Conductive Fault Zone Research Articles

Hydraulically conductive fault zone responsible for monsoon triggered earthquakes in Talala, India

The Deccan volcanic province (DVP) in the northwestern part of the Indian subcontinent is the largest continental volcanic province in the world. Talala, located in the middle of the DVP, experience seismicity with swarm-like behavior towards the end of, or shortly after, the annual monsoon period. Between the years 2007 and 2012, seismic activity was recorded with moment magnitudes up to Mw 5.1. Although there is a statistical correlation between seismic activity and monsoon rainfall, there is little information about the geometry or attributes of the potentially causative fault system to date. We conducted geophysical surveys in the Talala seismogenic region to investigate the structure of the causative fault, with two Time-Domain Electromagnetic (TDEM) profile measurements, and Magnetotelluric (MT) measurements across the fracture zone to characterize deeper structures. TDEM measurements in the E-W direction revealed an electrically conductive zone within highly resistive surroundings, and the MT measurements revealed high electrical resistance along a curved structure that correlates with the seismogenic area and is likely associated with fractured crystalline rock. These fracture systems or networks can act as conduits for channeling and focusing intense rainfall events and develop significant fluid over-pressures to drive the swarm sequences. Numerical modelling results of fluid pressure evolution indicate increased groundwater and reservoir levels are likely responsible for earthquake generation.

CSAMT constraints on the metallogenic mechanism of the Zhaishang gold deposit, West Qinling, China

Whether the ore-forming fluids and materials of Zhaishang Carlin/Carlin-like type gold deposit were derived from concealed Mesozoic intermediate–acidic intrusive rocks through deep faults is controversial. In this study, eight CSAMT profiles were used to investigate the electrical resistivity structure of this deposit using high-resolution 2-D inversion. A ∼1 km wide high-resistivity (>3000 Ωm) body was observed at the base of the resistivity model (∼600 m). Two slightly NE-dipping high-conductivity channels were also observed along two sides of the high-resistivity body that connected to the gold-bearing quartz veins at the surface. The data suggest that the high-resistivity body is a Mesozoic granite and the two high-conductivity channels are fault zone conductors, which may be the wings of an NE-trending anticline that formed under regional compressive stress during the Caledonian orogeny. Their reactivation during the Indosinian and Yanshanian periods likely formed conduits for rising deeply-sourced ore-forming fluids (likely magmatic water). These conduits also promoted ground and meteoric water infiltration and mixing with the rising fluids. Accordingly, at least some of the ore-forming fluids and materials in the Zhaishang deposit were derived from Mesozoic granites through concealed faults, which can provide new geophysical constraints for discussions of the deposit’s metallogenic mechanism.

Integrated interpretation of geophysical data from Zagros mountain belt (Iran)

Fluid composition and distribution, the key factors determining geoelectric structure in a seismically active region, are controlled by local and regional stresses and rheological contrasts. Two closely located magnetotelluric and seismic velocity profiles are jointly interpreted to recover more accurately the structural boundaries and fluid distribution within the crust in the central Zagros collision zone, one of the world’s most seismically active mountain belt. A multi-site and multi-frequency approach was used for the strike analysis of regional structure and decomposition of distortion effects on magnetotelluric data. Distortion corrected magnetotelluric data were then used for two-dimensional inversion modeling. The results indicate distinct conductive structures within the crust: (i) a thick conductive overburden in the southwest of the profile, (ii) high conductivities attributed to the fault zone conductors (FZCs) and (iii) an almost concave conductor extending from middle to lower crust in the central-eastern portion of the mountain belt, beneath the High Zagros (HZ). Comparison with the already available S-wave velocity structure, obtained by joint inversion of P-wave receiver functions and surface wave dispersion data shows that these main conductive features are spatially correlated with a low-velocity layer representative of the sedimentary cover overlying the Arabian platform and a velocity contrast bounded by the main Zagros thrust (MZT) fault, indicating the presence of fault zone fluids. The joint interpretation of magnetotelluric inverse modeling and seismicity data also shed light on fluid generation influencing rock deformation and seismicity in this region. It suggests that beneath the HZ, deep crustal fluids generated through metamorphism may promote aseismic deformations before high stresses are builtup and cause the north-eastern part of the Zagros Fold and Thrust Belt (ZFTB) to be seismically inactive compared to its southwestern part.

