As opposed to their sedimentary counterparts, crystalline rocks tend be characterized by very small matrix porosities, and hence fluid pathways are mostly associated with brittle deformation structures at scales ranging from micrometers to kilometers, such as fractures and fault/shear zones as well as their associated damage zones e.g.,. These structures not only dominate the hydraulic behavior but also act as zones of weakness and thus substantially affect the mechanical behavior of the rock mass. For many applications, a thorough understanding of the mechanical and hydraulic characteristics of the examined rock volume is critical. For example, in tunneling operations and nuclear waste storage repositories, fractures provide undesired zones of weakness and hydraulic conductivity, while the same characteristics represent desirable features for the exploration and creation of enhanced geothermal systems e.g.,. The overarching importance of fractures and fracture networks inspired a wealth of research on their geometrical, hydraulic, and mechanical properties e.g.,. Although it has been shown that geophysical borehole logs can provide petrophysical properties of individual fractures and fractures networks e.g.,, so far only a few studies systematically analyze fracture systems in crystalline rocks based on geophysical borehole logs e.g.,.

The structure of interest in this study is the Grimsel Breccia Fault (GBF), a major WSW–ENE-striking subvertical brittlely overprinted shear zone in the Southwestern Aar Granite of the central Swiss Alps, which exhibits evidence of both fossil and active hydrothermal activity . To this end, a shallow borehole has been drilled into the GBF, which acutely intersects the main brecciated fault core and is entirely situated in its surrounding damage zone. A comprehensive suite of geophysical borehole logs, comprising passive and active nuclear, full-waveform sonic (FWS), resistivity, self-potential (SP), and borehole radar (BHR) measurements, were collected in 2015, 2016, and 2017. In a previous study , hydrophone vertical seismic profiling (VSP) data in combination with some of the borehole log data were used to image and characterize the main fault core of the GBF. The resulting seismic image allowed a delineation of the targeted zone, and numerous tube waves in the hydrophone VSP data could be linked to hydraulically open fractures in the damage zone around the fault core. A quantitative analysis of the amplitudes of the hydrophone VSP data in terms of hydraulic conductivity was, however, not possible due to the abundance of tube wave events and the resulting interference of the various parts of the recorded seismic wavefield. performed a structural characterization of the GBF system and its evolution with a specific focus on porosity, permeability, fracture distribution, and fluid flow reconstruction based on the combined analysis of drill cores, optical televiewer (OTV) data, and geological mapping. Building on the results of these previous works, the focus of this study is a detailed characterization of the fracture network in the damage zone of the main fault core from geophysical borehole log data with a particular focus on the geometrical and petrophysical properties of the network as well as the links of the latter to brittle deformation.

In contrast to the classically utilized OTV data and core samples, which identify the location, orientation, and dip of fractures along the borehole e.g.,, geophysical borehole logs sample a more representative volume of the fractured rock mass away from the immediate vicinity of the borehole and, as such, are essential in intervals of core loss e.g.,. However, a drawback of the larger sampling volume is that individual fractures and other smaller-scale structures tend to be difficult to discriminate and detect. Moreover, geophysical borehole log measurements tend to be sensitive to a combination of petrophysical properties, thus rendering quantitative estimations from single log-type measurements difficult and ambiguous. Correspondingly, an integrated workflow utilizing a variety of geophysical borehole log measurements is necessary to mitigate these ambiguities. Such an integrated analysis, in combination with evidence from OTV and core data, is used in this study to constrain geometrical and petrophysical properties of the GBF damage zone. BHR reflection data are employed to image the damage zone and to infer the geometrical characteristics of its fracture network. To analyze the distribution of petrophysical properties in the damage zone and their link to brittle deformation, a cluster analysis is performed for a selection of geophysical borehole logs. Finally, to shed light on the hydraulic characteristics of the GBF, we examine SP and fluid resistivity log data. To test and verify our findings, we assess their compatibility with the detailed structural characterization of and the results of previous studies by and on the hydraulic nature of the GBF. The paper starts with a brief description of the geological setting, the challenges associated with the acquisition of the geophysical borehole logs in the intensely fractured crystalline environment, and their resulting impact on the quality and reliability of the data.

