Locating and characterizing seismic events in Los Humeros (Mexico) using time-reverse imaging

Locating and characterizing the seismicity in a reservoir is crucial for any geothermal project. This study is the first time that the seismicity in a geothermal reservoir is characterized using time-reverse imaging (TRI). The spatio-temporal distribution of events in combination with focal mechanism solutions may enable the mapping of existing fault networks, the estimation of local stress regimes and the distinction between tectonic and induced events. Combining these results with results from other methodologies will in the future lead to an informed understanding of the physical processes occurring in reservoirs.

TRI is a method for locating and characterizing seismic events. TRI uses the whole time-reversed waveform and a seismic wave propagation solver to locate and characterize events. Therefore, it does not rely on the identification of seismic events and their onsets in the traces. In contrast to common tools that provide hypocenters and focal mechanism solutions for seismic events, TRI does not assume any a priori knowledge about the sources. Since events are not picked in the seismic traces, no assumption is made about the number of sources recorded in a certain time window. Similarly, the characterization of events does not exclude any source type or put any constraints or assumptions on the sources, such as them being only of double-couple nature. Therefore, TRI may be especially well-suited when the overall type of sources is not known or if it is suspected that common localization and characterization tools are not adequately depicting the physical processes in the subsurface.

In the first part of this study, seismic events, that occurred in the geothermal field of Los Humeros in Mexico, are located using TRI. So-called sensitivity maps are used to enhance the localization capabilities and to determine the spatial variation in source-location accuracy. In the second part of this study, the located events are characterized by determining the full time-dependent moment tensor. Since no assumption about the source type is made, these moment tensors complement results obtained from more standardized tools.