Keith F. Evans

Stress field variations in the Swiss Alps and the northern Alpine foreland derived from inversion of fault plane solutions

[1]xa0This study is devoted to a systematic analysis of the state of stress of the central European Alps and northern Alpine foreland in Switzerland based on focal mechanisms of 138 earthquakes with magnitudes between 1 and 5. The most robust feature of the results is that the azimuth of the minimum compressive stress, S3, is generally well constrained for all data subsets and always lies in the NE quadrant. However, within this quadrant, the orientation of S3 changes systematically both along the structural strike of the Alpine chain and across it. The variation in stress along the mountain belt from NE to SW involves a progressive, counterclockwise rotation of S3 and is most clear in the foreland, where it amounts to 45°–50°. This pattern of rotation is compatible with the disturbance to the stress field expected from the indentation of the Adriatic Block into the central European Plate, possibly together with buoyancy forces arising from the strongly arcuate structure of the Moho to the immediate west of our study area. Across the Alps, the variation in azimuth of S3 is defined by a progressive, counterclockwise rotation of about 45° from the foreland in the north across the Helvetic domain to the Penninic nappes in the south and is accompanied by a change from a slight predominance of strike-slip mechanisms in the foreland to a strong predominance of normal faulting in the high parts of the Alps. The observed rotation can be explained by the perturbation of the large-scale regional stress by a local uniaxial deviatoric tension with a magnitude similar to that of the regional differential stress and with an orientation perpendicular to the strike of the Alpine belt. The tensile nature and orientation of this stress is consistent with the “spreading” stress expected from lateral density changes due to a crustal root beneath the Alps.

Observation and simulation of non‐Darcian flow transients in fractured rock

Two independent multirate flow experiments were conducted in 1994 in the open hole depth interval of a well bore at the hot dry rock (HDR) test site Soultz. The steady state and transient downhole pressure records gave clear indications of non-Darcian flow. A numerical model has been set up to evaluate these two measurements. An excellent fit of the transient pressure responses of the two flow tests could be achieved by assuming one model of simple geometry. The model predicts fluid transport along a conduit with substantial surface area in which fully turbulent flow is occurring. All the parameters required by our best-fit simulation fall into a physically reasonable range. Sensitivity analysis demonstrates a non-Darcian flow regime along highly conductive features. The existence of high capacity far-field faults as postulated in our model confirms earlier characterizations of the Soultz test site.

Hydraulic fracture reopening pressure and the estimation of maximum horizontal stress

Abstract In hydrofracture stress measurements, the magnitude of the maximum horizontal stress, S H , is commonly estimated from the borehole pressure required to reopen an induced axial crack. Examination of the processes which govern the borehole pressure history recorded during the reopening cycle of such tests indicates two sources of error in the estimates of S H derived using the conventional method proposed by Bredehoeft et al. [Bredehoeft JD, Wolff RG, Keys WS and Shutter E, 1976, Colorado. Geol. Soc. Amer. Bull., 87, 250–8]. The first arises from the failure to include a term arising from pressure penetration into the crack prior to reopening in the force balance acting across the mouth of the induced axial cracks. The problem can be remedied by using a modified ‘reopening equation’ which includes pressure penetration of the crack. The second source of error is more problematic and concerns the correct identification of the true reopening pressure from the borehole pressure records. Analysis of the process of reopening aided by numerical simulations shows that the true reopening pressure is generally less than the apparent (i.e. that detected) reopening pressure. The discrepancy between true and apparent reopening pressures increases with larger hydraulic compliance of the test equipment. The compliance in question refers to that of the fluid volume between the flow meter and the crack mouth(s). Simulation of a pair of 1xa0m high axial cracks with 2xa0μm residual hydraulic aperture in a 100xa0mm borehole, indicates that the system compliance must be reduced to 5×10 −7 m 3 /MPa to enable the true reopening pressure to be estimated to better than 10%, at flow rate is less than 10 −4 m 3 /s. This is several orders of magnitude less than applies to conventional hydrofracture systems, but is attainable for tests in small holes at shallow depth by making relatively minor system modifications. Tests at greater depth, however, would seem to require downhole measurement of flow at the packers. We validate our assertions with a field test in which reopening pressure was determined mechanically and hydraulically.

Correlation between magnetic anisotropy and fabric for Devonian shales on the Appalachian Plateau

