Shallow buried heterogeneities act as diffractors to an incoming surface wave. Experimental and numerical analyses are conducted to analyze the problem and to propose a methodology for mapping shallowly embedded objects. A layered system having a high impedance contrast at the base containing shallowly buried drums is tested experimentally. Stacked maps of frequency, velocity and lateral position show the presence of the drums. A hypothesis is posed that the most significant impact of the shallow obstacle on the incident wavefield is diffraction of Rayleigh wave energy. Synthetic seismograms generated through numerical modeling support the hypothesis. The interpretation of the data is complicated by factors such as acquisition footprint and frequency content. These factors remain to be investigated.
Capability to isolate the seismic effects of shallowly buried heterogeneous features is valuable in two opposing cases: when the feature is the target of interest; and when the feature is an obstacle to the goals of imaging reflections and for building a background velocity model (eg., Campman, et al., 2005; Nemeth et al., 2000; Blonk and Herman, 1996). The goals of the research described here are (1) to gain understanding about the problem of scattering of surface waves from shallow obstacles and (2) to investigate a direct, automated approach to processing multi-channel, single or multicomponent data from a closely-spaced linear array such that the shallowly buried heterogeneities are identified. Because it is automated, the system would be efficient, and any subjectivity in processing would be delayed until the final step. The method would ideally alleviate the necessity for extensive prior knowledge of the site, such as background stratigraphy.
The subject of scattering by surface waves has received considerable attention lately due to its potential for locating shallow obstacles and mapping heterogeneities (Nasseri- Moghaddam et al., 2007; Xia et al., 2007; Riyanti et al., 2005). In the context of geotechnical engineering, surface wave scattering finds applicability in the problems of mapping cemented lenses, faults, fissures, tunnels and caves.
In this abstract we present a conceptualization of the phenomenon we intend to test. Then experimental results of a study over a controlled site containing shallowly buried barrels follows. Results are compared to preliminary numerical modeling.
Consider a point source on the free-surface of a layered medium (Figure 1). In the far field, a Rayleigh wave develops, carrying the majority of the energy observed in the seismogram. A singularity described by a localized impedance contrast with the surrounding material acts as a secondary source upon incidence of the surface wave. We hypothesize that the most readily detectable feature of the singularity arises from the surface wave if the obstacle is sufficiently shallow that the Rayleigh wave contains significant energy at that depth. Then a diffraction appears superimposed upon the surface wave in the seismogram. Figure 2 shows a numerical simulation of simplified modeling carried out in the frequency domain where a dispersive Rayleigh wave represents the incident wavefield over a local heterogeneity. The total wavefield then is a summation of the incident wave field plus the scattered wavefield (Snieder, 1986) which can act constructively or destructively with respect to the incident wave.