Seismic Shear Wave Anisotropy Used To Locate F’Racture Zones In Limestone

Primary and shear wave studies were conducted over fractured and solutioned limestone<br>to determine the value of using these waves to find the orientation of fractures and solution<br>cavities. Data were analyzed to ascertain the directional velocity and attenuation anisotropy.<br>Transmission data were gathered in a N-S, E-W, N45”E and N45”W direction and analyzed for<br>velocity and attenuation differences. These data were then compared to a previous reflection<br>test across two oil-producing wells. This previous test located two nearly vertical faults trending<br>N6O”W. The faults were the producing zones. The study suggested that this fault trend should<br>be reflected as a fracture trend in the near surface limestone. If so, shear waves should<br>determine the orientation by velocity and attenuation anisotropy.<br>The primary wave transmission showed little velocity and attenuation anisotropy. On the<br>other hand, the shear waves did have significantly different velocities and attenuation. The line<br>oriented N45”E had the lowest bedrock velocity while the line oriented N45”W had the highest.<br>The N-S and E-W lines had intermediate velocities which support fracture paths in a N6O”W<br>direction. The velocities should be lowest perpendicular to the fractures and highest parallel to<br>the fractures. The highest rate of amplitude attenuation was in the N45”E and the N-S direction<br>whereas the N45 “W and the E-W direction had the lowest rate of attenuation. These data<br>demonstrate that shear waves transmitted across fractures will have lower velocity and a higher<br>rate of attenuation. As the transmission direction becomes parallel to the fractures the velocity<br>increases and the attenuation reduces. Therefore, shear wave studies can greatly improve the<br>delineation of faults, fractures and solutional zones.