Abstract
Micro-seismicity can be used to monitor the migration of fluids during reservoir production and hydro-fracturing operations in brittle formations or for studies of naturally occurring earthquakes in fault zones. Micro-earthquake locations can be inferred using wave-equation imaging under the exploding reflector model, assuming densely sampled data and known velocity. Seismicity is usually monitored with sparse networks of seismic sensors, for example located in boreholes. The sparsity of the sensor network itself degrades the accuracy of the estimated locations, even when the velocity model is accurately known. This constraint limits the resolution at which fluid pathways can be inferred. Wavefields reconstructed in known velocity using data recorded with sparse arrays can be described as having a random character due to the incomplete interference of wave components. Similarly, wavefields reconstructed in unknown velocity using data recorded with dense arrays can be described as having a random character due to the inconsistent interference of wave components. In both cases, the random fluctuations obstruct focusing that occurs at source locations. This situation can be improved using interferometry in the imaging process. Reverse-time imaging with an interferometric imaging condition attenuates random fluctuations, thus producing crisper images which support the process of robust automatic micro-earthquake location. The similarity of random wavefield fluctuations due to model fluctuations and sparse acquisition is illustrated in this paper with a realistic synthetic example.
Highlights
Seismic imaging based on the single scattering assumption, known as Born approximation, consists of two main steps: wavefield reconstruction which serves the purpose of propagating recorded data from the acquisition surface back into the subsurface, followed by an imaging condition which serves the purpose of highlighting locations where scattering occurs
This framework holds both when the source of seismic waves is located in the subsurface and the imaging target consists of locating this source, as well as when the source of seismic waves is located on the acquisition surface and the imaging target consists of locating the places in the subsurface where scattering or reflection occurs
This imaging procedure succeeds if several assumptions are fulfilled: first, the velocity model used for imaging has to be accurate; second, the numeric solution to the waveequation used for wavefield reconstruction has to be accurate; third, the data need to be sampled densely and uniformly on the acquisition surface
Summary
Seismic imaging based on the single scattering assumption, known as Born approximation, consists of two main steps: wavefield reconstruction which serves the purpose of propagating recorded data from the acquisition surface back into the subsurface, followed by an imaging condition which serves the purpose of highlighting locations where scattering occurs.This framework holds both when the source of seismic waves is located in the subsurface and the imaging target consists of locating this source, as well as when the source of seismic waves is located on the acquisition surface and the imaging target consists of locating the places in the subsurface where scattering or reflection occurs. If the velocity used for wavefield reconstruction is inaccurate, the wavefronts do not all overlap at the source position (Fig. 1c), leading to imaging artifacts.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