Mobility of Ba, Sr, Se and As under simulated conditions of produced water injection in dolomite

The volume of petroleum produced water (PPW) has increased dramatically over the last decade. PPWs are rich in salts and often contain high concentrations of potentially toxic trace elements such as Barium (Ba), Strontium (Sr), Selenium (Se), and Arsenic (As) which, under the right circumstances, may contaminate freshwater supplies. To avoid these issues, PPW is frequently disposed of in subsurface aquifers such as the dolomitic Arbuckle Group in the state of Oklahoma, USA. This may not be a permanent solution if the injected PPW migrates within the disposal sites to ultimately reach conductive fault zones and/or existing wells with casing and/or cementing failures. In these cases the water could pose an environmental risk to potable groundwater. In order to understand the mobility of these elements under conditions similar to produced water injection, we conducted a series of batch sorption experiments. We investigated the effect of brine salinity and temperature on the sorption of Ba, Sr, Se, and As by dolomite. The results revealed that the sorption of the tested elements on dolomite did not substantially change with increasing salinity from 18 to 90 g/L. However, the sorption for all elements did increase with increasing temperature from 22 to 150 °C. This is likely due to a combination of strong surface complexation or ion exchange reactions coupled with precipitation/co-precipitation on the dolomite mineral surfaces.

Electrical resistivity structure of the Xiaojiang strike-slip fault system (SW China) and its tectonic implications

It is widely accepted that large-scale strike-slip faults play an important role in accommodating the India‐Eurasia convergence, but the relationships between surface displacements and deep deformation along the faults remain largely uncertain. To address this issue, new magnetotelluric (MT) images were obtained along two profiles across the Xiaojiang strike-slip fault system (XJFS), one of the most tectonically active areas in southwest China. Although different in details, these two profiles show similar overall crustal structures. The upper crust is generally resistive but separated by several narrow, subvertical conductors that broadly coincide with the surface traces of the XJFS. These fault zone conductors (FZCs) are interpreted to represent fault damage zones that formed by strike-slip faulting/shearing along the faults and filled with deeply-sourced magmatic and/or metamorphic fluids. The lower crust is broadly characterized by enhanced conductivity and likely associated with partial melting at high temperatures. These observations are consistent with other geophysical measurements in this area and in favor of the development of ductile flow along the XJFS, although the pattern of flow is more complex than previously suggested. Based on these observations, we propose that active deformation along the XJFS can be better described, in the brittle upper crust, by strike-slip motion along the fluid-weakened faults, and by shear-enhanced ductile flow in the partially molten lower crust. These two mechanisms are not mutually exclusive but rather can be complementary to each other.

Magma Chamber and Crustal Channel Flow Structures in the Tengchong Volcano Area From 3‐D MT Inversion at the Intracontinental Block Boundary Southeast of the Tibetan Plateau

AbstractBased on magnetotelluric (MT) array data, we have obtained the first three‐dimensional (3‐D) electrical resistivity model at the Gaoligong intracontinental block boundary in southeastern Tibetan Plateau where the Quaternary intraplate Tengchong volcanism and seismic activities occur. Comparing with results of previous geophysical studies in the area, our MT model clearly reveals three conductive bodies in the depth ranges of 10–30 km in the Tengchong volcano area, which we interpret as three middle‐lower crustal magma chambers associated with the Tengchong volcanism. Seismogenic faults in the Gaoligong Shear Zone (GLGSZ) are characterized by subvertical conductive zones bounded by resistive upper crustal layer on both sides. Earthquakes of moderate magnitudes near the GLGSZ have all occurred within the conductive fault zones at the bottom of the upper resistive crust. More importantly, one large resistive body was imaged at middle‐lower crustal depth beneath the GLGSZ, which seems to block the previously proposed horizontal crustal channel flow along this intracontinental block boundary. Our 3‐D model indicates that crustal channel flow could take place east of the GLGSZ. The current study provides evidence from electrical resistivity structure for the middle‐lower crustal magma chambers in the Tengchong volcano area and detailed 3‐D electrical structure of crustal channel flow in this active tectonic region.

