Seismic Ground Roll Absorption and Reemission by Sand Dunes

Ground roll is a significant source of noise in land seismic data, with crossline scattered ground roll particularly difficult to suppress. This noise arises from surface heterogeneities lateral to the receiver spread, and in desert regions sand dunes are a major contributor. However, the nature of this noise is poorly understood, preventing the design of more effective data acquisition or processing techniques. Here, we present numerical simulations demonstrating that a barchan sand dune acts as a resonator, absorbing energy from ground roll and reemitting it over an extensive period of time. We derive and validate a mathematical framework that quantitatively describes the properties of the emitted waves, and demonstrate that wave amplitude is estimable from easily‐measurable bulk properties of the dune. Having identified regions in time, space, and frequency space at which noise will be more significant, we propose reducing dune‐scattered noise through careful survey design and data processing. In particular, we predict that seismic noise will be lower upwind of barchan dunes, and at frequencies far from a ‘resonant’ frequency 2cS/H, for dune height H and typical seismic velocity within the dune cS. This work is especially relevant to seismic acquisition in the vicinity of a dune field, where scattered noise appears incoherent and difficulties arise with alternative approaches to noise suppression.This work was performed at Schlumberger Cambridge Research, where M. I. Arran was a CASE Student/Intern and E. Muyzert is a Senior Research Scientist

Scholte-wave tomography for shallow-water marine sediments

We determine the 3-D in situ shear-wave velocities of shallow-water marine sediments by extending the method of surface wave tomography to Scholte-wave records acquired in shallow waters. Scholte waves are excited by air-gun shots in the water column and recorded at the seafloor by ocean-bottom seismometers as well as buried geophones. Our new method comprises three steps: (1) We determine local phase-slowness values from slowness-frequency spectra calculated by a local wavefield transformation of common-receiver gathers. Areal phase-slowness maps for each frequency used as reference in the following step are obtained by interpolating the values derived from the local spectra. (2) We infer slowness residuals to those reference slowness maps by a tomographic inversion of the phase traveltimes of fundamental Scholte-wave mode. (3) The phase-slowness maps together with the residuals at different frequencies define a local dispersion curve at every location of the investigation area. From those dispersion curves we determine a model of the depth-dependency of shear-wave velocities for every location. We apply this method to a 1 km2 investigation area in the Baltic Sea (northern Germany). The phase-slowness maps obtained in step (2) show lateral variation of up to 150 per cent. The shear-wave velocity models derived in the third step typically have very low values (60–80 m s−1) in the top four meters where fine muddy sands can be observed, and values exceeding 170 m s−1 for the silts and sands below that level. The upper edge of glacial till with shear-wave velocities of 300–400 m s−1 is situated approximately 20 m below sea bottom. A sensitivity analysis reveals a maximum penetration depth of about 40 m below sea bottom, and that density may be an important parameter, best resolvable with multimode inversion

Wavefield divergence via hydrophone measurement on land

ISSN:1029-7006ISSN:1607-796

Extension of the Spatial Autocorrelation (SPAC) Method to Mixed-Component Correlations of Surface Waves

Using ambient seismic noise for imaging subsurface structure dates back to the development of the spatial autocorrelation (SPAC) method in the 1950s. We present a theoretical analysis of the SPAC method for multicomponent recordings of surface waves to determine the complete 3 × 3 matrix of correlations between all pairs of three-component motions, called the correlation matrix. In the case of isotropic incidence, when either Rayleigh or Love waves arrive from all directions with equal power, the only non-zero off-diagonal terms in the matrix are the vertical–radial (ZR) and radial–vertical (RZ) correlations in the presence of Rayleigh waves. Such combinations were not considered in the development of the SPAC method. The method originally addressed the vertical–vertical (ZZ), RR and TT correlations, hence the name spatial autocorrelation. The theoretical expressions we derive for the ZR and RZ correlations offer additional ways to measure Rayleigh wave dispersion within the SPAC framework. Expanding on the results for isotropic incidence, we derive the complete correlation matrix in the case of generally anisotropic incidence. We show that the ZR and RZ correlations have advantageous properties in the presence of an out-of-plane directional wavefield compared to ZZ and RR correlations. We apply the results for mixed-component correlations to a data set from Akutan Volcano, Alaska and find consistent estimates of Rayleigh wave phase velocity from ZR compared to ZZ correlations. This work together with the recently discovered connections between the SPAC method and time-domain correlations of ambient noise provide further insights into the retrieval of surface wave Green’s functions from seismic noise