Acoustic measurements in unconsolidated sands at low effective pressure and overpressure detection
-
- Manika Prasad
- ∗Stanford University, Geophysics Department, 397 Panama Mall, Stanford, California 94305. E-mail: manika.prasad@stanford.edu.
書誌事項
- 公開日
- 2002-01-01
- DOI
-
- 10.1190/1.1468600
- 公開者
- Society of Exploration Geophysicists
この論文をさがす
説明
<jats:title>Abstract</jats:title> <jats:p>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 (Vp) can be caused both by overpressure and by presence of gas. The ratio of P-wave velocity to S-wave velocity (Vp/Vs), 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 Vs = 0, Vp/Vs 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 Vp but Vs will remain unaffected and VpVs will decrease. Analyses of ultrasonic P- and S-wave velocities in sands show that the Vp/Vs ratio, especially at low effective pressures, decreases rapidly with pressure. At very low pressures, Vp/Vs 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 Vp/Vs values can be invaluable indicators of low differential pressures.</jats:p>
収録刊行物
-
- Geophysics
-
Geophysics 67 (2), 405-412, 2002-01-01
Society of Exploration Geophysicists
