High-resolution seismic Tomographic Delineation of Ore Deposits

A crosswell tomography (CWT) experiment in a hard-rock environment was carried out to image the subsurface between two 240 m wells separated by 28 m. Detonators were used as downhole sources and a 24-channel hydrophone streamer provided the downhole receivers. The receiver spacing in the streamer is 2 m. An effective 1 m receiver spacing was achieved by two deployments of the streamer with a shift of 1 m. High quality seismic data were recorded at a sampling interval of 0.1 ms.

Seismic first-break times from the experiment were used to construct the velocity tomogram between the two wells by a least squares algorithm. Large source statics were identified in many receiver gathers. These statics can have strong adverse effects on the tomographic imaging. The statics can, however, be mostly recovered and compensated for in the analysis. In a tomographic inversion, they are treated as part of the model parameters and are reconstructed simultaneously with the velocity distribution.

Tomographic images agree well with the known geological structure obtained from the two imaging wells and a third one between them. They also agree with other independent geophysical survey results (e.g. Applied Potential and TEM). The difference between the CWT and the other geophysical methods is that the CWT can provide a high-resolution 2-D image between the wells rather than simple detection and localisation of the anomaly using the other geophysical techniques. CWT can offer vital information for further exploration and may have great economic potential.