Abstract
Enhanced geothermal systems (EGS) are promising for generating clean
power by extracting heat energy from injection and extraction of water
in geothermal reservoirs. The stimulation process involves hydroshearing
which reactivates pre-existing cracks for creating permeability and
meanwhile inducing microearthquakes. Locating these microearthquakes
provide reliable feedback on the stimulation progress, but it poses a
challenging nonlinear inverse problem. Current deep learning methods for
locating earthquakes require extensive datasets for training, which is
problematic as detected microearthquakes are often limited. To address
the scarcity of training data, we propose a transfer learning workflow
using probabilistic multilayer perceptron (PMLP) which predicts
microearthquake locations from cross-correlation time lags in waveforms.
Utilizing a 3D velocity model of Newberry site derived from ambient
noise interferometry, we generate numerous synthetic microearthquakes
and 3D acoustic waveforms for PMLP training. Accurate synthetic tests
prompt us to apply the trained network to the 2012 and 2014 stimulation
field waveforms. Predictions on the 2012 stimulation dataset show major
microseismic activity at depths of 0.5–1.2 km, correlating with a known
casing leakage scenario. In the 2014 dataset, the majority of
predictions concentrate at 2.0–2.9 km depths, consistent with results
obtained from conventional physics-based inversion, and align with the
presence of natural fractures from 2.0–2.7 km. We validate our findings
by comparing the synthetic and field picks, demonstrating a satisfactory
match for the first arrivals. By combining the benefits of quick
inference speeds and accurate location predictions, we demonstrate the
feasibility of using transfer learning to locate microseismicity for EGS
monitoring.