Optimization of heat extraction strategies in fault-controlled hydro-geothermal reservoirs

Hydrothermal reservoirs commonly exist in association with fault activities. This study discusses the geothermal energy exploitation strategies in a fault-controlled geothermal system based on the Zhacang geothermal field in the northeastern Tibetan Plateau. The influence of well placement (including production depth, injection depth and well distance) and operation parameters (including injection temperature and production backpressure) on the heat production performance are evaluated by coupled thermal–hydraulic numerical modeling. Results indicate that a geothermal energy production of 1.67 MW can be obtained in the Zhacang geothermal field in 50 years, when the injection rate is 2.5 kg/s and injection temperature is 10 °C, with the injection depth at 1500 m, production depth at 300 m and well distance at 300 m. This optimized strategy is weakly affected by the terrestrial heat flux, because the heat transport in permeable fault zone is dominated by convection rather than conduction. Heat production increases with permeability in the conductive fault zone. The relationship between geothermal energy production and the associated natural and anthropogenic factors discussed in this study can help optimize heat extraction management in the other fault-controlled hydro-geothermal reservoirs.

MT data inversion and sensitivity analysis to image electrical structure of Zagros collision zone

Magnetotelluric (MT) data from 46 stations on a 470-km-long profile across the Zagros fold-thrust belt (ZFTB) that marks the Arabia-Eurasia collision zone were inverted to derive 2-D electrical resistivity structure between Busher on the coast of Persian Gulf and Posht-e-Badam, 160km north east of Yazd. The model includes prominent anomalies in the upper and lower crust, beneath the brittle-ductile transition depth and mostly related to the fluid distribution and sedimentary layers beneath the profile. The conductivities and dimensions of the fault zone conductors (FZCs) and high conductivity zones (HCZs) as the major conductive anomalies in a fault zone conceptual model vary significantly below the different faults accommodated in this region. The enhanced conductivity below the site Z30 correlates well with the main Zagros thrust (MZT), located at the western boundary of Sanandaj-Sirjan zone (SSZ) and known as the transition between the two continents. The depth extent of the huge conductor beneath the south west of the profile, attributed to the thick sedimentary columns of the Arabian crust, cannot be resolved due to the smearing effect of the smoothness constraint employed in the regularized inversion procedure and the sensitivity of MT data to the conductance of the subsurface.We performed different tests to determine the range of 2-D models consistent with the data. Our approach was based on synthetic studies, comprising of hypothesis testing and the use of a priori information throughout the inversion procedure as well as forward modeling. We conclude that the minimum depth extent of the conductive layer beneath the southwest of the profile can be determined as approximately deeper than 15km and also the screening effect of the conductive overburden is highly intense in this model and prevents the deep structures from being resolved properly.

Numerical investigations of upward migration of fracking fluid along a fault zone during and after stimulation

This study investigates the potential contamination as a result of hydraulic fracturing operations in the vicinity of a conductive fault zone. The general aim is to determine and rank key parameters with the most important influence on upward transport of fracking fluid from a hydrocarbon bearing formation (source) towards a shallow groundwater aquifer (receptor) via a conductive fault (pathway). Simulation results show that the fault zone properties and hydraulic fracturing pressure strongly dominate fluid transport towards the aquifer horizon. Nevertheless, the probability of contamination of shallow groundwater from fracturing fluid migration is low.

