59 2. Refraction surveying For conventional studies, the refraction method only requires the measurement of arrival times of the first arrival waves (direct and refracted waves) to provide a geological model. Amplitudes are not commonly used in seismic refraction studies. A detailed analysis of head wave amplitudes did not generate useful results, however, the fact that head wave amplitude is a function of densities and of seismic velocities suggests that the joint inversion of seismic refraction travel times and head wave amplitudes should facilitate the determination of both seismic velocity and density models (Nikrouz, 2016). Where irregular interface occurs, the analysis of the distortion of the head wave arrival allows the detection of wave interferences, which can be associated with the presence of fractures (second field example). Seismic refraction is widely used for the study of near-surface layers. Its advantages are: • only simple equipment required, • good determination of velocities, • rapid depth determination. Unfortunately, it also presents several drawbacks: • Layer velocity must increase with depth; which is not always the case when dealing with compacted formations; • A particularly thin layer may completely evade detection; which is even more possible if the velocity contrast is insufficient. In such cases, the determined depths would be incorrect; • The method is unable to provide highly detailed mapping of structures. References Bernard S., Delay F., Porel G., 2006, A new method of data inversion for the identification of fractal characteristics and homogenization scale from hydraulic pumping tests in fractured aquifers, Journal of Hydrology, 328, 647-658. Bourbiaux B., Callot J.P., Doligez B., Fleury M., Gaumet F., Guiton M., Lenormand R., Mari J.L., Pourpak H., 2007, Multi-Scale Characterization of a Heterogeneous Aquifer Through the Integration of Geological, Geophysical and Flow Data: A Case Study, Oil and Gas Science and Technology, Rev IFP, 62, 347-373. Bourges M., Mari J.L., Jeannee N., 2012, A practical review of geostatistical processing applied to geophysical data: methods and applications, Geophysical prospecting, 60, 400-412. Coppens F., 1985, First arrival picking on common offset trace collection for automatic estimation of static corrections, Geophysical Prospecting, 33, 1212-1231.
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