The GBF is a major WSW–ENE-striking subvertical ductile shear zone in the Southwestern Aar Granite, which has been exhumed from 3 to 4 km depth and brittlely overprinted. Indeed, ductile and brittle deformation often coincide along the GBF . The GBF exhibits both fossil and current hydrothermal activity , the latter being evidenced by warm springs in the village of Gletsch (∼18 ∘C) and in the Transitgas AG tunnel ∼200 m (up to 28 ∘C) below the Grimsel Pass , which is the site for this study (Fig. ). The hydrothermal water in the tunnel consists of approximately equal parts of recent meteoric components and older geothermal components, which reached circulation depths of several kilometers and maximum temperatures of 230–250 ∘C . The GBF fault breccia exhibits an epithermal-style mineralization consisting of quartz, including microcrystalline varieties (locally chalcedony), adularia, hydrothermal clays, pyrite, marcasite, and molybdenite . For the section of the GBF drilled and under investigation in the current study, clay-rich gouge is not a major factor .

The location of the GDP1 borehole and a schematic cross section are shown in Fig. , which also illustrates the extreme topographic relief of the area. The borehole is orientated approximately SSE, orthogonal to geological strike, with a vertical deviation of 24∘, and acutely intersects the GBF main breccia core between 82 and 86 m borehole depth. The GDP1 borehole was diamond-drilled with a so-called HQ bit (96 mm outer diameter, 76 mm inner diameter) to a length of 125.3 m. During the drilling operations, fluid circulation was lost at 76 m borehole depth, and, hence, the borehole was cemented and redrilled between 71 and 76 m depth. Due to the intense fracturing, enlargements are encountered along the entire length of the borehole and inherently affect the quality of the logging data.

A list of all geophysical borehole datasets collected along the GDP1 borehole during 2015, 2016, and 2017 is shown in Table 1 together with the respective condition of the borehole at the time of their acquisition. Throughout this paper, all data are referred to and displayed with regard to measured depth along the borehole track. The most complete dataset was collected in 2015 directly after drilling under open borehole conditions. It comprises OTV, borehole caliper, passive and active nuclear, electrical resistivity, SP, temperature, and multiple center frequency FWS logs as well as constant offset BHR, ambient (and pumped) flow meter, and periodic pumping test measurements.

After completion of the drilling operations, water from the adjacent lake was pumped into the borehole to flush out the polymer-based drilling mud and to enable OTV measurements. Although, the water in the borehole cleared up, remnants of the drilling fluid within the adjacent formation were likely to be present throughout the 2015 logging campaign. As a consequence, the SP data acquired in 2015 differ significantly from those measured in 2016 and 2017 since the polymer-based drilling mud changed the viscosity and the chemical composition of the pore fluid in the rock volume with regard to its ambient state. In addition to this, the 2015 SP data were also affected by changes in the flow regime in and around the borehole induced by the pumping of lake water. In the following, we therefore only consider the SP data acquired in 2016 and 2017. The addition of the lake water and the remnant presence of drilling mud also affected the 2015 electrical resistivity measurements. These were therefore repeated in 2016 and 2017 through the slotted PVC casing, which was installed to prevent borehole collapse. To analyze the effects of this casing as well as for data calibration purposes, a small section at the bottom of the borehole was remeasured under open borehole conditions in 2017. Details with regard to the data corrections applied to remove the casing effects are given in Appendix  and . EM flow meter tests, recorded in 2015, are also affected by the severely disturbed flow system as well as the fact that the polymer drilling mud potentially kept clogging some of the flow paths. Thus, only the passive data will be shown here for completeness of information.

BHR measurements were repeated in 2016, as no zero-time correction was available for the data collected in 2015. However, the 2015 data were acquired with a smaller spatial sampling interval and thus produce more coherent signals. For this reason, we utilize the 2016 data for calibration of the 2015 dataset, which we then consider for the remainder of the paper. Corresponding details are given in Appendix . Additionally, more detailed temperature and fluid resistivity measurements were conducted in 2016. High-frequency (25 kHz nominal center frequency), high-gain three-receiver FWS data were acquired in 2016 for more reliable and robust compressional (P) and shear (S) wave velocity estimations by semblance analysis . In the intensely fractured zones, especially around the main fault core, the first arrivals are, however, still very weak, thus making reliable velocity estimations very difficult due to the inherent uncertainty and local variability in the picks. Therefore, the P-wave velocity estimates are smoothed, and the S-wave velocity is entirely discarded in this zone. Everywhere else, the P- and S-wave estimates are reasonably robust.

