Seismic and lithological near surface characteristics of an area in north-east Colombia

The seismic image of deep rock, interesting for the petroleum industry, can be distorted by the heterogeneous near-surface layers, characterized by low wave propagation velocity. The conventional methods used in counteracting this effect seem less effective in complex areas with rough topography such asthose commonly found in Colombia, which are also affected by stronger tropical weathering. Characterizationof the near-surface layer was conducted in this work with the purpose to investigate these relationships. Geological and geophysical methods were applied using data from a 2D seismic survey performed in the Catatumbo area of Colombia and seismic data and cutting samples analysis from a couple of 60 m depth wells (downhole surveys), drilled at rough surface locations. Wave propagation velocities were calculated by the application of tomography and refraction. Visual and laboratory assays such as granulometry and mineralogy were used in the analysis of the cutting samples. It was then possible to relate physical and lithological characteristics with properties of seismic response. Differences between the seismic response and the geological description were also observed and some uncertainties were identified.