Signal‐to‐noise estimates of time‐reverse images

Abstract

Locating subsurface sources from passive seismic recordings is difficult when attempted with data that has no observable arrivals or a low signal‐to‐noise ratio. Using time‐reversal techniques recorded energy can be focused at its source depth. However, when a focus cannot be matched to a particular event, it can be difficult to distinguish true focusing from artifacts. Artificial focusing could arise from numerous causes, including surface waves, local noise sources, acquisition geometry and velocity model effects. We present a method to more reliably locate subsurface sources that reduces the ambiguity of the results. Time‐reverse imaging techniques are implemented on both the recorded data and a noise model. In the data domain, the noise model only approximates the energy of local noise sources. After imaging, however, the result also captures the effects of acquisition geometry and the velocity model. The noise image is then used to correct the data image to produce an estimate of the signal‐to‐noise ratio. Synthetic data examples show the versatility of this technique to varying amounts of noise and to challenging velocity models. A field data example shows how this technique can be used to locate the source of low‐frequency energy collocated with an oil reservoir.