Acoustic measurements in unconsolidated sands at low effective pressure and overpressure detection

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 =O, 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.