Subsurface evaluation of copper ore deposits

Summary

Electrical resistivity tomography (ERT) and induced polarization (IP) was successfully used to investigate the occurrence of copper ore deposits in the region of Waziristan, north-western Pakistan. Possible sulfide ore deposits (often associated with copper) could be outlined with a combination of resistivity and IP measurements and visualized in 3D.

PROJECT

Method: Resistivity and IP
Solution: ABEM Terrameter LS 2, 4 cables with 21 take outs on each cable.
Measurement: Dipole-Dipole measurement protocol to achieve maximum depth of investigations with adequate power. Resistivity and IP. 10-meter electrode spacing.
Software for processing and interpretation: Res2DInv and Res3DInv from Seequent.

Challenge

The resistivity of the ground under investigation can vary for several reasons, not just due to the presence of ore deposits. For example, increased clay and water content can also decrease the ground resistivity. Additionally, ore deposits are often found at greater depths (down to several hundreds or thousands of meters) and in areas that can be difficult to reach with traditional drilling equipment.

Solution

The ABEM Terrameter LS 2 solution was used as it:

Measures resistivity and IP (chargeability) simultaneously, where a sulfide ore (often associated with copper) would result in low resistivity values and a high chargeability. When measuring two complementary physical parameters, such as these, low resistivity effects from the likes of clay or ground water can be discounted.
Has the possibility to efficiently make deep and detailed analysis of the geology with the ERT (Electrical Resistivity Tomography) technique, where multiple electrodes are placed in a line at a set distance (10 meters for depth). 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, in a 2D section, can be measured.

In the investigation area 3 profiles with a length of 800 meters were investigated.

Processing of data was made with Res2DInv, a 2D inversion software package for both ERT and IP data. The results were shown as 2D sections of the inverted resistivity and the chargeability of all investigated profiles. Data was also further processed to show results from the parallel 2D lines in 3D space.

Results & Conclusions

The ABEM Terrameter LS 2 solution could, in an efficient way, resolve the subsurface anomalous ore bodies at different sites and distinguish them from the surrounding rocks. The investigations indicated that the depth of the mineralized zone is generally below 25-50 meters, except at a few locations.

The project further recommended more investigations of both resistivity and IP, as well as test pits, to increase the level of detail and confirm the geophysical findings.

Example of an inverse model resistivity and chargeability section, where low resistivity zones associated with high chargeability values are considered a probable sulfide ore deposit.

When several parallel ERT profiles are measured, the data can be processed in 3D, giving the outline of the probable sulfide ore in a distinct way.

Acknowledgement

We would like to thank CMTL and Water and Power Development Authority, WAPDA (wadpa.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.