.e. is energetic enough), then the entire memory may be overwritten by data from the storm.

The master unit can be replaced by a PC running a simulation program, and in this configuration laboratory tests and calibration of all sensors can be performed, with data being displayed directly on the screen in real time in a graphic or text format and also written to hard disk.

Finally, post-experiment data archiving and dissemination amongst researchers are facilitated by comprehensive and largely automated post-experiment data-handling methods which have been developed around the concept of treating the data logger as the "front-end" of a complete system. An image of the data store is dumped to a PC and then immediately backed-up on magnetic tape (Fig. 4). This backup occurs prior to any manipulation. In this way the data can always be recovered if, for instance, errors in subsequent handling procedures are discovered. Since the data set at this stage are in their most compressed form, this is an efficient and robust security method.

The mission description record, which is appended to every burst, is then used to decode the image file, producing for each burst one file that contains the data for every sensor. The data in the files are ordered by slave and channel number and are stored as unscaled signed integers in ASCII format. It is at this point that the user accesses the data: a suite of BASIC programs is available which will extract data from any combination of sensors, apply nominal or real calibrations, and output data in a form specified by the end-user. All the data are available to the user within a few hours of recovery of the tetrapod. The signed- integer files are also archived on a mainframe computer at a later date, where they may be accessed by the user as necessary.

We have found that having fast access to the data is an invaluable tool in experimental design. For instance, trial configurations of current meters can be quickly assessed onboard ship by a series of short deployments prior to an important long-duration


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experiment. In the same way, gain settings that are appropriate to the in situ conditions may be established.

4. DESIGN OF THE INSTRUMENT-MOUNTING PLATFORM

The design of the tetrapod was largely conditioned by the requirement that the near-bed flow be as little disturbed as possible by the presence of the structure. Since it is obviously not possible to build a perfectly unobtrusive structure, some compromises were required in the design. What were decided upon as acceptable compromises varied depending upon the experimental design.

Two different types of experiment have been performed. In the first type, we seek to measure various time-averaged properties of the near-bed velocity and suspended- sediment fields. In the second type, we seek to resolve the wave and turbulent scales of sediment suspension.

Our objective in the first kind of experiment is to measure variations on the scale of tidal cycles in the sediment transport and the bed shear stress (Z'b) that drives that transport. The burst-sampling regime reflects that aim: typically, sampling is initiated every 60 min for a period of 10 min, and may extend for periods of a few months. Analyses of this kind of data typically draw on steady-state equilibrium theories such as the law-of-the-wall for inferring bed shear stress, eddy-viscosity type models for estimating the dynamical effect of unsteady motions on mean flows (e.g. GREEN et al., 1990), and Rouse-type equations for inferring suspended-sediment reference concentrations and vertical mixing scales (e.g. VINCENT and GREEN, 1990).

To apply the law-of-the-wall to velocity data for the purposes of inferring rb requires accurate estimates of the mean flow at several elevations above the bed. Thus, the mean flow must not be significantly altered by flow disturbances induced by the tetrapod structure. To comply with this requirement, a horizontal distance of at least 20D from each leg to each current meter, where D is the leg diameter, was specified, as was a clearance of 1.2 m from the uppermost current meter to the base of the octagonal framework within which the pressure housings are mounted. The former distance was chosen on the basis of observations from tow-tank tests that showed that turbulence shed from an obstructing rod maintains its structure for not more than 20 rod diameters downstream. The 1.2-m clearance was chosen to place the uppermost current meter beyond the influence of any convective accelerations induced by the presence of the pressure cases.

A more serious problem is the fact that the current-meter mounts disturb the flow in the immediate vicinity of the sensors. We have mounted each current meter on a 220-mm horizontal arm cantilevered from a 45-mm diameter pole that hangs vertically from the centre of the tetrapod. In the past, we have mounted one current meter at 90 ° to the other four sensors in order to achieve a log-spacing (Fig. 1), however, the wake shed from the misplaced current meter can contaminate data from the other elevations. In recent experiments, we have aligned all current meters in the one plane (Fig. 5a), thus sacrificing log-spacing, but yielding good data from all five elevations. We have used this mounting method in rectilinear tidal flows, where the instrument can be aligned upon deployment such that the current meters are rarely in the turbulent wake shed from the vertical pole (Fig. 5b). A digital compass mounted on the tetrapod measures rig orientation and this information is telemetered to the surface acoustically: if the orientation is unfavourable, the rig can be lifted, allowed to swing, and replaced on the seabed. As a worst case, 90 °


misalignment of the platform (relative to the principal axis of the tidal flow) would result in contamination of half of the current data by turbulence shed from the pole. Our experience with this problem has shown that the momentum deficit in the pole-induced wake is significant and that data contaminated in this way are not easily rehabilitated (Figs 6 and 7).

