Analysis of 3D Refraction Data of Ranipur Area, computation of Statics and Calculations of Engineering Properties

Abstract

Varying thicknesses and velocities of near-surface layers cause serious problems for seismic reflection imaging of the deeper subsurface. Static corrections, calculated from nearsurface velocity models, are uSl"d to remove the effects of the variable topographic near surface. The "refraction method "which is essential for the purpose as described earlier, utilizes the first arrival times of refracted waves propagating along the shallow subsurface layers. The main limitation of the method is that it requires the refractor velocity be higher than that in the overlying layers. The field acquisition, processing and interpretation of the refraction data are usually fast and simple, keeping the overall cost of the refraction surveys relatively low. The data is acquired by applying a source of seismic energy at a few points within and outside the spread of receivers. The processing and interpretation are performed using simple interactive softwares. The final product of refraction 3urveys is a layered depth section including information on layer geometry and seismic velocities within the layers. 3D Seismic Refraction survey is carried over Ranipur Area located in the Lower Indus Basin, Sindh province comprises of Uphole survey and surface shooting. In this thes is, near-surface layers are studied and characterized using two methods, first is a widely used and proven method for obtaining near-surface velocity models for both P and S waves and second is mainly used for geotechnical engineering purposes. Velocity, thickness and elevation are computed for both weathered and sub-weathered layers by using software that's K-tron~eiRA. It is further divided into SeiRaAutopick, Refract and StatCalc. Autopick is used for first break picking and amplitude analysis, Refract for establishing relation between time and depth (T-X graph) and StatCalc for static correction. Weathered layer and Sub-weathered layer velocities are used to calculate Elastic properties for civil Engineering purposes to estimate the strength of Soil/Rock of Study Area. Contours are drawn for each Elastic moduli using software Surfer.