Seismic exploration for metallic mineral deposits

Laletsang, Kebabonye (2001) Seismic exploration for metallic mineral deposits. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis examines the effects of geometry on the seismic response of mineral deposits. Massive mineral deposits have comparable velocities to silicate rocks but are denser. Hence they are characterized by higher acoustic impedances which should cause strong seismic anomalies. However, mineral deposits cause only weak diffractions in seismic data because of their small sizes and complex geometries. Conventional data processing may fail to detect mineral deposits because it attenuates the diffractions. -- P-wave reflection coefficients for a density-driven acoustic impedance contrast are large near normal incidence. The SV-wave has a stronger reflection coefficient with a reversed phase compared with the P-wave coefficient beyond normal incidence. Hence, large source to receiver offsets cannot record strong P-wave responses if the acoustic impedance contrast is density-driven. This finding is important in that, while deep targets are sought, source to receiver offsets must be small to record a strong P-wave response. -- 3D seismic data for an egg-shaped model show a seismic response comprising concentric, circular diffraction patterns in time slices. For a cylinder-shaped model with a rugged surface, the data show that the relief and dip impact strongly on the seismic response. Time slices through these data show circular but discontinuous diffraction patterns. 3D data for a disk-shaped model equal in size to the Fresnel zone, show that its surface structure cannot be mapped, but it can still be detected in time slices from the circular diffraction patterns. Using 2D numerical models, this thesis demonstrates that small, dipping targets produce diffraction seismic responses with amplitudes displaced down-dip, with phase reversals at large angles of incidence for different source locations. -- The modelling results are tested on 2D field data recorded on the Duck Pond deposit (NF), which dips 35-40° SW and comprises segments with different dips. In this case, little diffraction energy resides in the plane of the seismic section and clearly 3D data are required to obtain a better seismic response. -- It is concluded that massive mineral deposits can be directly detected or imaged with reflection seismic if the appropriate parameters are used, namely, 3D seismic surveys with small source to receiver offsets and processing tailored to preserve the weak diffractions.

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