Electrical conductivity of a locked fault: investigation of the Ganos segment of the North Anatolian Fault using three-dimensional magnetotellurics

This study attempts to reveal the fault zone characteristics of the locked Ganos Fault based on electrical resistivity studies including audio-frequency (AMT: 10,400–1 Hz) and wide-band (MT: 360–0.000538 Hz) magnetotellurics near the epicenter of the last major event, that is, the 1912 Mürefte Earthquake (Mw 7.4). The AMT data were collected at twelve stations, closely spaced from north to south, to resolve the shallow resistivity structure to 1 km depth. Subsequently, 13 wide-band MT stations were arranged to form a grid enclosing the AMT profile to decipher the deeper structure. Three-dimensional inverse modeling indicates highly conductive anomalies representing fault zone conductors along the Ganos Fault. Subsidiary faults around the Ganos Fault, which are conductive structures with individual mechanically weak features, merge into a greater damage zone, creating a wide fluid-bearing environment. This damage zone is located on the southern side of the fault and defines an asymmetry around the main fault strand, which demonstrates distributed conduit behavior of fluid flow. Ophiolitic basement occurs as low-conductivity block beneath younger formations at a depth of 2 km, where the mechanically weak to strong transition occurs. Resistive structures on both sides of the fault beneath this transition suggest that the lack of seismicity might be related to the absence of fluid pathways in the seismogenic zone.Graphical .

Exploring the potential linkages between oil‐field brine reinjection, crystalline basement permeability, and triggered seismicity for the Dagger Draw Oil field, southeastern New Mexico, USA, using hydrologic modeling

AbstractWe used hydrologic models to explore the potential linkages between oil‐field brine reinjection and increases in earthquake frequency (up to Md 3.26) in southeastern New Mexico and to assess different injection management scenarios aimed at reducing the risk of triggered seismicity. Our analysis focuses on saline water reinjection into the basal Ellenburger Group beneath the Dagger Draw Oil field, Permian Basin. Increased seismic frequency (>Md 2) began in 2001, 5 years after peak injection, at an average depth of 11 km within the basement 15 km to the west of the reinjection wells. We considered several scenarios including assigning an effective or bulk permeability value to the crystalline basement, including a conductive fault zone surrounded by tighter crystalline basement rocks, and allowing permeability to decay with depth. We initially adopted a 7 m (0.07 MPa) head increase as the threshold for triggered seismicity. Only two scenarios produced excess heads of 7m five years after peak injection. In the first, a hydraulic diffusivity of 0.1 m2 s−1 was assigned to the crystalline basement. In the second, a hydraulic diffusivity of 0.3 m2 s−1 was assigned to a conductive fault zone. If we had considered a wider range of threshold excess heads to be between 1 and 60 m, then the range of acceptable hydraulic diffusivities would have increased (between 0.1–0.01 m2 s−1 and 1–0.1 m2 s−1 for the bulk and fault zone scenarios, respectively). A permeability–depth decay model would have also satisfied the 5‐year time lag criterion. We also tested several injection management scenarios including redistributing injection volumes between various wells and lowering the total volume of injected fluids. Scenarios that reduced computed excess heads by over 50% within the crystalline basement resulted from reducing the total volume of reinjected fluids by a factor of 2 or more.

New data on the deep structure of the northern part of the Gulf of Tonkin in the South China Sea based on the results of magnetotelluric soundings

