Downhole seismic logging for high-resolution reflection surveying in unconsolidated overburden

Downhole seismic velocity logging techniques have been developed and applied in support of high-resolution reflection seismic surveys. For shallow high-resolution reflection surveying within unconsolidated overburden, velocity-depth control can sometimes be difficult to achieve; as well, unambiguous correlation of reflections with overburden stratigraphy is often problematic. Data obtained from downhole seismic logging can provide accurate velocity-depth functions and directly correlate seismic reflections to depth. The methodologies described in this paper are designed for slim-hole applications in plastic-cased boreholes (minimum ID of 50 mm) and with source and detector arrays that yield similar frequency ranges and vertical depth resolutions as the surface reflection surveys. Compressional- (P-) wave logging uses a multichannel hydrophone array with 0.5-m detector spacings in a fluid-filled borehole and a high-frequency, in-hole shotgun source at the surface. Overlapping array positions downhole results in redundant first-arrival data (picked using interactive computer techniques), which can be processed to provide accurate interval velocities. The data also can be displayed as a record suite, showing reflections and directly correlating reflection events with depths. Example applications include identification of gas zones, lithological boundaries within unconsolidated sediments, and the overburden-bedrock interface. Shear- (S-) wave logging uses a slimhole, well-locked, three-component (3-C) geophone pod and a horizontally polarized, hammer-and-loaded-plate source at ground surface. The pod is moved in successive 0.5- or l-m intervals downhole with no redundancy of overlapping data as in the P-wave method. First-arrival data can be obtained by picking the crossover onset of polarized energy or by closely examining particle-motion plots using all three components of motion. In unconsolidated sediments, shear-wave Velocity contrasts can be associated with changes in material density or dynamic shear modulus, which in turn can be related to consolidation. Example applications include identification of a lithological boundary for earthquake hazard applications and mapping massive ice within permafrost materials.