Abdullah AlTheyab

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 ...

Imaging Near-surface Heterogeneities by Natural Migration of Back-scattered Surface Waves

We present a surface-wave natural migration method that does not require velocity models to migrate backscattered surface waves to their projected locations on the surface. This method uses recorded Greens functions instead of simulated Greens functions. The key assumptions are that the scattering bodies are within the depth interrogated by the surface waves, and the Greens functions are recorded with dense receiver sampling. This natural migration takes into account all orders of multiples, mode conversions, and non-linear effects of surface waves in the data. The natural imaging formulas are derived for both active source and ambient-noise data, and computer simulations show that natural migration can effectively image near-surface heterogeneities with typical distributions of ambient-noise sources. We also present the results of applying natural migration to Long-Beach and US-Array passive data. The migration images highlight known discontinuities in surface-wave tomograms and correlate well with some of the prominent geological boundaries at two different scales: (1) the tectonic scale such as the edge of the Atlantic Plain Province in southeastern US and (2) the regional scale structure under Long Beach, California. The migration images provide complementary high-wavenumber information to the smoother surface-wave tomograms and can be used to refine the tomographic models.

Imaging near-surface heterogeneities by natural migration of surface waves

We demonstrate that near-surface heterogeneities can be imaged by natural migration of backscattered surface waves in common shot gathers. No velocity model is required because the data are migrated onto surface points with the virtual Green’s functions computed from the shot gathers. Migrating shot gathers recorded by 2D and 3D land surveys validates the effectiveness of detecting near-surface heterogeneities by natural migration. The implication is that more accurate hazard maps can be created by migrating surface waves in land surveys.

Subwavelength imaging using seismic scanning tunneling macroscope: Field data example

We present the first results for a controlled source seismic experiment where we demonstrate that subwavelength scatterers can be imaged with a resolution of λ/8 using seismic data recorded in the far field of the scatterer. Using the Time Reverse Mirror (TRM) operation, we show that an image resolution Δx of 0.6 m can be obtained at the source location from data with seismic wavelengths ≥ 5 m. In other words, we show that it is possible to extract 220 Hz information from 55 Hz data using the TRM operation. These results also validate the theory of seismic superresolution associated with a seismic scanning tunneling macroscope.