Downhole seismic logging for high-resolution reflection surveying in unconsolidated overburden
- Geological Survey of Canada, Ottawa, Ontario (Canada)
- Millsaps College, Jackson, MS (United States). Dept. of Geology
- Univ. of Geneva (Switzerland). Dept. of Geology and Paleontology
- Russian Academy of Sciences, Moscow (Russian Federation). Inst. of Earth Cryosphere
- Russian Committee of Geology, Noginsk (Russian Federation). Research Inst. for Hydrogeology and Engineering Geology
Downhole seismic velocity logging techniques have been developed and applied in support of high-resolution reflection seismic surveys. 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 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. 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.
- OSTI ID:
- 669914
- Journal Information:
- Geophysics, Vol. 63, Issue 4; Other Information: PBD: Jul-Aug 1998
- Country of Publication:
- United States
- Language:
- English
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