Batakrishna Mandal

Shear Wave Anisotropy from a Single Dipole Source

Summary This study presents a method of determining shear wave anisotropic parameters from a single dipole source with a multilevel receiver array. The method is based on the properties of wave propagation in an anisotropic medium. A key feature is using pressure transducer receivers instead of conventional dipole receivers. Multiple receivers (four or more) at one level are used to analyze source propagation effects in an anisotropic medium. The waveforms at any angle within an adjacent receiver pair are obtained under the assumption of circular radiation. Dipole waveforms at any azimuthal direction are then constructed from the waveforms at and between the receivers at one level from one source. If the single source acts along the symmetry plane, then the method determines only slow or fast shear wave velocity. The source need not align with receiver directions. This method is applied to synthetic waveforms generated using a 3D finite difference algorithm in an anisotropic medium that includes a borehole. Preliminary application to field data shows that this method has significant advantages over conventional cross-dipole array shear wave logging techniques.

Borehole Acoustic-Array Processing using Model-Based Adaptive Filtering

Summary Acoustic array processing is common for estimating formation properties (compressional and shear slownesses) from both Wireline and LWD tools. The most common method uses an inversion method to obtain formation shear waves from an array-based, non-dispersive processing method. This work proposes a model-based adaptive filtering method applied prior to the standard semblance processing to minimize the dispersion of the different modes of propagation along the borehole. In this approach, numerous theoretical models, including borehole diameters, borehole fluid properties, and formation properties were computed to establish a relationship between the cut-off frequency (useful starting energy of the mode) and the peak excitation frequency (associated Airy phase of the mode) with all borehole and formation parameters for a particular mode of propagation. A model-based fitting polynomial is established to obtain a frequency window for any borehole sizes and formation types. An adaptive filtering algorithm is developed and implemented to the real-time and postprocessing work flow. The results from theoretical modeling and field data show the usefulness of this

Determination of shear‐wave velocities from crosshole measurements in low‐velocity formations

An important goal of cross-hole seismic surveys is to obtain both compressional and shear wave velocity distributions between the holes. Shear waves are generally observed in high velocity formations. When formation shear velocity is less than about 4500 ft/sec, shear wave amplitudes are generally small. In this situation, if the source borehole is cased and the source has the capacity to generate tube waves, then a “Mach” wave phenomenon occurs. The tube wave propagating with super shear velocity radiates conical or “Mach” waves that obliterate the shear waves. This paper provides the theoretical background, synthetic seismograms and suggests methods for determining the formation shear wave velocities in the presence of Mach waves.