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.

The OBS on deck is ready for operation.
The OBS on deck is ready for operation. (Photo: Kasper Fischer, RUB and Mechita Schmidt-Aursch, AWI)