Overall, the logging data are affected by strong borehole enlargements, which complicates their quantitative analysis. The enlargements are primarily due to the intensely fractured nature of the rock volume rather than being purely drilling-induced damage. Most of the enlargements can indeed be associated with distinct fractures or cataclastic features along the borehole track and thus present structural and petrophysical indicators in their own right. Geochemically the rock mass of the damage zone surrounding the GBF core is relatively homogeneous consisting of metagranite. The main heterogeneities are variations in fabric, ranging from granitic through gneissic, all the way to mylonitic as well as fractured and cataclastic zones, which are due to different degrees of ductile and/or brittle deformation . In the following, we seek to link these geological features inferred from the OTV and core data to the responses of the geophysical borehole logs. In a first step, we utilize the BHR data to image the fracture network situated in the damage zone. Then, the response of selected borehole log data is compared to different degrees of deformation encountered.

In contrast to most other geophysical borehole logging techniques, which have a relatively limited range of investigation, the BHR reflection method allows one to image individual fractures, clusters of fractures, and cataclastic zones outside of the immediate vicinity of the borehole e.g.,. As such, these data are much less affected by borehole enlargements than most other logging data. In the considered setting, the reflection coefficient is governed by the pronounced contrast of the dielectric permittivity between the host rock and the fluid-filled fractures, while the bulk electrical conductivity of the rock volume determines the attenuation of the BHR signals. This, in turn, makes resistive crystalline environments ideal sites for BHR measurements in general and for imaging fractures in particular. In the following, we first describe the processing of the BHR data and then proceed to analyze the migrated images.

Although the borehole logging data are clearly related to the degree of fracturing, which is expected to be a proxy for fluid flow, a quantitative analysis in terms of key hydraulic properties, such as porosity and permeability, in the studied environment is challenging. The reasons are manyfold. As previously mentioned, the Stoneley wave data are not of sufficient quality to allow for corresponding permeability estimations. The gamma–gamma and neutron–neutron logs, which are classically used to determine the porosity via the density and the hydrogen content, respectively, are affected by the large variations in the borehole diameter. These enlargements make a calibration of the gamma–gamma log to obtain density and subsequently porosity essentially impossible. The neutron–neutron log is less affected by the caliper variations than the gamma–gamma log, but calibrating it is still difficult due to its nonlinear relation to the hydrogen content and the lack of representative core material over a large enough porosity range. The resistivity logs, cannot not be converted into porosity, since the fluid resistivity is very high compared to common groundwater, the matrix porosity is quite low, and the surface conductivity can possibly not be ignored, which renders Archie's law inapplicable e.g.,. This is illustrated in Appendix  by comparing the resistivity logs with Archie's law. To circumvent these problems associated with the commonly used porosity logs, we use the 2015 BHR velocity measurements to derive porosity estimates. BHR measurements are not commonly utilized in oil and geothermal reservoirs, but there use is quite common in groundwater investigations to estimate water content or equivalently porosity e.g.,. The approach is quite robust, since the method is strongly sensitive to the water content and has a large support volume, which makes it less susceptible to borehole enlargements. The resulting smooth porosity profile will be then used to calibrate the neutron–neutron log, which provides a detailed downscaled version of the porosity distribution along the borehole. In a subsequent step, we examine the fluid flow characteristics of the subsurface region based on a combined analysis of the SP, temperature, and resistivity logs.

The BHR reflection image reveals a network of fluid-filled fractures in the damage zones above and below the main fault core. A projection of the reflections identified in this image onto the borehole track is shown in Fig. . Reflections with relative dips above 55∘ are well captured by the image and in good agreement with the interpretation of the OTV data. Conversely, events with smaller dips are more difficult to detect due to the single-hole setup and the limited offset. The extent and continuity of reflections away from the borehole can only be assessed in zones with low signal attenuation, as evidenced by high first-cycle amplitudes. In these zones, fractures can be tracked up to 5 m into the formation, revealing an interconnected fracture network. High signal attenuation, as evidenced by low first-cycle amplitudes, occurs in several zones characterized by intense brittle deformation. This also prevents a more quantitative analysis of the reflection amplitudes, which could be theoretically related to fracture apertures in a more favorable environment.

