1

3D-Electrical Resistivity Tomography

Surendra Raj Pant

Engineer-Geophysicist

Geophysical Research & Consultancy Service (P) Ltd

E-Mail: grcs25@yahoo.com; srpant99@gmail.com

1. Introduction

Geophysical methods are being increasingly used for the subsurface exploration of

different projects and construction sites in Nepal. Nowadays the realm of near-surface

geophysics has developed towards 2D and 3D imaging. One such development has

occurred in the field of electrical methods known as electrical imaging. Electrical

imaging technique represents a re-emergence of an old technology. The technology

was hampered due to high cost compared to other geophysical methods. A recent

development in automatic equipment which has multichannel receiving option has

made the technology feasible for commercial application. The development of

computing technology has also made the inversion of 3D field data possible.

In the inversion of 1D electrical resistivity methods the resistivity values are allowed

to change along the depth axis only. So, the model is viewed as 1D. In 2D electrical

resistivity methods the resistivity values are allowed to change both along the depth

axis and along the line of investigation. In 2D methods offline resistivity variations

are not modelled. In other words offline effects may include and could be interpreted

as inline geological structures. However in 3D resistivity inversion resistivity values

are allowed to vary in all three directions. This provides an opportunity to resolve the

subsurface in greater degree of resolution.

2 Possible study objectives

3D Electrical resistivity tomography (3D-ERT) is useful to map complex geological

features. It is useful to map the variations in the material type in the overburden,

complex geological features such as karst, undulation in the bedrock head, sheared

2

zone and water table and variation in the material type in the overburden. Since there

are no offline effects in 3D inversion the spatial resolution of the subsurface is much

higher than that of the 2D method. In Hydropower projects the method can be useful

where detailed coverage of the subsurface is needed. Such area could be damsite and

powerhouse.

3 Data acquisition

Data acquisition can be carried out in different layout systems. A large number of data

are collected (much larger data volume than in 2D-ERT) along and across closely

spaced profiles. For this it is essential to have multichannel receiving and switching

system.

Data acquisition should be carried out by using good quality multichannel receiving

system and high power transmitter. One such equipment system is known under the

brand name SYSCAL PRO SWITCH 48, manufactured by IRIS Instruments, Orleans,

France. The equipment is designed to make high precision and high productivity

measurement of electrical current and voltages (primary and decay). The electrical

voltages are measured during the current pulse transmission and after the switch off of

the current pulse. These measured current and voltages are used to calculate resistivity

and chargeability of the subsurface. The equipment has 10 simultaneous receiving

channels and automatic switching capacity of 48 groups of electrodes. Number of

channels available and switching capacity makes instrument ideal for use in imaging

of subsurface. Specially designed shielded multi-core cables are connected between

the instrument and the electrodes. The equipment has power of 250 Watts, maximum

available voltage 800 Volts and maximum available injection current 2.5 Amperes.

The equipment is suitable to explore 200-400 m depths depending on the geology and

topography of the area. It has power line noise rejection capacity. The input

impedance of the receiving channels is 100 MOhm. The voltage resolution is 1 µV

and the precision of the measurement is 0.2 %. The equipment has stacking facility

which helps to secure good quality data. Furthermore, the quality control indicator Q

(standard deviation) between the successive measurements helps to secure the quality

3

of the data. It can make measurement of induced polarization (chargeability) up to 20

automatic slices, which can provide detail nature of the voltage decay curve.

Stainless steel electrodes are used for both current transmission and voltage receiving.

Six stainless steel electrodes each of 30 cm long are grounded in each station. Strings

of 48 electrodes groups are connected to the SYSCAL PRO Switch-48 through high

quality shielded multi-core cable. The switching of these electrodes groups is carried

out automatically by the system included within the instrument.

The measurement sequences are created by using software ELECTRE PRO supplied

by IRIS Instruments. Large number of sequence can be created manually or

automatically for conventional and non conventional electrode arrangements. For

shallow depth of investigation sequences are created to make measurement along

forward and reverse direction. All the sequences are uploaded to the memory of

SYSCAL PRO SWITCH 48. During the measurement data are automatically stored in

the memory of SYSCAL PRO SWITCH 48. After the measurement is completed data

are downloaded from the memory of the instrument to the computer by the help of

software PROSYS II. PROSYS II is designed for data transfer, data visualization,

editing and preliminary processing and data export into the format of the code of

inversion.

