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

Abstract

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 slimhole 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 1-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.