Аннотация:Shallow water flows and over‐pressured zones are a major hazard in deepwater drilling projects. Their detection prior to drilling would save millions of dollars in lost drilling costs. I have investigated the sensitivity of seismic methods for this purpose. Using P‐wave information alone can be ambiguous, because a drop in P‐wave velocity (V p ) can be caused both by overpressure and by presence of gas. The ratio of P‐wave velocity to S‐wave velocity (V p /V s ), which increases with overpressure and decreases with gas saturation, can help differentiate between the two cases. Since P‐wave velocity in a suspension is slightly below that of the suspending fluid and V s =0, V p /V s and Poisson's ratio must increase exponentially as a load‐bearing sediment approaches a state of suspension. On the other hand, presence of gas will also decrease V p but V s will remain unaffected and V p /V s will decrease. Analyses of ultrasonic P‐ and S‐wave velocities in sands show that the V p /V s ratio, especially at low effective pressures, decreases rapidly with pressure. At very low pressures, V p /V s values can be as large as 100 and higher. Above pressures greater than 2 MPa, it plateaus and does not change much with pressure. There is significant change in signal amplitudes and frequency of shear waves below 1 MPa. The current ultrasonic data shows that V p /V s values can be invaluable indicators of low differential pressures.
