Compressional wave character in gassy, near-surface sediments in southern Louisiana determined from variable frequency cross-well, borehole logging, and surface seismic measurements
- Argonne National Lab., IL (United States)
- Gas Research Inst., Chicago, IL (United States)
Velocity and attenuation data were used to test theoretical equations describing the frequency dependence of compressional wave velocity and attenuation through gas-rich sediments in coastal Louisiana. The cross-well data were augmented with velocities derived from a nearby seismic refraction station using a low-frequency source. Energy at 1 and 3 kHz was successfully transmitted over distances from 3.69 to 30 m; the 5 and 7-kHz data were obtained only at distances up to 20 m. Velocity tomograms were constructed for one borehole pair and covered a depth interval of 10--50 m. Results from the tomographic modeling indicate that gas-induced low velocities are present to depths of greater than 40 m. Analysis of the velocity dispersion suggests that gas-bubble resonance must be greater than 7 kHz, which is above the range of frequencies used in the experiment. Washout of the boreholes at depths above 15 m resulted in a degassed zone containing velocities higher than those indicated in both nearby refraction and reflection surveys. Velocity and attenuation information were obtained for a low-velocity zone centered at a depth of approximately 18 m. Measured attenuations of 1.57, 2.95, and 3.24 dB/m for the 3-, 5-, and 7-kHz signals, respectively, were modeled along with the velocity data using a silt-clay sediment type. Density and porosity data for the model were obtained from the geophysical logs; the bulk and shear moduli were estimated from published relationships. Modeling results indicate that gas bubbles measuring 1 mm in diameter occupy at least 25% to 35% of the pore space.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE, Washington, DC (United States); Gas Research Inst., Chicago, IL (United States)
- DOE Contract Number:
- W-31109-ENG-38
- OSTI ID:
- 80987
- Report Number(s):
- ANL/ES/CP-85486; CONF-950450-11; ON: DE95013493; CNN: Contract 5088-252-1770; TRN: AHC29520%%54
- Resource Relation:
- Conference: 8. annual symposium on the application of geophysics to environmental and engineering problems, Orlando, FL (United States), 23-27 Apr 1995; Other Information: PBD: [1995]
- Country of Publication:
- United States
- Language:
- English
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