Crosswell tomography of a sedimentary foundation at an iron foundry was affected by very high background noise; nevertheless, high‐resolution velocity images were obtained between wells separated by long distances (120 to 250 m). A piezoelectric source in a water‐filled well used long sequences (4095 cycles) of pseudorandom binary codes at high carrier frequencies (1 to 10 kHz). A 24‐channel hydrophone array in another well received the signal. Beamforming of common‐source data selected the directions and arrival times of multiple raypaths and tube waves and further enhanced the signal‐to‐noise ratio. Inversion of first‐arrival times by damped least squares imaged the compressional wave velocities. Assuming the normal consolidation condition, the porosity and shear strength images are predicted from the compressional wave velocity image. The direct measurements of porosity and shear strength conducted on the cores and boreholes were used to verify the tomographic predictions. The slight differences in the compressional wave velocity images obtained using different carrier frequencies can be used to determine the permeability image of sediments based on the Biot theory.
A narrow‐band pseudorandom binary sequence (PRBS) code was used to generate an acoustic pulse that approximates a single laser‐like frequency. Measurements of crosswell tomography under a Florida limestone aquifer were made using four different PRBS frequencies: 250, 1 000, 2 000, and 3 000 Hz. Velocity images created by different PRBS frequencies showed different velocity values in the permeable layers but no velocity difference in the impermeable layers. For selected source‐receiver pairs across these layers, crosswell experiments were conducted with more PRBS frequencies ranging from 200 to 5 000 Hz. Pulse propagation calculations using a layered elastic model indicate that the velocity‐frequency dispersion is not a result of geometrical dispersion. The original procedure to extract porosity, permeability, and shear strength from seismic velocity images has been extended to the more general case of mixed lithified and unlithified sediments. The velocity‐frequency dispersion data for the limestone layer were compared with the Biot theory and the Biot‐squirt flow (BISQ) theory. The permeability value of the cavity‐filled limestone layer inferred from BISQ theory is 200 darcies, whereas the average permeability measured from pumping tests was 331 darcies.
The hydraulic structure of the seabed (porosity, permeability and shear strength) is important with regard to many physical processes including the storage and transport of pore fluids, the consolidation and subsidence of man-made islands, the liquefaction of foundations, and the occurrence of catastrophic earthquakes and tsunamis. Propagation of acoustic waves through sediments is affected by the hydraulic properties of the sediments which may be modeled by poroelastic theories by Biot (1956), Dvolkin etal. (1993), and Chesnokov et al. (1991). Crosswell tomography measurements enable one to image the structure of sound speed and attenuation within the earth, from which the hydraulic structure is found based on the poroelastic theories. The hydraulic structures of the seabed revealed from four case studies will be presented and their implications to the earth processes of storage and transport of pore fluids, consolidation, liquefaction, and earthquake will be discussed. Acoustic waves are scatted from the seabed. It will be shown that the seabed structure can be extracted from measured acoustic bottom scatter. It will be also shown that the disturbances made in the seabed for instrumentation can act as vertical waveguides which may be mistaken as a false Biot slow wave. [Work sponsored by ONR, NSF and KSC.]
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