4 Data processing and interpretation

Resistivity data collected in the profiles can be processed by using a software

RES3DINV designed by Dr. M. H. Loke. Preliminary processing could be carried out

at field. The output of the software is electrical tomograms such as horizontal or

vertical slices at different depth levels and different horizontal distances. Modelled

data could be exported to different 3D plotting program such as Slicer Dicer.

4

5 Some Examples

5.1 3D-ERT Model of Powerhouse, Bijayapur Small Hydropower Project, Pokhara

Fieldwork was carried out to make detailed coverage of the subsurface in the Powerhouse

Area of Bijaypur Small Hydropower Project, Pokhara. The measurements were carried out in

closely spaced parallel lines. The electrode spacing was 3 m and the distance between profiles

was 4 m. Total number of profiles was seven. Initially the aim of the profiling was to get 2D

resistivity model of the subsurface. If the measurements are carried out in closely spaced 2D

parallel lines same data sets can also be used for 3D-ERT inversion. The results obtained from

such 3D-ERT inversion have better resolution than provided by 2D-Inversion. During 3D

inversion the resistivity is allowed to vary in all three directions. True 3D data acquisition has

high demands in instrumental requirement and logistic support than 2D data acquisition. So,

3D data are acquired similar to 2D data acquisition but with much denser parallel profiles

with some tie up profiles across.

The measurement in seven 2D profiles was combined to make a single file for 3D-ERT

inversion. This combined profile was inverted by using 3D-ERT inversion code RES3DINV.

Basic guidelines for the geological/hydrogeological interpretation of the model is as follows:

Predominantly unsaturated gravel >150 Ohm.m

Predominantly silt 50-80 Ohm.m

Fine to medium grained sand 80-110 Ohm.m

Predominantly coarse sand and gravel >120 Ohm.m

Spring water in Tailrace area 25 Ohm.m

Here are some examples of the presentation of the 3D-ERT model. The model can be

viewed as block and cross-sections in different directions. In these models surface

layer which is shown by yellow to red colour is the indication of unsaturated gravel

with some inclusion of fines. The model indicates that silt layers are not consistent in

different parts of the area. They frequently pinched out in different parts of the area.

The depth portion indicated by intermediate resistivity is possibly the indication of the

5

fine to medium grained sand. High resistivity in some parts at depth is the indication

of the gravel dominated layer.

6

7

8

9

5.2 3D-ERT Model of River Valley (metamorphic bedrock environment)

Generally three-layered geological setup can be expected in a river valley of Lesser Himalaya and Higher Himalaya of Nepal. The first layer is unsaturated granular material (coarse-grained material above water table), the second layer is saturated granular and highly weathered and highly fractured bedrock, and the third layer is fresh metamorphic bedrock. In the assumed model the thickness of the first layer is 8 m and its electrical resistivity is given to be 2000 Ohm.m, the second layer is assumed to extend from depth 8 m to 20 m and its electrical resistivity is given to be 200 Ohm.m and the third layer extends beyond 20 m depth and its resistivity is given to be 3000 Ohm.m. The resistivity of the third layer is the typical for the fresh phyllite.

For the model of above assumed parameters apparent resistivity values have been calculated by using forward modelling software RES3DMOD. The electrode arrangement chosen was pole-dipole. This electrode arrangement is known for higher resolution and optimised for multichannel receiving equipment. Two percent of Gaussian noise has been added to the apparent resistivity. It is known that equipment with high power transmitter and high quality receiving circuit should have effects of the noise less than 2%. The repeatability of the measurement of such high quality equipment is much lower than 1%. These data has been inverted by using code

10

RES3DINV. The results of the modelling have been exported in the format of the program SLICER DICER. Block models and slices are presented below. The model indicates that the depth to water table (thickness of unsaturated layer) is 8 m. The depth to the bedrock is slightly increased. Bedrock is at depth of around 22 m instead of model depth of 20 m. Furthermore modelled resistivity of the bedrock is decreased. It ranges between 1200 to 1500 Ohm.m or slightly more than 1500 Ohm.m (initial model value was 3000 Ohm.m).

11

12