However, if the fluid is air, as in most of structural–acoustic interaction problems, this requirement is not compulsory because the acoustic pressure is not sufficient to modify the structural motion. On the other hand, especially if the fluid is a liquid, the pressure produced in the fluid can significantly affect the structural motion and, in this case, a full coupled analysis is necessary, i.e., one should load the structure with the external exciting pressure and the acoustic pressure generated in the fluid. In fact the (flexural) vibrating structure generates fluctuating pressures in the fluid, so that the Helmholtz equation must be solved with suitable boundary conditions specifying the motion of the vibrating surfaces. In general, the fluid–structure interaction implies the contemporary solution of the equations governing both the structural and acoustic phenomena. However, an effective exchange of energy between structure and fluid is only generated from structural flexural waves, because it is only in this case that the structural particle velocities are orthogonal to the direction of the fluid wave propagation. While fluids can only sustain compressional waves, solids can store energy in shear and compression. Sestieri, in Encyclopedia of Vibration, 2001 Interaction Between Sound Waves and Vibrating Structures STRUCTURE-ACOUSTIC INTERACTION, LOW FREQUENCIESĪ. Using this behavior, overpressure can be identified, as shown in Fig. 8.20, where a very high shear transit time (at depth of 27,900–28,000 ft) corresponds to a high pore pressure. Therefore, shear transit time or shear velocity can be used as an indicator of overpressure. ![]() The gas in the formation has little effect on shear transit time however, an overpressured formation causes both compressional and shear transit time to increase ( Fig. 8.19). The shear transit time can be used to identify whether a higher compressional transit time is caused by overpressures or by gas effect ( Chilingar et al., 2002). A lower compressional velocity or higher transit time may not only correspond to an overpressured formation but may also be related to a gas-bearing formation because the gas slows the compressional velocity down or increases the compressional transit time. The challenge for pore pressure prediction is to distinguish between the presence of overpressure and gas-saturated formation from the velocity response. ![]() These facts offer an opportunity to predict pore pressure and fluid content using seismic or sonic velocities. The shear and compressional waves respond differently to reservoir fluids and pressures.
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