An extensive campaign to characterize rock stresses on the decameter scale was carried out in three 18–24 m long boreholes drilled from a tunnel in foliated granite at the Grimsel Test Site, Switzerland. The survey combined stress relief methods with hydrofracturing (HF) tests and concomitant monitoring of induced seismicity. Hydrofracture traces at the borehole wall were visualized with impression packer tests. The microseismic clouds indicate sub-vertical south-dipping HFs. Initial inversion of the overcoring strains with an isotropic rock model yielded stress tensors that disagreed with the HF and microseismic results. The discrepancy was eliminated using a transversely isotropic rock model, parametrized by a novel method that used numerical modelling of the in situ biaxial cell data to determine the requisite five independent elastic parameters. The results show that stress is reasonably uniform in the rock volume that lies to the south of a shear zone that cuts the NNW of the study volume. Stress in this volume is considered to be unperturbed by structures, and has principal stress magnitudes of 13.1–14.4 MPa for σ1, 9.2–10.2 MPa for σ2, and 8.6–9.7 MPa for σ3 with σ1 plunging to the east at 30–40°. To the NNW of the uniform stress regime, the minimum principal stress declines and the principal axes rotate as the shear zone is approached. The stress perturbation is clearly associated with the shear zone, and may reflect the presence of more fragmented rock acting as a compliant inclusion, or remnant stresses arising from slip on the shear zone in the past.

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Stress Measurements for an In Situ Stimulation Experiment in Crystalline Rock: Integration of Induced Seismicity, Stress Relief and Hydraulic Methods

. Surface electrical resistivity tomography (ERT) is a widely used tool to study seawater intrusion (SWI). It is noninvasive and offers a high spatial coverage at a low cost, but its imaging capabilities are strongly affected by decreasing resolution with depth. We conjecture that the use of CHERT (cross-hole ERT) can partly overcome these resolution limitations since the electrodes are placed at depth, which implies that the model resolution does not decrease at the depths of interest. The objective of this study is to test the CHERT for imaging the SWI and monitoring its dynamics at the Argentona site, a well-instrumented field site of a coastal alluvial aquifer located 40 km NE of Barcelona. To do so, we installed permanent electrodes around boreholes attached to the PVC pipes to perform time-lapse monitoring of the SWI on a transect perpendicular to the coastline. After 2 years of monitoring, we observe variability of SWI at different timescales: (1) natural seasonal variations and aquifer salinization that we attribute to long-term drought and (2) short-term fluctuations due to sea storms or flooding in the nearby stream during heavy rain events. The spatial imaging of bulk electrical conductivity allows us to explain non-monotonic salinity profiles in open boreholes (step-wise profiles really reflect the presence of freshwater at depth). By comparing CHERT results with traditional in situ measurements such as electrical conductivity of water samples and bulk electrical conductivity from induction logs, we conclude that CHERT is a reliable and cost-effective imaging tool for monitoring SWI dynamics.

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Time-lapse cross-hole electrical resistivity tomography (CHERT) for monitoring seawater intrusion dynamics in a Mediterranean aquifer

High-resolution 3D geological models are crucial for underground development projects and corresponding numerical simulations with applications in e.g., tunneling, hydrocarbon exploration, geothermal exploitation and mining. Most geological models are based on sparse geological data sampled pointwise or along lines (e.g., boreholes), leading to oversimplified model geometries. In the framework of a hydraulic stimulation experiment in crystalline rock at the Grimsel Test Site, we collected geological data in 15 boreholes using a variety of methods to characterize a decameter-scale rock volume. The experiment aims to identify and understand relevant thermo-hydro-mechanical-seismic coupled rock mass responses during high-pressure fluid injections. Prior to fluid injections, we characterized the rock mass using geological, hydraulic and geophysical prospecting. The combination of methods allowed for compilation of a deterministic 3D geological analog that includes five shear zones, fracture density information and fracture locations. The model may serve as a decameter-scale analog of crystalline basement rocks, which are often targeted for enhanced geothermal systems. In this contribution, we summarize the geological data and the resulting geological interpretation. Machine-accessible metadata file describing the reported data (ISA-Tab format)

