f The impact of very accurate mapping of shallow velocity layers, determined by combining surface and refracted waves on static corrections. An example from the Llanos Basin, Eastern Colombia, SA

Estimating an accurate P-wave velocity of near-surface subsoil is necessary for reliable static corrections of reflection data used in seismic exploration, crucial for exploration surveys of areas with very low relief structures less than 25 milliseconds like Sub Andean Llanos basin, in Eastern Colombia. We found that combining Raleigh wave inversion together with P wave refraction tomography give us a very precise subsurface layered model that is quite useful for accurate static correction. To carry out this subsurface model, seismic data was acquired using a multi-channel seismograph, with 10-Hz Geospace GS-One geophones, and low-power explosive charges that acted as the seismic source. The seismic data was collected at a sample interval of 1 millisecond, and a trace length of 1000 milliseconds. Following the seismic data acquisition, the first breaks of the refracted wave were picked and inverted to obtain a refraction tomography through an iterative process of ray tracing and smooth inversion. The refraction tomography provided a P-wave velocity model of the subsoil to a depth of 10 meters approximately. Once the refraction tomography was obtained, the S wave profile was estimated by generating overtone images, which are cross plots of phase-velocities and frequencies of surface waves, usually considered as noise and filtered from the data. Each overtone image was picked to extract dispersion curves, and inverted together with the P-wave velocity model to obtain an S-wave velocity model. Some results obtained from data acquired and processed by GMAS are included.