Ambient noise tomography with non-uniform noise sources and low aperture networks: case study of deep geothermal reservoirs in northern Alsace, France

Abstract

We developed and applied a method for ambient noise surface wave tomography that can deal with noise cross-correlation functions governed to first order by a non-uniform distribution of the ambient seismic noise sources. The method inverts the azimuthal distribution of noise sources that are assumed to be far from the network, together with the spatial variations of the phase and group velocities on an optimized irregular grid. Direct modelling of the two-sided noise correlation functions avoids dispersion curve picking on every station pair and minimizes analyst intervention. The method involves station pairs spaced by distances down to a fraction of a wavelength, thereby bringing additional information for tomography. After validating the method on synthetic data, we applied it to a set of long-term continuous waveforms acquired around the geothermal sites at Soultz-sous-Forêts and Rittershoffen (Northern Alsace, France). For networks with limited aperture, we show that taking the azimuthal variations of the noise energy into account has significant impact on the surface wave dispersion maps. We obtained regional phase and group velocity models in the 1-7 s period range, which is sensitive to the structures encompassing the geothermal reservoirs. The ambient noise in our dataset originates from two main directions, the northern Atlantic Ocean and the Mediterranean Sea, and is dominated by the first Rayleigh wave overtone in the 2-5 s period range.

Reservoir Imaging Using Ambient Noise Correlation From a Dense Seismic Network

“…The asymmetry between causal and acausal sides is also consistent in this period range. Higher amplitudes are observed on the causal side due to prominent microseismic sources located in the northern Atlantic Ocean for back azimuths approximately 290–310°N (Lehujeur et al, , ). For waveforms of both networks, we compare the ratio between the peak amplitude of direct arrivals (green time window, Figure ) and the root‐mean‐square of the NCF in the coda part (blue time window, Figure ).…”

“…We compare the EstOF NCFs with the results of a previous study by Lehujeur et al (2016), who used the ambient noise cross-correlation technique in the same region on a seismic network of 34 stations devoted mainly to the monitoring of seismicity (Figure 1, yellow squares). This sparse network is composed of both permanent and long-term temporary stations.…”

“…A comprehensive study of the properties of the ambient seismic noise recorded in the area has revealed a strong directivity of the ambient seismic noise at periods between 1 and 5 s that mainly originates from the Northern Atlantic Ocean and the Mediterranean Sea (Lehujeur et al, ). Such directivity patterns in the ambient seismic noise may affect the tomographic models obtained with sparse seismological networks (Lehujeur et al, ).…”

Ambient Seismic Source Inversion in a Heterogeneous Earth: Theory and Application to the Earth's Hum

“…Previously developed localization methods for ambient sources include spectral analysis (e.g., Anthony et al, ; Bromirski, ; Bromirski & Duennebier, ), polarization analysis (e.g., Chevrot et al, ; Schimmel et al, ; Schulte‐Pelkum et al, ), cross correlation‐based imaging (e.g., Ermert et al, ; Stehly et al, ; Tian & Ritzwoller, ; Yang & Ritzwoller, ), beamforming (e.g., Behr et al, ; Gal et al, ; Gerstoft & Tanimoto, ; Juretzek & Hadziioannou, ; Kurrle & Widmer‐Schnidrig, ; Landès et al, ; Reading et al, ; Rhie & Romanowicz, ), migration of cross‐correlation signals (Brzak et al, ; Dales et al, ; Retailleau et al, ), grid searches for fitting particular observables (Gaudot et al, ; Rhie & Romanowicz, ; Shapiro et al, ), and inversion based on simplified plane‐wave models of noise cross correlations (Harmon et al, ; Lehujeur et al, ; Sadeghisorkhani et al, ; Yao & van der Hilst, ). Nishida and Fukao () inverted for the seasonal source of the Earth's hum using a model of cross correlations based on normal modes and a spherically symmetric Earth model.…”

Direct Modeling of the Mechanical Strain Influence on Coda Wave Interferometry

“…When applied to coda waves reconstructed with ambient seismic noise, this technique has found a significant application for monitoring structures at various scales. Examples include monitoring of volcanoes, mud landslides (Brenguier et al, , ; Duputel et al, ; Mainsant et al, ), and also a geothermal reservoir in the area of Rittershoffen (northeast of France) where a deep geothermal plant (2,500 m) is installed (Lehujeur et al, , ). As discussed above, the physical causes at the origin of the relative time shifts are multiple and complex and require different models to interpret the CWI measurements.…”