We present a geoelectric model for the northern part of the Gulf of Tonkin in the South China Sea, which was developed for the first time based on the data of magnetotelluric studies carried out in 2012- 2013. The existence of a regionally developed aniso� tropic conducting fractured horizon and a steeply dip� ping electrically conductive fault zones is a distin� guishing property of this model, which has a layer� block structure. The nature of the zones of electric conductivity is related to the accumulation of a water� graphite mixture during longterm propagation of the flow of deep gases, which include H 2 and CH4. This allows us to consider the outcrops of the deep fluid sat� urated fractures as potential oil bearing regions. In the conditions when traditional oil and gas deposits are being exhausted, the search for new objects of accumulation of hydrocarbons has become one of the present pressing problems. In recent years, research into fissured formations in the upper level of the basement, which is a transition complex between the crystal basement and the sedimentary cover, has become widely developed (1). The rocks related to the transition basement are almost not changed by the processes of regional metamorphism. They are dislo� cated more greatly than the overlying columns of the sedimentary cover, transformed by recrystallization processes and catagenesis, and have a different struc� tural position. The oilandgas-bearing properties of such complexes have been established in different regions of the world: East Siberia (2), Venezuela, US, Libya, Egypt, and India (3). In 1988, a unique deposit was discovered in the granites of the Mesozoic base� ment on the shelf of South Vietnam (4). Deep geophysical processes that give us the possi� bility to make a primary estimate of the structural and material characteristics of the basement in the study regions play an important role in the study of these structures. The estimate includes the existence of fis� sured complexes and locations of permeable zones in the lithosphere that provide pathways for migration of deep fluid fluxes, which are sources of saturations of the fissured complexes with hydrocarbons. All this allows us to forecast the oilandgas-bearing potential of the study region. One of the geophysical methods used for the solution of these problems is magnetotel� luric sounding, which makes it possible to study the differentiation of electric resistivity of the tectono� sphere deep section depending on its structural mate� rial composition.

Fault zone conductors in Northwest Turkey inferred from audio frequency magnetotellurics

A comparison of the electrical properties of the two main branches of the North Anatolian Fault is made based on audio-frequency magnetotelluric (AMT) surveys carried out along three typical profiles across the North Anatolian Fault Zone (NAFZ). These two branches are (i) the active northern branch (Izmit-Adapazari Fault (IAF)) where the 17 August, 1999, Izmit earthquake took place, and (ii) the less active southern branch (Iznik-Mekece Fault (IMF)). In this paper, we focus on the shallow depths (surface–1.5 km) in search for conductors near the fault zone for the purpose of comparing these two branches with each other, as well as other strike-slip faults. Phase tensor analyses and Groom-Bailey decompositions were applied to check the dimensionality of the AMT data. Following the tensor analyses and decompositions, two-dimensional inversions were carried out to yield models based on the transverse electric (TE), and transverse magnetic (TM), modes. These models suggest that the North Anatolian Fault exhibits signs of fault zone conductors (FZC) regardless of being active or less active.

Geometry and faults tectonic activity of the Okavango Rift Zone, Botswana: Evidence from magnetotelluric and electrical resistivity tomography imaging

We used Magnetotelluric (MT) and Electrical Resistivity Tomography (ERT) to investigate the geometry and nature of faults activity of the Okavango Rift Zone (ORZ) in Botswana, an incipient rift at the southern tip of the Southwestern Branch of the East African Rift System. The ORZ forms a subtle topographic depression filled with Quaternary lacustrine and fluvio-deltaic sediments and is bounded by NE-trending normal faults that are more prominent in the southeastern portion of the rift basin. An MT model from a regional (∼140km) NW–SE trending MT transect shows that much of the rift basin is underlain by a broad asymmetrical low resistivity anomaly that slopes gently (∼1°) from NW to SE reaching a depth of ∼300m. This anomaly suggests that faults in the southeastern part of the rift form a NW-dipping border fault zone and that the lacustrine and fluvio-deltaic sediments contain brackish to saline water filling the broad half-graben structure. Furthermore, MT and ERT models from detailed (4–13kmlong) MT transects and resistivity profiles show that one border fault (Thamalakane) and two within-basin faults (Lecha and Tsau) in the southeastern part of the ORZ are characterized by a localized high conductivity anomaly while another border fault (Kunyere) lacks such an anomaly. These localized anomalies are attributed to channelized fresh surface water and saline groundwater percolating through these faults forming “fault zone conductors” and suggest actively displacing faults. The lack of a “fault zone conductor” in the Kunyere fault is interpreted as indicating diminishing displacement on this fault, and that strain was transferred to the Thamalakane fault further to the east. The fluids provide lubricant for the ORZ faults, hence preventing infrequent large magnitude earthquakes, but favoring frequent micro-seismicity.

