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A Seismic Method for Estimating Subsurface Vp/Vs Ratio Based on Converted Waves: A Case Study From Arabian Gulf


Vp/Vs ratio is a critical property for geologist to identify subsurface rocks better. Because elastic properties of rocks, such as Young's modulus and Poisson's Ratio, can be calculated on the basis of P and S wave velocity ratio. It can indicate the kind of rock and fluids, the sand/shale lithologic boundary and identify conglomerate and anhydrate/dolomite. This study proposes a seismic method to estimate Vp/Vs values for some subsurface reflectors and get a more accurate subsurface image, using the seismic converted-wave data. In previous studies, geophysicists assume the curved trace of converted wave is aligned vertically in one stacking bin, calculated by an approximate Vp/Vs value. For the deep part of the image space, Common Conversion Point (CCP) image trace is almost vertical, but the upper part is father away from the vertical trace. The image error increases as a reflector goes closer to the water bottom. Inaccurate P-wave and S-wave velocity ratios and subsurface imaging produce an effect on interpreting geologic structures. In this study, the Vp/Vs value related to a subsurface layer can be estimated. The field data used for this study are 4C Ocean Bottom Cable (OBC) data, acquired by two 2D test lines around the Umm Al Lulu offshore fields of the U.A.E.. The ray paths of converted waves are asymmetric, so locating converted points is an important step to process converted waves. In this study, one Vp/Vs value was tested at a time. The theory proposed is that the Vp/Vs ratio determines the location of converted points. Because the asymmetric feature of converted waves, if Vp/Vs is incorrect, the records from positive and negative offsets represent different subsurface points. There will be no correlation between the positive- and negative-offsets stacking results. However, if Vp/Vs is close to the rock properties in some subsurface layers, there will be a strong correlation in specific times. This seismic approach can estimate Vp/Vs values for different depths. A set of Vp/Vs values was tested by processing seismic data to get positive- and negative-offsets stacking results, and then doing correlation between these stacks. By correlating events in different depth for each Vp/Vs ratio, some reflection events are enhanced in certain value, which means this Vp/Vs represents the rock properties at this depth. Then the best-correlated part of each stacking can be combined to get a more accurate converted-wave result about subsurface structures.