LOCATION OF SEISMIC VELOCITY CHANGES DURING GEOTHERMAL OPERATIONS AT DOMO DE SAN PEDRO GEOTHERMAL FIELD IN NAYARIT, MEXICO
The Domo de San Pedro Geothermal field, located in the westernmost part of the trans-Mexican volcanic belt, is part of a silicic volcanic complex. Geothermal energy is exploited since 2015 from 5 wells between the surface and 3 km deep. We used a continuous seismic dataset of a seismic temporary network composed of 20 broadband stations operating from March 2021 to January 2022. We investigated the seismic response of the geothermal field to the stimulation, production, and fluid re-injection operations across the geothermal wells.
First, we applied classical seismic methods for microseismicity location, producing a catalog of earthquakes using a dedicated velocity model for the geothermal field, alongside combining ambient noise imaging and the stations collapse sensitivity method to identify the spatial distribution of velocity anomalies.
Next, we implemented seismic ambient noise interferometry to analyze the apparent velocity changes dv/v measured from the coda of noise cross-correlations. We identified three periods of seismic activity that coincided with re-injection of fluids, significant variation in the production rate of the geothermal wells, and prominent changes of velocity (dv/v). We located these velocity changes by defining the velocity kernel between station pairs that relate travel-time changes in given time windows. These velocity transients may be associated with variations in the properties of rocks, reservoir fluid content, and changes in the effective stress beneath the volcanic system. We propose that changes in rock properties and fluid overpressure might be responsible for induced microseismicity. Our study shows that implementing dv/v quantifications as an additional tool for geothermal seismic monitoring can unveil seismic phenomena not visible by classical methods and may lead to better controls on geothermal operations.