Magnetotelluric Studies at the San Andreas Fault Zone: Implications for the Role of Fluids

Fluids residing in interconnected porosity networks have a significant weakening effect on the rheology of rocks and can strongly influence deformation along fault zones. The magnetotelluric (MT) technique is sensitive to interconnected fluid networks and can image these zones on crustal and upper mantle scales. MT images have revealed several prominent electrical conductivity anomalies at the San Andreas Fault which have been attributed to the presence of saline fluids within such networks and which have been associated with tectonic processes. These models suggest that ongoing fluid release in the upper mantle and lower crust is closely related to the mechanical state of the crust. Where fluids are drained into the brittle crust, and where these fluids are kept at high pressures, fault creep is supported. Fluid fluxes from deeper levels, in combination with meteoric and crustal metamorphic fluid inflow, and in response to fault creep, leads to high-conductivity zones developing as fault zone conductors in the brittle portion of crust. In turn, the absence of crustal fluid pathways may be characteristic for mechanically locked segments of the fault. Here, MT models suggest that fluids are trapped at depth and kept at high pressures. We speculate that fluids may infiltrate neighboring rocks and in their wake induce non-volcanic tremor.

Integrated geophysical investigations in the Hisar geothermal field, Demirci, western Turkey

The shallow, low-temperature geothermal field of Hisar, in western Turkey, was studied by means of vertical electrical sounding (VES), Wenner, self-potential (SP) and very low-frequency electromagnetic (VLF-EM) profiling surveys. The VES survey, conducted along the valley in an E–W direction, provided lithological and structural information that is in good agreement with the well data, and suggests that the field is characterized by low-resistivity values (≤30 Ω m). Because the resistivity structure in a N–S direction could not be investigated due to the rough topography towards the north, SP, Wenner and VLF-EM measurements were made to identify and characterize a possible hot-fluid carrying fault/fracture zone that forms one of the boundaries of the valley. There is a good correlation between the results of the SP, VLF-EM and Wenner surveys, which confirm the existence of the NE–SW striking fault zone that had been inferred from geologic information. Tensional fractures that developed perpendicular to the fault zone were also identified. Consistent with the results of the SP and Wenner profilings, the two-dimensional model derived from VLF-EM data and a Karous–Hjelt current density pseudo-section detected the conductive fault zones bearing the geothermal fluids.

High resolution electrical imaging of fault zones

Two-dimensional numerical modelling studies of a generic conductive fault zone indicate that very closely spaced TM mode magnetotelluric (MT) data are capable of providing high-resolution images of the fault structure. The degree of resolution that is possible is largely controlled by the conductance of the overburden and is also dependent on the spacing of measurement sites. In the absence of any overburden, or in the case of very thin overburden, the resolution is limited by the minimum electrode dipole length which is practicable. As an example, the results of two-dimensional inversion of closely spaced data measured across the Alpine Fault are compared with an earlier model derived from more widely spaced measurement sites. Much higher resolution of the fault zone is obtained from the closely spaced data and some features of the earlier model appear to arise due to the sparseness of the data. In the case of a very narrow fault zone, it is shown that the MT data exhibit very little lateral variation in phase and therefore independent control of static-shift is necessary.

On the Generation of ULF Magnetic Variations by Conductivity Fluctuations in a Fault Zone

