Compressional Wave Character In Gassy, Near-Surface Sediments In Southern Louisiana Determined From Variable Frequency Cross-Well, Borehole Logging, And Surface Seismic Measurements

Velocity and attenuation data were used to test theoretical equations describing<br>the frequency dependence of compressional wave velocity and attenuation through gas-rich<br>sediments in coastal Louisiana. The cross-well data (obtained from a variable-frequency,<br>cross-well seismic experiment using source frequencies of 1, 3, 5, and 7 kHz) were<br>augmented with velocities derived from a nearby seismic refraction station using a lowfrequency<br>(~20 Hz) source. Velocities obtained from the borehole-sonic tool (18 kHz)<br>were not used, because it is unclear at this time what signal phase was being detected.<br>Energy at 1 and 3 kHz was successfully transmitted over distances from 3.69 to 30 m; the<br>5- and 7-kHz data were obtained only at distances up to 20 m.<br>Velocity tomograms were constructed for one borehole pair and covered a depth<br>interval of lo-50 m. Results from the tomographic modeling indicate that gas-induced<br>low velocities are present to depths of greater than 40 m. Analysis of the velocity<br>dispersion suggests that gas-bubble resonance must be greater than 7 kHz, which is<br>above the range of frequencies used in the experiment. Washout of the boreholes at<br>depths above 15 m resulted in a degassed zone containing velocities higher than those<br>indicated in both nearby refraction and reflection surveys.<br>Velocity and attenuation information were obtained for a low-velocity zone<br>centered at a depth of approximately 18 m. Measured attenuations of 1.57, 2.95, and<br>3.24 dB/m for the 3-, 5-, and 7-kHz signals, respectively, were modeled along with the<br>velocity data using a silt-clay sediment type. Density and porosity data for the model<br>were obtained from the geophysical logs; the bulk and shear moduli were estimated from<br>published relationships. Modeling results indicate that gas bubbles measuring 1 mm in<br>diameter occupy at least 25% to 35% of the pore space.