Anomalies in P wave velocities in the Tampico embayment, Gulf Coast of Mexico

The objective of this study was to discover relationships between seismic velocities and stratigraphic and tectonic zones of possible oil accumulation, by extrapolating velocity relations from known geologic features and oil fields in the Tampico Region. The original velocity data from the Tampico Region, consisting of vertical time-depth relations for longitudinal waves, was obtained by shooting wells and long reflection profiles for velocities. This data was converted to velocity functions that could be used in the computation of velocity maps for single formations, groups of formations, and 250 meters thick depth-intervals. The velocity functions were computed by fitting polynomials by Least Squares methods to the vertical time-depth data. The analysis of the velocity data was done by computing the regional trend of the velocity maps in each case, and then computing the residuals, which show the relationships between velocities and geologic features. The regional was computed according to the concept of a smooth regional. Thus a low order polynomial in x and y - a surface - was fitted by Least Squares method to the non-anomalous irregularly distributed data to represent the regional. The fitting of the velocity functions and of the surfaces, and the graphing and mapping was done using the IBM-7040 computer of the Rice Computer Center. Regional maps computed on formation datum show the general trend of variations of velocities. Velocities augment toward the Sierra Madre Oriental. The residual maps show good correlation with the tectonics of the area. The results are difficult to interpret as functions of known geologic features on the 250-meter depth-interval maps. The regional maps show the same tendency for the velocity to increase toward the Sierra. However, the residual maps do not show features consistently ascribable to the known geologic features, and it is difficult to hypothesize about the features present on the maps without additional information. No conclusion has been made regarding better prospective zones for oil accumulation; this is due mainly to the insufficiency of the velocity data. A density of about one velocity determination for each 20 square kilometers or less is desirable. Nevertheless, it has been proved that definite relations exist between velocities and structures in the Tampico Region