— Prior to the October 18, 1989 Loma Prieta M s 7.1 earthquake, Fraser-Smith et al. (1990) recorded a 10–100 fold increase in ultra-low frequency (ULF) magnetic fields near the earthquake epicenter. Several mechanisms for generation of these ULF fields by fluid flow in the earth have been advanced, but all appear to require unrealistic fluid velocities or hydraulic permeabilities to match the observations. As an alternative explanation, Merzer and Klemperer (1998) proposed that the increase in ULF magnetic fields could result from induced electric currents flowing in a fault-zone made temporarily much more electrically conductive by stress-induced reorganization of pore geometry. Using a numerical model we show that while this mechanism could produce a significant increase in ULF variations, mutual induction between the fault zone and the surrounding crust would probably limit the amplitude increase to levels well below those observed at Loma Prieta. We consider a variant on this quasi-static conductive fault zone model in which low frequency telluric currents are modulated by small higher frequency variations of bulk fault zone conductivity. We show that because the spectrum of natural EM variations is red, substantially larger relative increases in ULF magnetic fields could be produced by this mechanism with even small conductivity fluctuations at these frequencies. These variations would be easy to detect with a well-designed experiment, if they occurred. In principle this mechanism could explain the Loma Prieta ULF observations, however the magnitude of conductivity fluctuations that would be required to match the very large reported amplification factors still appears to be too large to be physically plausible.

Surface seismic and electrical methods to detect fluids related to faulting

In the absence of drilling, surface‐based geophysical methods are necessary to observe fault zones and fault zone physical properties at seismogenic depths. These in situ physical properties can then be used to infer the presence and distribution of fluids along faults, although such observations are by nature indirect and become less exact with greater depth. Multiple observations of a range of such geophysical properties as compressional and shear seismic velocity (Vp and Vs), Vp/V5 ratio (related to Poisson's ratio), resistivity and attenuation in and adjacent to fault zones offer the greatest hope of making inferences of the fault zone geometry, fluids in the fault zone, and fluid reservoirs in the surrounding crust. For simple geometries, fault zone guided waves can provide information on fault zone width and velocities for faults of the order of 200 m wide. To address the question of whether a narrow fault zone can be imaged well enough at depths of seismic rupture to infer the presence of anomalously high fluid/rock ratios, we present synthetic seismic tomography and magnetotelluric examples for an ideal case of a narrow fault zone with a simple geometry, large changes in material properties, and numerous earthquakes within the fault zone. A synthetic 0.5‐km wide fault zone with 20% velocity reduction is well imaged using local earthquake tomography. When sequential velocity inversions are done, the true fault width is found, even to 9 km depth, although the calculated amplitude of the velocity reduction is lower than the actual amplitude. Vp/Vs is as well determined as Vp. Magnetotelluric imaging of a synthetic fault zone shows that a conductive fault zone can be well imaged within the upper 10 km. Further, a narrow (1 km) very low resistivity (3 ohm m) fault core can be imaged within a broad (5 km) low resistivity (10 ohm m) fault zone, illustrating that regions of a fault containing large quantities of interconnected fluids within a broader, conductive fault zone should be detectable. Thus variations in fluid content and fluid pressure can be inferred from electrical and seismic methods but there will always be uncertainty in these inferences due to the trade‐off with other factors, such as intrinsic variations in porosity, mineralogy, and pore geometry. The best approach is combined modeling of varied seismic and electrical data.

Analysis of induced seismicity for stress field determination and pore pressure mapping

The focal mechanisms of some one hundred microseismic events induced by various water injections have been determined. Within the same depth interval, numerous stress measurements have been conducted with the HTPF method. When inverted simultaneously, the HTPF data and the focal plane solutions help determine the complete stress field in a fairly large volume of rock (about 15×106 m3). These results demonstrate that hydraulically conductive fault zones are associated with local stress heterogeneities. Some of these stress heterogeneities correspond to local stress concentrations with principal stress magnitudes much larger than those of the regional stress field. They preclude the determination of the regional stress field from the sole inversion of focal mechanisms. In addition to determining the regional stress field, the integrated inversion of focal mechanisms and HTPF data help identify the fault plane for each for each of the focal mechanisms. These slip motions have been demonstrated to be consistent with Terzaghi's effective stress principle and a Coulomb friction law with a friction coefficient ranging from 0.65 to 0.9. This has been used for mapping the pore pressure in the rock mass. This mapping shows that induced seismicity does not outline zones of high flow rate but only zones of high pore pressure. For one fault zone where no significant flow has been observed, the local pore pressure has been found to be larger than the regional minimum principal stress but no hydraulic fracturing has been detected there.