Microseismic Event Location with Dual Vertical DAS Arrays: Insights from the FORGE 2022 Stimulation

We investigate the resolvability of a microseismic event location given a recording array composed of vertical distributed acoustic sensing (DAS) boreholes. We use a modified source-scanning algorithm that takes into account both P and S waves. We transform the brightness maps it produces into probability density functions (PDFs), over which we carry out a resolution and uncertainty analysis. We apply this approach to microseismic events recorded by two vertical DAS boreholes as part of the Frontier Observatory for Research in Geothermal Energy (FORGE) project. We show that for the specific acquisition geometry in FORGE, the horizontal location of the events cannot be determined, but their depth can, similar to results obtained with a single borehole. Using synthetic examples, we show that the recording array’s geometry is the limiting factor in the determination of the horizontal location. We investigate various possible recording geometries composed of idealized DAS-like vertical boreholes with varying locations and depths. We find that, besides the number of recordingd boreholes, their depth is the main factor influencing the location estimation uncertainty. The number and position of the boreholes mainly influence the spatial distribution of the PDF, whereas the boreholes’ depth mainly influences its size. Despite the simplicity of our analysis, it highlights the influence of the monitoring array design for microseismic events’ locating using vertical DAS arrays.