We present a promising new seismic imaging method that uses Mach waves radiated by waves such as tube waves traveling up and down the borehole. The beauty of the technique is that a true virtual downhole source at all points in the borehole is generated using only a real source at the top of a borehole. However, the technique only works when the formation velocity (either P or S) is less than the velocity of the wave traveling in the borehole. Another disadvantage of the technique is that due to the plane wave nature of the source (conical in three dimensions), at any individual depth the source only has one takeoff angle. Consequently only one receiver position for each borehole azimuth is recorded.
In this paper, we briefly summarize the physics of the Mach waves and discuss its applicable environment, imaging coverage as well as potential applications. Then presentative acquisition configuration is simulated using an axisymmetric 2.5D elastic forward modeling code. Characteristics of the Mach wave are identified through the synthetic data.
Presentation Date: Tuesday, September 26, 2017
Start Time: 9:45 AM
Location: Exhibit Hall C/D
Presentation Type: POSTER
Albertin, Uwe (Chevron Energy Technology Company) | Shen, Peng (Chevron) | Sekar, Anusha (Chevron) | Johnsen, Thor (Chevron) | Wu, Chunling (Chevron) | Nihei, Kurt (Lawrence Berkeley Laboratory) | Bube, Ken (University of Washington)
We formulate the theory of 3D orthorhombic full-waveform inversion using adjoint state techniques for recovery of shear and p-wave elastic stiffness coefficients from acquired pressure data. In formulating the adjoint of forward Born scattering, we use a novel transformation of wavefields that allows us to use the same high performance elastic propagation kernel for the adjoint that we use for forward elastic modeling. We further derive inversion equations suitable for recovery of p-wave and shear velocities in fixed inhomogeneous density media with fixed inhomogeneous anisotropic coefficients, through the use of a chain rule formulation. We demonstrate that for a simple model with a strong Class 2 AVO signature involving phase reversal, a sequence of short-offset p-wave elastic stiffness recovery (equivalently velocity or impedance) folllowed by long offset shear elastic stiffness recovery produces a significant uplift in data fitting when compared to simple acoustic inversion for p-wave velocity. We also demonstrate effective recovery in 2D using similar techniques, for a more complex model with thin sands exhibiting strong AVO behavior.
Presentation Date: Tuesday, October 18, 2016
Start Time: 8:00:00 AM
Presentation Type: ORAL