Figure 1. SuperSting R8 IP with its main appliances.

I. METHODOLOGY2.1. Preparation

The main job in this stage in relation with this equipment is to make sure that the equipment is working properly, then to pack all appliances to be several boxes so that they are ready to deliver by air cargo (in this case from Jakarta to Medan). Another preparation work is simulation. This job is such trial and error step to decide some parameters that will be applied while data recording in the field (such as depth penetration, electrode spacing, electrode array) that relate one to each other.

Using the AGI Admin package, the simulation result can be performed as Figure 2.

Figure 2. Sample of simulation result and the configuration of recorded data

If the length of the line to be measured (in the first spread) is 825 m and electrode spacing is 15 m, Wenner array is applied, the effective penetration depth is about 200 m. The second spread of the electrode shall be 75 m overlapped. Figure 3 shows a general illustration of survey line, spreading of electrodes and penetration depth.

a aa

st-1st-2 .2

st-3 .3

Targe t a re a(s*d )

Be twe e n0.5D and D

Targ e t a readepth

d

Betwe e n0.5d a n d d

Pen etratio ndep th

D

Survey lin e leng th L

Targ e t a re a le ng th s Betwe e n0.5D and D

st-2 .1a

a = S pa si e lec trodast-1st-1 = K od e s ta tion

= station= targ et keda lam an

Figure 3. General illustration of survey line, spreading of electrodes and penetration

Depth

In rough terrain, topographic survey is required to decide the fix position and elevation of every electrode along the proposed line. Later this data is neccessary for topographic correction to create the inverted resistivity sections.

2.2. Field Work

During field work some important points which should be done, are:

Cable spreading; Receiver test; Contact resistant test; Data recording; Data down loading, and Data proccessing.

2.2.1. Cable Spreading

It is a hard work especially when the topographic condition is very rough. Each porter carried a roll of electrode cable of 200 m long and about 30 kg on his shoulder. Ideally it needs 14 men, the same number as the total cable rolls, while four others brought and arranged the electrode stakes in certain spacing. This SuperSting R8 is equiped with 56 electrode stakes. In this project there are totally 20.1 km long of survey line with penetration target at 200 m depth. So that 40 – 41 times of electrodes spreading are needed. Every electrode spreading takes several hours to prepare especially in rough terrain. Figure 4 shows the configuration and arrangement of cables, electrodes, switch box, battery and receiver.

Figure 4. Configuration of cables, electrodes, switch box, battery and receiver.

2.2.2. Receiver Test

Every time before measurement the receiver test should be run to make sure that 8 (eight) receiver work well. It takes a moment then the result will appear on the monitor. In the normal condition there value result of each receiver is about 0.5 Ohm meter.

2.2.3. Contact Resistant Test

It is part of important tests before running this equipment. This test is to ensure that all electrodes are in a good contact to the ground or earth. If there is a problem with this contact, a man who is usually one of the operators or porters should do some treatment to improve the contact like to blow the stakes deeper or add mixed water with salt as the electrolite around the stakes. It takes quite long time when the improper electrode is quite away from the man position.

2.2.4. Data Recording

When the result of the mentioned tests are quite well, the data recording can be started. There are some parameters that should be chosen before starting measurement, such as:

Electrode array; Current needed; Tolerance error; Reading time, etc.

All are computer controlled from the centre. The length of the data recording depends on the last mentioned parameters, but usually about one and half an hour. The number of recorded data relies on the type of electrode array selected. Recorded data are stored in the machine, and they are ready to be transfered to the PC computer or laptop. Using the Wenner array, the number of recorded data is about 500 points and it takes about 71 minutes to fully record for each line.

2.2.5. Data Downloading

Data can be downloaded after each recording has finished, daily, or after the field work has completed. Using a special transfer cable connecting the SuperSting Unit to the computer and AGI Admin software, this job can be done very quickly. Data is in the *.stg extension format that can be converted to other ASCII format.

2.2.6. Data Processing

This step can be completed very quickly after the data has been transfered from the SuperSting Unit, mainly if the quality of recorded data is good enough. When the data is some what poor, it can also be proccessed after some manual or statistical correction. This corection module is available in the software. The next step is to apply inversion technique to produce the inverted resistivity section. These procedures are then applied to other lines. These proccesses are conducted using the Earth Imager, the accompanying software of the SuperSting R8. The first result of the inversion usually consists of section from the top-down, with no topographic correction. Figure 5 shows example of Geo-resistivity survey result.

Figure 5. Example of Geo-resistivity survey result.