Although there are uncertainties in the absolute values of the measurements due to borehole enlargements, the relative variations in the petrophysical properties are clearly dominated by the different degrees of brittle deformation. Table 3 summarizes the range (25th and 75th percentiles of the median) of porosity and P- and S-wave velocities for the groups identified in the cluster analysis and compares them to core measurements and average matrix and micro-fracture porosity estimates of . These authors differentiate matrix porosity types by their mineralogy and texture. Quartz-dominated zones are governed by inter- and intragranular porosity (0.8 %), feldspar-dominated areas by solution porosity (1.4 %), fine-grained mica (75 µm) and coarse grained mica by inter- and intragranular porosity (2.8 % and 4.6 %), whereas breccia porosity is significantly higher. A summary of their estimates for mean matrix and fracture porosity is provided in Table 3. These estimates obviously do not account for larger fractures, which are captured by the log data. Nevertheless, for the cataclasites and the fault breccia the mean porosity estimates fall within the range of values obtained for clusters 1 and 2, which are categorized as the main fault core and regions of high fracture density or cataclastic zones, respectively. The values for clusters 3 and 4, which cover sections of the borehole with moderate to low fracture density, are quite similar to mean matrix and micro-fracture porosity values of the granites, gneisses and mylonites. However, a more detailed differentiation between these tectonic groups based on the log data is not possible. Ultrasonic velocity measurements performed on selected core samples generally provide higher values than the sonic log measurements. However, the maximum velocity values of the sonic log data in cluster 4, which comprises the most intact rocks, fall within the range of these laboratory measurements. The chosen core samples cover different types of fabric variations (granitoids, gneiss, and mylonites) and are from the more intact parts of the recovered drill core. Nevertheless, even these core samples still seem to contain fractures, as evidenced by the non-negligible difference between dry and saturated ultrasonic P-wave velocity measurements. This suggests that the elastic response of the damage zone of the GBF is dominated by the mechanical compliance of fractures at multiple scales.

The energy deficit of the Stoneley wave is related to the hydraulic transmissivity. At lower frequencies, this wave type is referred to as a tube wave and commonly encountered in hydrophone VSP surveys. linked the tube waves along the GDP1 borehole to hydraulically open fractures. Figure  compares the tube wave energy along the borehole with the energy deficit of the low-frequency FWS data. Both measures respond to hydraulically open fractures along the borehole. For the cataclastic zones and some of the large-scale fractures, we observe a good correlation between the two energy measures. In the zone around the main fault core, the agreement is less good, since borehole enlargements and the rugosity of the borehole wall prevent the propagation of tube waves. However, tube waves are still created in this region as shown by . Thus suggesting that these are zones/features of increased hydraulic transmissivity. Furthermore, the distinct anomalies in the wave energy deficit in the damage zone below the GBF show a very good correlation with the anomalies encountered in the SP data. This supports our interpretation that the SP anomalies are related to fluid flow governed by hydraulically open fractures.

