A crosswell seismic experiment for nickel sulphide exploration
A crosshole seismic tomography experiment was conducted in the Kambalda nickel district to image the space between two dipping exploration boreholes separated by 80 m. A downhole electromagnetic (EM) survey had disclosed a conductor (possible mineralisation) between the holes. Excellent data quality was obtained from explosive sources at 2-m increments and recorded on a hydrophone array at 2-m spacing. Over 3200 travel times were converted into a velocity tomogram. A velocity low was found to coincide with the EM anomaly. Laboratory measurements on rock and core from Kambalda suggested that the velocity low was massive nickel sulphide. A subsequent drillhole was targeted on the seismic and EM anomaly. Unfortunately, no mineralisation was found. However, the sonic log of the hole gave an almost identical velocity profile to that extracted from the tomogram, and an inductive log run in the hole revealed a high conductivity. The cause of the velocity and conductivity anomaly was found to be due to chemical alteration within the host ultramafics.