The electrical resistivity of competent/unfractured rock depends mainly on porosity and theconductivity of the pore fluid. In fractured/jointed rocks a further degree of dependence on thedegree of fluid saturation and permeability is observed; The audiomagnetotelluric surveydescribed in this report is primarily concerned with mapping resistivity variations (i.e. rock/fluidproperties) through the critical depth range (2 to 8 km) of geothermal energy extraction withinand across the Carnmenellis granite.This report describes the audiomagnetotelluric experiment that was carried out during 1988.Two field surveys were performed. A main survey to assess the presence of a refractor (awide-angle reflector) at about 7 to 8 km (Brooks et al., 1984) and a secondary survey to assessthe depth e_xtent of a surface lineament. The principal results of the field experiment are nowsummarised.A seven-site investigation of a NW-SE surface lineament was carried out at spacings of 50m. Toour knowledge, this is the first audiomagnetotelluric experiment involving such small spacings.The survey data were partially marred by an additive noise component in one of the electric fieldchannels. Subsequent detailed analysis of these data revealed electric fence noise as the cause.Although such hindsight detection is unfortunate, the uncontaminated data offer valid constraintsin relation to the detection of a fluid-filled fracture zone. The results presented indicate that it ishighly unlikely that the target lineament can be associated with a conductive zone possessing areasonable resistivity contrast which intersects the surface or near-surface. The results of themain experiment extend this null interpretation to the majority of surface lineaments as describedbelow.The main granite survey consisted of 17 soundings in total. An E-W profile across the graniteoutcrop comprised 12 soundings and two further soundings were conducted off the outcrop forcontrol. The two off-granite, soundings provided very different 'dimensional' characteristics tosoundings on the granite. They therefore confirm the broad homogeneity of the geoelectricanisotropy on the granite. The two off-granite soundings also provided information on theresistivities of the Devonian cover rocks and the depth to granite. A further 3 soundings wereperformed in the vicinity of the Hot Dry Rock (HDR) reservoir at Rosemanowes quarry.Overall the Carnmenellis granite, as defined by its broad resistivity characteristics, appearspredominantly homogeneous. A very consistent set of resistivity values are found for the wholegranite structure below depths of 2 km. The majority of resistivity values are in the range 1000to 10,000 ohm.m. When laboratory analyses of thermal and pressure dependence are taken intoaccount the granite is found to correspond to a 'wet' granite saturated with several weight-percentof free water down to at least IO km. 路 The consistency of the resistivity values below 2 kmacross a major portion of the granite indicates that lateral geoelectric effects are likely to beconfined to the near-surface (i.e. < 2 km). An examination of the lateral anisotropic behaviour across the granite confirms this general conclusion. We conclude that with the exception of onelocation the surface lineations, within the area surveyed, do not appear to represent deep verticalor sub-vertical zones with rock/fluid properties that would distinguish them from 'background'properties.Judging by the hydrogeological models for the near-surface granite, a spatially-complexhydrothermal circulation system can be considered to operate at depth (Gregory and Durrance1987). The spatial distribution of such a system is likely to be closely tied to the distribution ofmajor water-conducting fractures. Although the scale and degree of resistivity contrasts shouldbe considered, the main survey profile has detected only one such near-surface feature.Significantly this conductive zone is spatially localised and is directed NE-SW. The zone appears路to be correlated with a lineament and a main arterial alluvial fan of the survey region. The E-Wsurvey profile intersects the NE-SW trending zone 0.5 km directly south of the village ofPorkellis. There is an indication of a conductive layer at a depth of I km in the vicinity of thislineament.The sites defining the SE portion of the granite display anisotropic features which are differentfrom those to the west. It is suggested that the cause is associated with boundary or off-granitevariations rather than with variations across the outcrop although this cannot be ruled out. Thefeatures observed in the SE appear to define a broad, large-scale effect. Resistivity valuesthrough the granite also appear slightly larger in this region of the outcrop.The lateral geoelectric anisotropy transfers (i.e. azimuths rotate) from local and near-surfacepenetrations (e.g. 1 to 2 km) to a regional scale anisotropy at large-volume penetrations (e.g.in excess of 25 km). The rotation pattern is consistent at the majority of survey locationsbeginning NE/ENE at depths of about l km to NINE at depths of 6 to 10 km and thence toNW/N at depths in excess of 25 km. The information on geoelectric anisotropy has beencompared with the principal joint and stress directions of the granite in order to identify themechanism controlling the resistivity variations. The main conclusion is that the directions ofresistivity anisotropy do not display any persistent alignment with the principal horizontal stressdirections. Such a conclusion assumes that the present indicators of stress directions arerepresentative of the in situ stress at depths in excess of 2.5 km. On this basis then themechanism of aligned microcracks does not appear to control the observed geoelectric anisotropy.The results indicate that within the upper 1.5 km (at least), resistivity is controlled by one of thetwo principal joint systems of the granite. The results identify the fracture system parallel to theNW-SE master joints as being preferentially 'open' and containing enhanced concentrations offluids. A definitive interpretation (from geoelectric anisotropy) at greater depths appears to berestricted by the observed 'intermediate' rotations as other larger-scale (regional) effects becomemore dominant. The vertical field is, however, influenced by a major resistivity contrast strikingNW-SE beyond, and to the SW, of the Carnmenellis outcrop.Below a depth of 2 km resistivity values increase slowly with depth attaining maximum values by about 6 km. The anticipated linear decrease of resistivity with increasing temperature is notobserved and a more dominant pressure/stress dependence must control the spatially-consistent.depth dependence observed. Laboratory measurements on a wide range of granitic rocks indicatethat transfer from crack-dominated behaviour to pore-dominated behaviour will be complete by anapplied pressure of 200 MPa. The extrapolated overburden and stress magnitudes, from theHDR borehole measurements. suggest that this will be achieved within the Carnmenellis by about6 km. The resistivity profiles are therefore consistent with the completion of crack closure by adepth of 6 km and a transfer to a pore-dominated resistivity mechanism below this depth. Thus,in simple terms, if a 'joint' can be defined as a feature that is capable of 'closing' (and closedhere means an inability to support ionic conduction of interstitial fluids) the observations suggestthe absence of such joints below 6 km.A comparison of the vertical geoelectric profiles across the granite with the boundary reflector(RI) of the deep low-velocity zone modelled by Brooks et al. (1984) is possible. The resistivityprofiles however do not reveal any spatially-consistent major discontinuities in the upper 12 km.The depth interval of the low-velocity zone appears merely to be associated with an interval ofapproximately constant and maximum resistivity. We conclude that no detectable geoelectricvariations in rock/fluid properties can be identified at a depth associated with the Rl reflector.Thus RI does not appear to represent the upper surface of a fractured zone with an associatedenhancement of conducting fluids.The three sounding sites above and around the HDR reservoir provide interesting results in theupper 5 km. The identical resistivity profiles at two of the sites display a more conductiveprofile when compared with a third site some 1.1 km away. The two sites, 500 m apart, appearto define a 'reservoir-influenced' section down to depths of 4 to 5 km. This depth appearsconsistent with the termination depth of the microseismic zone defined during hydrofracturing.The finite lateral extent of the low-resistivity reservoir volume is also apparent in the azimuthal(anisotropic) behaviour across the three locations. Thus the zone of enhanced fluid concentration(i.e. the reservoir) has been shown to have finite dimensions both laterally and vertically.
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