For specifically targeted zones, indicated in Fig. , performed conventional and periodic pumping tests in the GDP1 borehole in 2015. The resulting transmissivity estimates are shown in Fig. . The highest values are obtained for the intervals i2 and i5, which feature a large aperture fracture of 16.4 mm at 105 m borehole depth. This fracture can be associated with a distinct anomaly in the SP data and the energy deficit. However, no clear trend can be established between the magnitude of the anomalies in the two log attributes and the estimated transmissivity from the pumping tests. One reason is that the pumping test only provides a few values of transmissivity averaged over relatively large intervals. The logs analyzed in this study do, however, point to a compartmentalized system with distinct hydraulic zones with water of different origins. A hypothesis is the existence of a shallow water reservoir above 95 m depth where meteoric water penetrates into the formation through the exposed fracture network, whereas below 95 m depth, a deeper and more saline hydrothermal water reservoir may prevail. Even though, there is no clear evidence of a hydraulic barrier between the two zones, below 95 m depth, a zone of increased ductile deformation with fracture apertures smaller than 3 mm followed by a zone of weakly deformed granite may act as such a barrier. Furthermore, the results of suggest a complex and variable flow geometry on the decameter scale in the studied subsurface region of the GBF associated with a heterogeneous system dominated by steeply dipping structures with a pipe-like hydraulic behavior. No chemical analysis of the water in the GDP1 borehole was conducted. However, the interpretation of different water sources is consistent with a recent hydrogeochemical study carried out in the Transitgas AG tunnel situated 200 m below the GDP1 borehole. The study identified two types of springs along the tunnel: (i) cold springs with a low mineralization (total dissolved solids TDS < 100 mg L-1) and (ii) warm springs with elevated mineralization (TDS > 250 mg L-1). The composition of the warm springs can be well explained by a binary mixture between a geothermal component characterized by elevated salinity and temperature and weakly mineralized cold meteoric water from the surface.

All of this points to the presence of distinctively different sources and passages of fluid flow in the studied subsurface region of the GBF (GDP1 borehole), thus suggesting a compartmentalization of fluid pathways in the hydraulic system along steeply dipping intersecting fractures and cataclastic zones. The main flow paths are associated with the intensely fractured nature of the exhumed ductile shear zone. Fracture permeability is expected to decrease with depth due to its inherent pressure-dependence, but cataclastic zones may retain a part of their porosity due to hydrothermal processes. Furthermore, suggested that fault intersections act as regions of high permeability in the Grimsel Pass hydrothermal zones. These intersections inferred from the drill hole data and observed in the BHR data may prevail on a larger scale along the GBF, effectively feeding a hydrothermal reservoir with meteoric water and allowing localized upflow of hydrothermal water along cataclastic zones (subsidiary fault cores) and fault intersections . This interpretation is supported by the study of , which analyzed the architecture and hydrothermal activity of the GBF on a larger scale and concluded that the hydraulic characteristics are controlled by localized subvertically oriented pipe-like upflow zones. Furthermore, a recent study of based on 3D thermal-hydraulic modeling constrained by surface observations of warm springs and fossil hydrothermal mineralization, suggested that the thermal anomaly at the Grimsel Pass may reach down to 9 km depth. The authors did, however, argue that, this specific anomaly is not a suitable candidate for petrothermal power production due to its comparatively low flow rates.

With the objective to characterize the fracture network of the damage zone surrounding the GBF and its petrophysical properties, we have performed an integrated analysis of the geophysical borehole log measurements. Although the log data are affected by challenging borehole conditions, notably numerous and large enlargements, the dataset contains a multitude of valuable information, which is in agreement with previous studies and adds to their findings. The BHR reflection data in combination with the OTV data suggest a complex network of fluid-filled fractures in the damage zone surrounding the main fault core of the GBF. Larger-scale fractures can be tracked several meters into the formation in the BHR reflection image, which, in turn, confirms that the borehole enlargements can generally be related to geological features and are not primarily drilling-induced damage. A comparison of the borehole logs to the fracture characteristics inferred from the OTV data confirms that the response of the logs, and thus the variations in petrophysical properties, are predominantly governed by the intensity of brittle deformation. A clear influence of ductile deformation on the petrophysical properties cannot be discerned. Overall, the measured petrophysical properties correlate very well with each other. Many of the petrophysical properties show distinctly different values than those expected for intact granitic formations, such as P-wave and S-wave velocities ranging between 2600 to 5600 and 1600 to 3200 m s-1, respectively, and porosities ranging between 3 % and 15 %. The compliant high-porosity zones associated with cataclastic structures and fractures are the main fluid pathways of the system. The SP data show an abundance of anomalies, which can be linked to fractures and are most likely of electrokinetic origin. The results of the SP probability tomography suggest that above 95 m inflow is dominating, and below inflow and outflow occurs with a dominant outflow region around 120 m borehole depth. Furthermore, the distinct layering observed in the fluid resistivity points to a compartmentalized pipe-like hydraulic behavior dominated by the steeply dipping geological structures as well as to the inflow of water from various different sources.