Seismic sensors and seismic imaging have been widely used to monitor the
geophysical properties of the subsurface. As subsurface engineering
techniques advance, more precise monitoring systems are required.
Seismic event catalogs and seismic velocity structures are two of the
major outputs of seismic monitoring systems. Although seismic event
catalogs and velocity structure are often studied separately, published
reports suggest constraining them simultaneously can lead to better
results. We conducted a double-difference seismic tomography analysis to
constrain both the seismic event locations and the 3D seismic velocity
structure. Passive seismic data collected from a geothermal research
project in Lead, South Dakota were used to image a 3D volume on the
scale of tens of meters. Specifically, around 18,500 P-wave and 8,900
S-wave arrival times from 1,874 seismic events were used. Checkerboard
tests showed that the observed data can image the seismically active
region well. We compared tomography results with fixed seismic event
locations against those with updated event locations. Tomography results
with updated event locations showed better fits to the observations and
improved the seismic event catalog, showing sharper patterns compared to
the original one. These patterns helped us monitor the seismically
active fractures since the seismic events were mostly due to hydraulic
stimulations. Two parallel fractures revealed by the updated seismic
event catalog spatially correlated with independent borehole temperature
observations. The average seismic velocity values of the
well-constrained volume agreed to the first order with core sample
measurements and active-source seismic surveys.