Resistivity Investigations for a Water Storage Dam project in Pakistan

Summary

Electrical resistivity tomography (ERT) measurements were successfully used to investigate the geological setting for a new water storage dam on the Chenab river in Pakistan. The ERT investigations could identify subsurface characteristics such as lithological units, geological structures, and the presence of groundwater within the study area. The geophysical investigations were interpreted together with information from drillings and test pits made along the geophysical profiles.

PROJECT

Method: Resistivity
Solution: ABEM Terrameter LS 2, 4 cables with 21 take outs on each cable.
Measurement: Multiple Gradient array measurement protocol. 2-meter electrode spacing.
Software for processing and interpretation: Res2DInv from Seequent.

Solution

As the investigation area was extensive and detailed information on the geological setting was needed to larger depths (around 20-50 meters), the ABEM Terrameter LS 2 was used, in 2D mode (so called ERT, Electrical Resistivity Tomography). The ABEM Terrameter LS 2 solution offers the possibility to measure IP (‘chargeability’ of the ground) at the same time as the resistivity.

For efficient data collection along profiles, multiple electrodes were placed in a line at a set distance (2 meters for high resolution). The system then automatically selects which electrodes to use for current injection and voltage readings. In a short time, a high number of data points (with varying X and Z coordinates, but a fixed Y coordinate) could be measured.

The resistivity and IP measurements were made with ABEM Terrameter LS 2, with a four-cable set up and 81 electrodes.

As the surface conditions during measurements were very dry, electrodes were watered to improve electrical contact with the ground and, by that, improve data quality.

27 ERT profiles were collected across the investigation area, with a total length of approximately 6500 meters.

Processing of data was made with Res2DInv, a 2D inversion software for both ERT and IP data. The results were shown as 2D sections of the inverted resistivity and chargeability for all 27 investigated profiles. Data were further interpreted by categorizing the resistivity values with customized colors, for different geological settings.

Results

All 27 profiles investigated with the Terrameter LS 2 system resulted in usable information on the subsurface conditions. Analyses of the ERT data, together with traditional geotechnical investigations (i.e. drilling and test pits) gave a detailed picture of the geological setting, mapping features such as soil and rock horizons. Groundwater deposits and potential aquifer zones, such as broad areas of geological fracturing, could also be delimited.

For geological interpretations, first a correlation between borehole and nearby resistivity data was undertaken, this allowed the geological sequences to be associated with a particular resistivity range. For example, 1 to 113 Ωm was considered a saturated silty sand, whereas higher values deeper down corresponded to the bedrock.

Example of an inverted resistivity section from the project. Darker green colors: Lean clay/clayey silt with silty sand from 0-1.5 m depth. Red and yellow colors: Dry silty sand with gravels and boulders from 0 to 3.5 m depth. Lighter green colors: Unsaturated silty sand 0 to 9 meters depth. Blue colors: Saturated silty sand from 9 up to 30 meters depth.

Example of a measured resistivity pseudosection, calculated apparent resistivity pseudosection and inverted resistivity section. Blue colors: Clayey silt with sand/lean clay from 0-1 m depth. Green and yellow colors: Saturated silty sand, 0.3 to 32 meters depth. Red colors: Arenaceous slate, 0.3 to 32 meters depth.

Conclusions

The project concluded that the area of the water storage dam is highly heterogenous, and further detailed investigations with geotechnical methods is strongly recommended. The ERT data can be used to position these drill holes in the most efficient way and the results from the planned drilling should be used to further confirm the geophysical interpretation.

Acknowledgement

We would like to thank WAPDA (wapda.gov.pk) for sharing the information above.

Pakistan’s Water and Power Development Authority, WAPDA was established through an act of parliament in February 1958 for integrated, rapid, development and maintenance of water and power resources across the country. This includes controlling soil salinity and water logging to strengthen the predominantly agricultural economy of the country.

WAPDA has responsibility for investigation, planning and execution of projects for:
• Generation, transmission and distribution of power.
• Irrigation, water supply and drainage.
• Prevention of waterlogging and reclamation of saline land.

The fieldwork at the water storage dam was carried out by the Directorate of Geology, Office of General Manager, Hydro Planning Organization at WAPDA.