Refraction seismology

In the seismic refraction method, seismic waves are recorded that propagate along layer boundaries or as arcuate “diving waves” mainly sub-horizontally. The seismic signals, produced by controlled seismic sources, are recorded along lines of seismometers deployed on land or in the ocean (ocean-bottom seismometers). Although this method can also be used in engineering geology for near-surface investigations, we apply it to analyze deep crustal structures, the crust-mantle boundary and the uppermost mantle. Wide-angle reflections recorded at large distances between source and receivers are part of this data analysis scheme. Geophysicists often use the abbreviation WARRP (wide-angle reflection / refraction profiling) for these techniques. The main advantage of WARRP is that the inversion procedure directly results in relatively accurate seismic velocity-depth sections of the subsurface, while its disadvantage is the relatively low structural resolution. Therefore, a combined seismic reflection and seismic refraction survey is mostly conducted across an area of interest.

Example of a seismic refraction record from an ocean-bottom seismometer (OBS) with travel-times that are reduced with 8 km/s to better display the data (top), picked refraction phases (middle), and a crustal cross-section model with traced rays corresponding to the picked refraction phases (bottom). Pc1 and Pc3 are refraction phases from the middle to lower crust, while PmP denotes a wide-angle reflection from the crust-mantle boundary (Moho discontinuity). Distance is from the OBS to the moving seismic source along the profile.