Fractured aquifers detection using the ERT method: application to the Okvau Gold Deposit, Cambodia

Abstract

In any exploration area, it is useful to assess water conditions in future pits by investigating fractures networks present in the subsurface. In hard rocks, groundwater occurs in secondary porosity that result from fracturing and faulting (Emerson and Yang, 1997).

Nine 2D lines of electrical resistivity tomography (ERT) measurements were realized over the Okvau Gold Deposit in Cambodia to determine the locations of suspected fractures zones within the crystalline ore body. Lines extend to about 300 and 400 m and span an area of 40 hectares. A five meter electrodes spacing allowed to reach depth of about 100 m.

The collected data, given as apparent resistivity values, was inverted using the Res2Dinv software to get the 2D resistivity models of the subsurface. Aberrant data points were eliminated and reciprocal errors was incorporated in the inversion process to increase confidence in the results. Reliability of the inverted data was appraised by calculating the DOI indexes.

Borehole logs data was used in intenting to relate geological features to the resistivity models calculated with the inversion. The analysis did not show any significant correlation with the measured resistivity. The most substantial correlation was noted for gold and sulphides abundances. Anyhow, boreholes informations helped in interpreting the resistivity results. Low resistivity anomalies ( < 50 ohm.m), seemingly unrelated to mineralization, were detected in the northern part of the study area.