High-resolution ambient noise imaging of geothermal reservoir using 3C
dense seismic nodal array and ultra-short observation
Abstract
Tomographic imaging based on long-term ambient seismic noise
measurements, mainly the phase information from surface waves, has been
shown to be a powerful tool for geothermal reservoir imaging and
monitoring. In this study, we utilize seismic noise data from a dense
nodal array (192 3C nodes within 20km2) over a ultra-short observation
period (4.7 days) to reconstruct surface waves and determine the
high-resolution (0.2km) three-dimensional (3-D) S wave velocity
structure beneath a rural town in Zhejiang, China. We report the
advantage of cross-coherence over cross-correlation in suppressing
pseudo-arrivals caused by persistent sources. We use ambient noise
interferometry to retrieve high quality Rayleigh waves and Love waves.
Body waves are also observed on the R-R component interferograms. We
apply phase velocity dispersion measurements on both Rayleigh waves and
Love waves and automatically pick more than 23,000 dispersion curves by
using a Machine Learning technique. 3-D surface wave tomographic results
after depth inversion indicate low-velocity anomalies (between -1% and
-4%) from the surface to 2km depth in the central area. Combined with
the conductive characteristics observed on resistivity profile, the
low-velocity anomalies are inferred to be a fluid saturated zone of
highly fractured rock. Joint interpretation based on HVSR measurements,
and existing temperature and fluid resistivity records observed in a
nearby well, suggests the existence of the high-temperature geothermal
field through the fracture channel. Strong correlation between HVSR
measurements and S wave velocity model sheds light on the potential of
extraction of both amplitude and phase information from ambient noise.