In a combined wave and current flow, which is the normal case on the continental shelf, the instantaneous velocity vector continuously rotates and thus it may be impossible to align the tetrapod such that the current meters are never in the wake shed by the vertical pole. We designed an alternative method of mounting the current meters which facilitates the use, in combined wave and current flows, of the law-of-the-wall method as well as the inertial dissipation technique for estimating r b. Our intention was to produce a mounting structure that disturbs the flow only in accountable ways.

An enclosing cage-type current-meter mount was designed, which was constructed of cylindrical rods (Fig. 8). The idea was to choose the rod size such that the turbulent wake shed from each rod has a characteristic frequency higher than the Nyquist frequency (fN). Measured velocity spectra will thus still extend into the inertial subrange, provided that the sampling rate is fast enough, but shed turbulence is not aliased since it is properly averaged by electronic filters in the current meters which are matched to the sampling rate (see PEARSON and THOMAS, 1991). Choosing a maximum sampling rate of 5 Hz to resolve turbulent motions in at least a part of the inertial subrange of wavenumbers (e.g. GRANT et al. , 1984) and given that a cylindrical rod of diameter D sheds turbulence of characteristic frequency fST, where fST----0.2U/D is the Strouhal frequency and U is the mean flow speed, then D = 0.5 cm gives fs T > fN for all U > 6.25 cm s -1. Since this speed is at the low end of the range of speeds that are likely to be of interest, the cage was therefore constructed from 0.5-cm diameter cylindrical rods (Fig. 8).

Shown in Fig. 9 is an example of a spectrum of downstream turbulent velocity measured with the current meter mounted in the cage shown in Fig. 8. The expected wavenumber extent of the inertial subrange is marked on the plot (2:r/z to 2er/Ls, where L s is the sensor-averaging length), as is the theoretical -5/3 rolloff that should apply in the subrange. The measured spectrum falls off at k -~58, which is close to the theoretical rate, and there is no evidence of turbulence shedding from the cage at measured wavenumbers.

To apply the log-profile method and thus obtain alternative estimates of rb, a correction must be applied to the measured time-averaged velocity to account for the momentum defect in the wakes shed from the cage constituents. These corrections, which were determined by tests in a tow-tank, are on the order of only 5 % of the mean flow (cf. Fig. 6). We have applied these corrections to measurements of time-averaged velocity in a combined wave and current flow with some success at reproducing log-velocity profiles.

The pole method of current-meter mounting is used wherever possible (i.e. in pure, rectilinear tidal flows, or where the dominant wave direction aligns with the tidal flow), since even the small corrections needed to account for turbulent wakes shed by the cage constituents add uncertainty to velocity measurements.

Our objective in the second type of experiment is to measure directly the vertical turbulent fluxes of horizontal momentum and suspended sediment, rather than make inferences about turbulent fluxes from measurements of the time-averaged properties of the flow. To achieve this, measurements of all three components of the fluctuating velocity vector are required together with simultaneous (i.e. precisely synchronized) measurements of the suspended-sediment concentration at the same elevation. This type of


Fig. 1. The tetrapod rigged for deployment. The pressure cases that contain electronics and power supplies are mounted within an octagonal framework, which is protected by two crash rings.

554 M . O . GREEN et al.

Fig. 5. (a) The current meters arranged on the vertical pole in a single plane at the expense of log-spacing. The clearances to the legs and to the bases of the pressure housings are evident.


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~4M TIDAL FLOW Fig. 5. (b) The 5-cm pathlength t ransmissometer , the camera and the flashgun are mounted on legs such that they will not be directly ups t ream of the current meters when the tetrapod is correctly oriented on the seabed. The ABS is moun ted on the central pole, on the opposite side to the current meters in this case; the MOBS are mounted directly on the current meters in order that coincident

volumes are sampled.


