Guide to the Interpretation of Crustal Refraction Profiles

Abstract

Using the reflectivity method, synthetic, vertical-component seismograms have been computed for simple velocity transitions appropriate to crustal models. Record sections of the synthetic seismograms including refracted and reflected phases aid in elucidating the fine structure of the crust since they illustrate the effects of various velocity models. The computed record sections indicate the importance to interpretation of secondary arrivals such as near-vertical incidence and wide-angle reflections and illustrate the following: (1) an upper crustal low velocity layer (LVL) can best be recognized from reflections from the top of the LVL; (2) wide-angle reflections are nearly always of larger amplitude than head waves and have apparent velocities indicative of the layer velocities; (3) head wave amplitudes are greatly affected by velocity gradients within the refracting layer; (4) near-vertical incidence reflections are of significant amplitude principally for reflections from first-order velocity discontinuities; and (5) comp!ex wave patterns result from relatively simple upper crustal models which contain an LVL and serve to emphasize the need for detailed amplitude studies so that amplitudes of head waves and near-critical reflections may be considered. Synthetic seismograms which closely approximate observed seismograms for a detailed refraction line in the eastern Basin and Range province are used to infer fine structure of the crust. Synthetic record sections computed for a Basin and Range and an eastern United States crustal model illustrate different amplitude-timedistance features which are characteristic of observed seismic data from these regions.

Interaction between a hotspot and a fracture zone: The crustal structure of Walvis Ridge at 6° E

“…5). This reflection pattern might be caused by an alternating layering of basaltic rocks and sediments, volcaniclastic material or pillow lavas (Inoue et al, 2008), which can amplify amplitudes for certain frequencies due to constructive wave interference (Braile and Smith, 1975). We therefore interpret the upper crust as an inter-layered basalt-sediment sequence.…”

The Oblique Seismic Experiment on Deep Sea Drilling Project Leg 52

“…Braile and Smith (1975) show that compressional wave amplitudes in a critical region (e.g., Region A) can be increased by replacing a first-order discontinuity with a gradient. They also demonstrate that a low amplitude head wave (e.g., Region E) can be produced by introducing a negative velocity gradient below the interface.…”

Sn propagation in the Western United States from common midpoint stacks of USArray data

“…The type of head wave that is observed up to teleseismic distances is typically described as a sum of whispering gallery or interference waves which travel in the mantle lid between the Moho and the low-velocity zone of the upper mantle [e.g., Menke and Richards, 1980]. Attenuation behavior of such waves is generally quite complicated and different from the behavior of pure head waves, and the amplitudes will depend on the velocity gradient in the uppermost mantle [Hill, 1973;Braile and Smith, 1975]. Because of these complications, we do not attempt to formally model the Sn attenuation but try to determine the broad variations of the propagation efficiency.…”