Opportunities and pitfalls in surface-wave interpretation

Abstract

CitationSchuster GT, Li J, Lu K, Metwally A, AlTheyab A, et al. (2017) Opportunities and pitfalls in surface-wave interpretation. AbstractMany explorationists think of surface waves as the most damaging noise in land seismic data. Thus, much effort is spent in designing geophone arrays and filtering methods that attenuate these noisy events. It is now becoming apparent that surface waves can be a valuable ally in characterizing the near-surface geology. This review aims to find out how the interpreter can exploit some of the many opportunities available in surface waves recorded in land seismic data. For example, the dispersion curves associated with surface waves can be inverted to give the S-wave velocity tomogram, the common-offset gathers can reveal the presence of near-surface faults or velocity anomalies, and back-scattered surface waves can be migrated to detect the location of near-surface faults. However, the main limitation of surface waves is that they are typically sensitive to S-wave velocity variations no deeper than approximately half to one-third the dominant wavelength. For many exploration surveys, this limits the depth of investigation to be no deeper than approximately 0.5-1.0 km. IntroductionTucker and Yorston (1973) wrote a book titled Pitfalls in Seismic Interpretation that aimed to reveal how processed seismic reflection sections could lead to misinterpretation of the subsurface geology. They summarize the content of their book in their preface: "The geometry or shape of the reflecting surface is equally tricky. It can turn synclines into anticlines, reverse the throw of faults, superimpose one structure on another by sideswipe, and create a diffraction-anticline. Our latest and perhaps most serious pitfall is computer-derived. The recording and playback can distort both the structure and stratigraphy. Here real structures can be suppressed, false bedding created, faults smeared, and all of the geology lost. Only through constant rapport between the geologist, the interpreter, and the processing engineer will these recording and playback errors be avoided."In the spirit of Tucker and Yorston (1973), this paper presents a review on interpreting surface waves in recorded data, not as noise but as signal. It will demonstrate how surface waves can be used to estimate the locations of near-surface S-wave velocity anomalies and their extent in depth. Similar to reflection migration, we show how surface waves can be migrated to estimate the locations of near-surface velocity anomalies. The examples in this paper also reveal some pitfalls leading to misinterpretation of surface waves, and their avoidance by using an integrative interpretation of P-wave velocity tomograms, surface-wave migration images, and S-wave velocity tomograms.

Application of HVSR to estimating thickness of laterite weathering profiles in basalt

“…Yaede et al (2015) and Sowards et al (2018) have employed the MASW (multichannel analysis of surface waves; Park et al, 1999;Schuster et al, 2017) method in modified form to image the base of the LWP and underlying basalt in the Hawaiian Islands to depths of up to 70 m. Although standard P-wave reflection surveys are able to image relict igneous textures within LWPs (Sowards et al, 2018) it could be difficult to detect the gradational transition from saprolite to bedrock. Drilling is expensive.…”

“…Targets of opportunity such as stream banks and road cuts may not universally expose the base of the LWP and outcrop may be obscured by dense tropical vegetation. Yaede et al (2015) and Sowards et al (2018) have employed the MASW (multichannel analysis of surface waves; Park et al, 1999;Schuster et al, 2017) method in modified form to image the base of the LWP and underlying basalt in the Hawaiian Islands to depths of up to 70 m. Although standard P-wave reflection surveys are able to image relict igneous textures within LWPs (Sowards et al, 2018) it could be difficult to detect the gradational transition from saprolite to bedrock. Yaede et al (2015) concluded that MASW works well and generally detects the base of the weathering front to within a few meters accuracy, where depths to bedrock were known from drillers logs, geologic logs, and measured section from outcrop.…”

Joint inversion of Rayleigh wave fundamental and higher order mode phase velocity dispersion curves using multi‐objective grey wolf optimization

“…Figure 31(a) displays one of the five raw shot gathers used to generate the phase velocity dispersion curves. Figure 31(a) presents the unprocessed shot gathered with the refraction energy, while the refractions Schuster et al, 2017). Since we observe a mean phase velocity of approximately 1050 m/s at 11 Hz for the the large spread of P-wave solutions for the last layer, despite having an S-wave velocity of 1230 m/s and a depth of roughly 15 m for that layer, there is no conclusive evidence that the layer below 15 m is a basaltic layer.…”

The development of laterite weathering profiles as a function of rainfall and time: A geophysical approach

“…Measurements of T were made using well‐established geophysical methods, including multi‐channel analysis of surface waves (MASW; Park et al, 1999, 2007; Schuster et al, 2017; Sowards et al, 2018; Yaede et al, 2015) and in particular horizontal‐to‐vertical spectral ratio (HVSR; Nakamura, 1989; Nelson & McBride, 2019; Nelson et al, 2020). These were systematically employed to estimate LWP thicknesses across substrates of differing age such as Kohala on Hawai'i Island (0.303 Ma), Molokai (1.32 Ma), Oahu (2.0 Ma) and Kauai (4.0 Ma) (Ozawa et al, 2005; Sherrod et al, 2007; Sowards et al, 2018).…”