The magnetic anisotropy of Devonian black shale samples was measured from two cores drilled in the Appalachian Plateau. The mineralogy of the shales is predominantly clay, with small quantities of quartz and minor amounts of opaques and chlorite. Magnetite is the predominant ferromagnetic mineral present in the samples. The magnetic fabric was measured at both room temperature and liquid-nitrogen temperature and is dominated by a well-defined bedding (vertical) compaction and a lesser defined magnetic lineation. Measurements of the anisotropy of magnetic susceptibility (AMS) at liquid-nitrogen temperature, which enhances the paramagnetic contribution in the rock, showed a strong increase in both the bulk susceptibility and susceptibility differences. This increase suggests that the AMS is controlled by the paramagnetic minerals, particularly the clays and chlorite. Strain was measured from the orientation of basal planes of the chlorite crystals by texture goniometry. Good correlations have been found (1) between the orientation of the magnetic lineation and the long axes of the chlorite crystals, and (2) between the degree of magnetic foliation and the amount of vertical compaction. The magnetic lineation also agrees well with the direction of seismic anisotropy over the Plateau. The anisotropy of the anhysteretic remanence, which expresses the anisotropy due to the ferromagnetic component in the rocks, shows a weaker correlation with the amount of vertical compaction. A weak magnetic lineation suggests that the magnetite grains were aligned during a deformation phase which post-dates the main Alleghanian orogeny. The magnetic anisotropy of the Devonian shales mirrors the compaction and tectonic fabric on the Appalachian Plateau.

Water table effects on the measurement of earth strain

Abstract Forced changes in the water head within a granite-penetrating borehole were found to induce anomalously large free-surface strains and tilts in the vicinity of the hole. This deformation is shown to be due to fluid-pressure-induced changes in the aperture of a compliant hydraulically-conductive fracture at a depth of 100 m and a quantitative analysis of the deformation data is performed to recover fracture characteristics. The importance of the effect for secular earth strain measurements is discussed in the light of the ubiquitous nature of both water table fluctuations and fractures in the shallow crust.

Stress and rock mechanics issues of relevance to HDR/HWR engineered geothermal systems: review of developments during the past 15 years

This paper reports the findings of the Stress and rock mechanics working group of the Academic Review of Hot Dry Rock/Hot Wet Rock (HDR/HWR) Engineered Geothermal Systems convened in Sendai, Japan in 1997. Key developments in the fields of stress and rock mechanics that are relevant to the development of HDR/HWR systems and that have occurred since the last Academic Review in 1982 are described. Rock mechanics is here taken to include basic studies of fluid flow through fractures. Key unresolved issues that are important for HDR/HWR systems are also discussed.

Geophysical log responses and their correlation with bed‐to‐bed stress contrasts in Paleozoic rocks, Appalachian Plateau, New York

A 1-km profile of in situ stress and geophysical log data was acquired in the Wilkins well to study the relationship between rock properties and in situ stress contrasts. The Wilkins well penetrates Devonian clastic rocks on the Appalachian Plateau near the town of South Canisteo, New York. Open hole hydraulic fracture stress measurements were made in stratigraphic sequences where geophysical logs indicated significant bed-to-bed variations in elastic and lithologic properties. Analysis of stress magnitudes and interval-averaged geophysical data shows that principal horizontal stress magnitudes correlate directly with elastic stiffness and inversely with clay content. A similar relation is found for older Paleozoic strata penetrated by a well at Auburn, New York. Correlations between stress magnitude and geophysical properties observed in the Wilkins and Auburn wells provide strong evidence that bed to bed stress variations arise from a uniform ENE-WSW directed strain acting on beds of different Youngs modulus rather than from variations in rock shear strength. Because of their high Youngs modulus, sandstones, siltstones, and limestones in the northern Appalachian Basin are likely to be stronger barriers to hydraulic fracture propagation than shales. Porosity logs in the Wilkins well show that the large decrease in horizontal stress found at the base of the Rhine street Formation occurs where shales are less compacted. The correlation with undercompaction is consistent with a paleo-overpressure drainage mechanism as the cause for the stress decrease.

A comparison of the strain of crinoid columnals with that of their enclosing silty and shaly matrix on the Appalachian Plateau, New York

Abstract A pole-figure goniometer was used to measure the preferred orientation of the basal planes of chlorite in 87 samples from the Upper Devonian clastic rocks of the Appalachian Plateau, New York. Interpretation of the preferred orientation according to the theory of March yields an estimate of how much an element of the sediment has changed shape since deposition; this deformation can be interpreted according to three distinct models for the strain history. All begin with compaction, followed by one of three types of tectonic strain: (1) a plane strain, that conserves bedding-plane area by compensating for horizontal shortening with horizontal elongation at right angles; (2) one that uniaxially shortens a horizontal line normal to the fold axes without compensating elongation: and (3) a plane strain again, which conserves the area in the vertical plane containing a shortened horizontal direction by stretching it vertically. For lack of suitable names, we will simply call the three total strains type (b), (u) and (v) from the initials of the distinguishing features of their tectonic increment. The horizontal strain components estimated from preferred orientation are compared at a number of localities with those implicit in the principal horizontal diameters of the elliptical outlines of deformed crinoid columnals found on bedding planes. Expressed for constant bedding-plane area, or type (b) total strain, the strain in the direction of greatest elongation measured by preferred orientation was found to range from no recorded strain to 0.19, with a median of 0.04. As a rule, however, the more intensive strains were recorded in shales with a fine-grained phyllosilicate content exceeding 85% by volume, in which the mean was 0.07. Deformed crinoid columnals in the same set of samples have elongations parallel to their long axes that range from 0.04 to 0.10, with a median and average of 0.07. The discrepancy between the two sets of strain measures, at least in the coarser and more permeable rock types, may either be real and caused by easier flushing of dissolved ions or may be caused by imperfect strain recording in coarse matrix materials.