Fig. 8. Vertical array of current meters mounted in the cage. The cage is constructed of thin cylindrical rods; care is taken to bind only one current-meter cable to each rising member of the

cage.


Fig. 10. Annula r -head electromagnetic current meters mounted on a vertical pole. Two horizontal components and redundant vertical components of the turbulent velocity vector at each

elevation above the bed are rendered.


experiment usually results in large amounts of data and is directed at answering specific questions about the mechanisms of sediment suspension.

To measure the three velocity components , we have used annular-head electromagnetic current meters mounted in pairs on a vertical pole (Fig. 10). With this arrangement , the horizontal velocity components plus redundant vertical components are rendered. Either the acoustic or optical sensors are mounted in such a way that they are sampling the same volume as the current meters. For the ABS this means the sensor is mounted above the current meters in the same plane, and for the optical measurements the sensor is mounted on the current meter itself, looking out towards the centre of the annulus. We have not adapted the cage concept for these types of measurements ; the high data-return rate means that these experiments usually are per formed over less than a tidal cycle and thus the rig can be oriented with respect to the tidal current in order to negate wake- contamination problems.

5. CONCLUSIONS

The design of the microprocessor-controlled data logger was based on a master-s lave principle. We have not simply used new technology to provide larger and faster-accessed data stores; instead, we have examined the fundamental constraints imposed by switch- selectable, tape-based data logging and have tried to overcome them. The result is a system that offers great flexibility and potential for imaginative experimental design.

The design of the instrument-mounting platform was guided by scientific demands. Design criteria were identified by examining typical methods of analysis and assessing how the presence of the structure would contaminate the data to be used in the analyses. The

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Fig. 6. Calibration curve for a single axis of a Marsh-McBirney 3.8-cm diameter spherical-head current meter. The solid line is the calibration determined in unobstructed flow, and the dotted line is the calibration determined with the current meter mounted in the wake of the 45-mm diameter mounting pole. The gain (units: cm s 1 volt-l) suffers an apparent increase of -35% in the wake.

(In reality, the momentum defect in the wake is approximately 35% of the mean flow.)


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Fig. 7. A sequence of hourly vertical profiles of t ime-averaged horizontal velocity in which the current meter in the second position from the bot tom was in the wake shed from the vertical mount ing pole. The m o m e n t u m deficit in the wake is clearly evident in the deviations of the profiles from the ideal logarithmic form (but note that profiles formed by excluding the affected

measu remen t are close to that ideal). See GREEN et al. (1990) for an analysis of this data set.

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k / 2 ~ [- 1/cm3 Fig. 9. W a v e n u m b e r energy spect rum of the downst ream turbulent velocity measured at z = 54 cm with the current meter mounted in the cage shown in Fig. 8. The velocity was sampled at 5 Hz, and a frozen-turbulence assumpt ion has been used to convert from frequency to wavenumber . The mean flow at z = 54 cm was 14 cm s -1, and the water depth was 25 m. The spect rum roUoff is shown between 1/z and 1/Ls, where L s = 11 cm is the sensor-averaging length, which has been determined by inspection of spectra obtained over several tidal cycles. The measured rolloff compares closely with the theoretical value, and there is no plateau or peak in the spect rum that would indicate energetic turbulence shed from the cage. A frequency-dependent gain correction

has been applied to the spectrum (PEARSON and THOMAS, 1991).


result is a platform that is not invisible to the flow, but that disturbs the flow in known and accountable ways.

Acknowledgements This work was conducted as a part of the Northeast Coast Cohesive Sediment Dynamics Programme, which is partly funded by the Ministry of Agriculture, Fisheries and Food [Project BEC3; R. R. Dickson (MAFF), Principal Investigator] and partly by the Department of the Environment [contract F22; I. N. McCave (University of Cambridge), Principal Investigator]. C. E. Vincent (University of East Anglia) and P. D. Thorne (Proudman Oceanographic Laboratory) participated in the development and calibration of the ABS; M. Mason (Bullard Laboratories) developed the optical backscatter sensor. P. A. Gurbutt (MAFF) guided the design of the post-experiment data-handling and archiving system. Contribution 2070, Department of Earth Sciences, University of Cambridge. The references to proprietary products should not be taken as official (MAFF) endorsement of those products, nor is any criticism intended of other products or manufacturers not mentioned.

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Documents

Design of a data logger and instrument-mounting platform for seabed sediment-transport research