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Comprehensive geological dataset describing a crystalline rock mass for hydraulic stimulation experiments

/pdf/6-c-polarization-analysis-using-point-measurements-of-4jncpnwytd.pdf

6-C polarization analysis using point measurements of translational and rotational ground-motion: theory and applications

Multiple meter-scale hydraulic fracturing (HF) experiments were executed in the crystalline rock at the Grimsel Test Site, Switzerland. The effect of the HF on the rock transmissivity has been quantified with hydraulic tests before and after each HF experiment. We observe transmissivity enhancement of 2 to 3 orders of magnitude and a change in the dominant flow regime after most of the HF tests. From microseismicity induced by the HF, we do not observe a systematic correlation between transmissivity enhancement and event numbers, frequency-magnitude distribution, or maximum magnitude. However, the radii of hydraulic fractures inferred independently from seismicity clouds and hydraulic responses coincide, implying that slip along fractures is the common underlying mechanism for transmissivity increase and seismicity.

/pdf/transmissivity-changes-and-microseismicity-induced-by-small-4wd0bj0c30.pdf

Transmissivity Changes and Microseismicity Induced by Small-Scale Hydraulic Fracturing Tests in Crystalline Rock

Geostatistical regularization operators for geophysical inverse problems on irregular meshes

\n Structural joint inversion of several data sets on an irregular mesh requires appropriate coupling operators. To date, joint inversion algorithms are primarily designed for the use on regular rectilinear grids and impose structural similarity in the direct neighbourhood of a cell only. We introduce a novel scheme for calculating cross-gradient operators based on a correlation model that allows to define the operator size by imposing physical length scales. We demonstrate that the proposed cross-gradient operators are largely decoupled from the discretization of the modelling domain, which is particularly important for irregular meshes where cell sizes vary. Our structural joint inversion algorithm is applied to a synthetic electrical resistivity tomography and ground penetrating radar 3-D cross-well experiment aiming at imaging two anomalous bodies and extracting the parameter distribution of the geostatistical background models. For both tasks, joint inversion produced superior results compared with individual inversions of the two data sets. Finally, we applied structural joint inversion to two field data sets recorded over a karstified limestone area. By including geological a priori information via the correlation-based operators into the joint inversion, we find P-wave velocity and electrical resistivity tomograms that are in accordance with the expected subsurface geology.

Structural joint inversion on irregular meshes

Frequency-dependent traveltime tomography using fat rays: application to near-surface seismic imaging

Karstified areas are known to be difficult ground for seismic exploration. We conducted a combined numerical-modeling and field-experiment study with the objectives to study the impact of karst on seismic wave propagation and to advance geophysical characterization of karst with seismic as well as non-seismic methods (electric and electromagnetic techniques). Finite-difference simulations using models with realistic topography illustrate the pronounced impact of topographic variations in high-velocity carbonate-bedrock environments on the scattered surface/guided waves. Wavefield complexities such as strong lateral changes in the strength of surface/guided waves, which were observed in a Vibroseis gather from a karst terrain in the Middle East, were also evident in our data recorded in Switzerland. In the latter case, amplifications of surface/guided waves could be correlated with low-velocity zones, which are probably due to more intensively karstified zones. Our study demonstrates that because of the strong heterogeneity of karst terrains, dense sampling is required to properly comprehend and disentangle the observed wavefield. Furthermore, we observed in our field study that the electrical-resistivity models correlate more closely with the mapped lithology, whereas karstification seems to more strongly affect the P-wave velocity models.

Claudio Jordi

Papers

Geostatistical regularization operators for geophysical inverse problems on irregular meshes

Structural joint inversion on irregular meshes

Frequency-dependent traveltime tomography using fat rays: application to near-surface seismic imaging

Understanding the Impact of Karst on Seismic Wave Propagation - A Multi-method Geophysical Study

Characterizing fractures and shear zones in crystalline rock using anisotropic seismic inversion and GPR imaging