Estimating aquifer parameters from analysis of forced fluctuations in well level: An example from the Nubian Formation near Aswan, Egypt: 3. Diffusivity estimates for saturated and unsaturated zones

Atmospheric pressure variations provide a broadband signal that may force a sympathetic response in well water levels. In this paper, time series analysis techniques are used to estimate the response as a frequency-dependent admittance function, which is then modeled to provide estimates of the fluid transport properties of strata. The data derive from five cased piezometer wells sampling aquifers in the Nubian Formation southwest of Aswan, Egypt. Three shallow wells (100–140 m deep) sample a water table aquifer; a fourth (“W3”; 400 m deep) samples a basal aquifer in the same area that behaves in a confined manner up to a period of several years. The fifth well samples another basal aquifer and shows evidence of partial blockage. Nontidal water level variations in the shallow wells are due almost entirely to barometrically driven flow of air and water. Using a simple model to fit the observed barometric admittance spectra, we obtain estimates of horizontal and vertical permeabilities (for water) in the saturated zone. Local horizontal permeability is constrained by modeling the effects of flow-induced pressure gradients near the screen. For the W3 deep well sampling the basal aquifer, the resulting values (0.15–0.3 μ2) are marginally lower than the large-scale (5 km) estimates (0.32–0.43 μ2) derived in a previous paper. However, the values for the three wells sampling the water table aquifer, although consistent among themselves (0.2–0.5 μ2), are significantly lower than the large-scale estimate (1.0–1.5 μ2). This is contrary to what might be expected given that the wells are preferentially screened in clean sandstones. Vertical permeability, estimated by modeling partial confinement effects, is constrained for only one well. A low value was obtained because of the presence of claystone beds in the diffusion path between screen and water table at this well. The effects of air wave diffusion are clearly manifest in the spectra of one well where the water table lay at a depth of about 40 m. The form of the spectra was well fit by ascribing a uniform pneumatic diffusivity of 1.75×10 m−3m2/s to the unsaturated zone. However, it was also necessary to include an apparent attenuation of the air wave at the capillary fringe of about 0.5. We propose that the effect is due not to attenuation but that it reflects “compression” of the phreatic surface arising from the presence of trapped air pockets in the underlying saturated zone. A 40-m rise in the water table at the site during the decade prior to the measurements might explain the presence of significant quantities of trapped air. This rise in water table, together with the arid climate, might be taken to suggest that the moisture content of the unsaturated zone is negligible (except within a meter or so of the water table). However, calculation of the intrinsic rock permeability from pneumatic diffusivity assuming zero moisture content yields an estimate which is considered to be too low. The likely explanation is that the assumption of zero moisture content is in error, despite conditions which are as favorable as are ever likely to be realized under field conditions.

Estimating aquifer parameters from analysis of forced fluctuations in well level: An example from the Nubian Formation near Aswan, Egypt: 2. Poroelastic properties

We discuss the in situ estimation of aquifer poroelastic properties from observed water level fluctuations in five wells sampling the 0.30 porosity Nubian sandstone near Aswan Reservoir, Egypt. The study area is close to ideal for the application of poroelastic theory, because of the relatively simple stratigraphy and the absence of cultural and climatological noise. In this paper we use approximately 2 years data sampled at 0.2-hour intervals to calculate frequency-dependent admittances between the well levels and the recognized forcing functions of the earth tides and barometric fluctuations; the effect of variations in lake level is treated in a companion paper. The time series analysis techniques that we employ are described in detail. Three of the wells show equilibrium-confined response at tidal frequencies, which allows various in situ poroelastic parameters of the aquifer to be derived from the tidal and barometric admittance values. We make extensions to earlier treatments of poroelastic theory in order to include the effect of horizontal strain resulting from barometric loading; the modifications proved of minor importance for our data set because of the high compliance of the aquifer material and the shallow depth to basement. In situ shear modulus μ is determined as 5.4 - 6.4 × 109 Pa, and uniaxial strain loading efficiency as 0.33. Each of these parameters depends on the field measurements of well level and air pressure, and μ also depends strongly on a theoretical estimate of the solid earth areal strain tides; the principal source of error in μ lies in these areal strain estimates. We measured Poissons ratio in the laboratory and found that it lies in the range 0.13–0.18. This, together with the admittance values, allows estimates of uniaxial strain modulus (13 - 19 × 109 Pa, with the smaller values at shallower depth), specific storage (1.5–1.9×10−6 m−1, with the larger values at shallower depth), porosity (0.2–0.3), Skemptons constant (0.54–0.58), and Biots constant (0.78–0.85). Laboratory measurements of uniaxial strain modulus are about 50% higher than the in situ estimates, probably reflecting scale effects. Despite the high porosity, use of a theory of aquifer response in which the solid constituent is considered incompressible produces significant errors in these estimates.