Virtual source applications to imaging and reservoir monitoring

Seismic Interferometry from Correlated Noise Sources

It is a well-established principle that cross-correlating seismic observations at different receiver locations can yield estimates of band-limited inter-receiver Green’s functions. This principle, known as Green’s function retrieval or seismic interferometry, is a powerful technique that can transform noise into signals which enable remote interrogation and imaging of the Earth’s subsurface. In practice it is often necessary and even desirable to rely on noise already present in the environment. Theory that underpins many applications of ambient noise interferometry assumes that the sources of noise are uncorrelated in time. However, many real-world noise sources such as trains, highway traffic and ocean waves are inherently correlated in space and time, in direct contradiction to the these theoretical foundations. Applying standard interferometric techniques to recordings from correlated energy sources makes the Green’s function liable to estimation errors that so far have not been fully accounted for theoretically nor in practice. We show that these errors are significant for common noise sources, always perturbing or entirely obscuring the phase one wishes to retrieve. Our analysis explains why stacking may reduce the phase errors, but also shows that in commonly encountered circumstances stacking will not remediate the problem. This analytical insight allowed us to develop a novel workflow that significantly mitigates effects arising from the use of correlated noise sources. Our methodology can be used in conjunction with already existing approaches, and improves results from both correlated and uncorrelated ambient noise. Hence, we expect it to be widely applicable in ambient noise studies.

Multiple Geophysical Techniques for Investigation and Monitoring of Sobradinho Landslide, Brazil

Geophysical methods have a varying degree of potential for detailed characterization of landslides and their dynamics. In this study, the application of four well-established seismic-based geophysical techniques, namely Ambient Noise Interferometry (ANI), Horizontal to Vertical Spectral Ratio (HVSR), Multi-Channel Analysis of Surface Waves (MASW) and Nanoseismic Monitoring (NM), were considered to examine their suitability for landslide characterization and monitoring the effect of seasonal variation on slope mass. Furthermore, other methods such as Ground Penetrating Radar (GPR) and DC Resistivity through Electrical Resistivity Tomography (ERT) were also used for comparison purpose. The advantages and limitations of these multiple techniques were exemplified by a case study conducted on Sobradinho landslide in Brazil. The study revealed that the geophysical characterization of the landslide using traditional techniques (i.e., GPR, ERT and MASW) were successful in (i) the differentiation between landslide debris and other Quaternary deposits, and (ii) the delineation of the landslide sliding surface. However, the innovative seismic based techniques, particularly ambient noise based (HVSR and ANI) and emitted seismic based (NM), were not very effective for the dynamic monitoring of landslide, which might be attributed to the short-time duration of the data acquisition campaigns. The HVSR was also unsuccessful in landslide site characterization i.e., identification of geometry and sliding surface. In particular, there was no clear evidence of the light seasonal variations, which could have been potentially detected from the physical parameters during the (short-time) ambient noise and microseismic acquisition campaigns. Nevertheless, the experienced integration of these geophysical techniques may provide a promising tool for future applications.

Virtual reflector representation theorem (acoustic medium)

The virtual reflector method simulates new seismic signals by processing traces recorded by a plurality of sources and receivers. The approach is based on the crossconvolution of the recorded signals and makes it possible to obtain the Green's function of virtual reflected signals as if in the position of the receivers (or sources) there were a reflector, even if said reflector is not present. This letter presents the virtual reflector theory based on the Kirchhoff integral representation theorem for wave propagation in an acoustic medium with and without boundary and a generalization to variable reflection coefficients for scattered wavefields.