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Measurements of linear strain, ultrasonic velocity, and attenuation (Q−1) were made simultaneously as functions of confining pressure on core and outcrop samples from the Moodus, Connecticut, area. Strain measurements indicate the core samples contain cracks which formed in part by stress relief during recovery (Meglis et al., 1991). The outcrop samples have a small crack porosity compared with the cores. Closure of cracks with increasing confining pressure causes an increase in velocity and a decrease in attenuation. We present a form for the pressure dependence of the crack density parameter e (the number of cracks of unit radius per unit volume), which was used to incorporate the influence of crack closure with pressure into models of wave velocity and attenuation in cracked solids. The crack density parameter is represented as an exponentially decreasing function of confining pressure. The pressure dependence of e was determined from the strain measurements using the non-self-consistent effective modulus approach of O'Connell and Budiansky (1974), from the velocity data using the solutions of Garbin and Knopoff (1973) and Hudson (1981), and from the attenuation measurements using the frictional attenuation model of Walsh (1966). All of the models fit the data reasonably well using an exponentially decaying e described by one decay constant τ. However, some of the data are better fit by a crack density parameter with two decay constants, reflecting a rapid decrease of e at low pressure and a slower decrease at higher pressure. There is considerable variation among the predicted decay constants for a given sample from the different data sets. Several factors contribute to this variation. For example, the two velocity models predict a different dependence of velocity on e, which results in a different dependence of e on pressure. For the Q−1 data, approximating de/dP by a function with a single decay constant results in lower τ values for Q−1 than for strain or velocity. Finally, a large anisotropy in attenuation measured in the deepest core samples indicates that scattering is a significant source of wave energy loss in these samples, and therefore a frictional attenuation mechanism alone cannot account for all the observed attenuation.

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 Young's modulus rather than from variations in rock shear strength. Because of their high Young's 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.

To examine the amount of rock dissolution accompanying changes in joint permeability, deionized water was forced through axially split cylindrical samples of Vermont marble, subjected to a confining pressure of 60 MPa. Surface preparation and loading conditions varied among experiments during which permeability and fluid chemical composition were monitored with time. Time dependent permeability and fluid composition for freshly polished joints differed from joints which had been “etched” by the fluid during previous runs. For freshly polished surfaces, permeability decreased and fluid chemical concentrations increased during the first 50 to 100 hours of fluid flow. For the water-etched surfaces, permeability was not time dependent, and a steady state chemical composition was reached after 20 to 40 hours. A calculation of the steady state chemical concentrations for coexisting calcite and dolomite in deionized water, at a fluid pressure of 10 MPa and a confining pressure of 60 MPa, shows that the steady state values reached in our experiments were supersaturated. Supersaturation was probably due to enhanced solubility at contact points together with precipitation inhibition. Our data indicate that fluid flow through fresh fractures causes dissolution of contact points and permeability reduction, depending on the morphology and state of the fracture surfaces.

We made 29 in situ doorstopper strain relaxation measurements distributed among eight sites spaced on a 35-km transect running from the foothills of the San Gabriel Mountains, across the San Andreas fault, into the western Mojave desert southeast of Palmdale, California. This was a pilot study to test if such measurements can detect the regional stress field and any modification of it in a tectonically complex area. Strain was measured by overcoring strain gauge rosettes which had been bonded to the flattened bottom of a shallow borehole. Measurements of stress were repeatable at each site. We found NNE trending maximum compressive stress (σ1) at our sites farthest from the fault. This orientation is parallel to the σ1 inferred from fault plane solutions of major southern California earthquakes and compares favorably with deep hydrofracture stress measurements made near our sites. Nearer the San Andreas fault, the orientation of σ1 was approximately east-west north of the fault, and northwest-southeast to northsouth south of the fault. The repeatability of measurements at a site and the favorable comparison of our measurements with Tullis' [1977] near-surface stress measurements suggest that we reliably determined the stress field orientation present at a site. It was not possible, using the available data, to distinguish the stress caused by residual or topographic effects from tectonically applied stress or to account for modification of the stress field by decoupling across fractures.

To compare the orientation and development of jointing with the orientation and magnitude of finite strain recorded in the Upper Devonian rocks of the Appalachian plateau, New York, we mapped systematic joint sets on an area of 20,000 km2. In this area, Wedel [1932] mapped folds with limb dips of less than a degree and axes that change strike by 30° from 090° in the east to 060° in the west. We observed two different cross-strike joint sets that maintain their approximate cross-strike position from east to west. Yet, in detail the angle between the sets is 18°±2° in the east and 30°±4° in the west. In many outcrops one joint set parallels the direction of maximum compressive strain (eƒ) as recorded by deformed fossils, whereas the other joint set never parallels eƒ. Rare calcite-filled joints are oriented parallel to the direction of eƒ. In addition, the calcite-filled joints both cut and are cut by solution cleavage. These observations suggest that the joint set paralleling eƒ formed during the deformation event represented by the deformed fossils. The joint set that does not parallel eƒ somehow reflects a deformational event other than that producing fossil distortion, as suggested by strain relaxation experiments. In an outcrop of the Machias formation, where the direction of eƒ is 15° from the trace of a cross-strike joint set, the tensor average from 14 subsurface strain relaxation tests shows in situ maximum comprehensive strain (eoc = 10−4) parallel to cross-strike joints but not the direction of eƒ. Strain relaxation is coaxial with a fabric anisotropy indicated by sonic velocity tests. Our idea is that the orientations of cross-strike joints parallel to eoc were controlled by the same rock property that causes strain relaxation on overcoring, whereas the orientations of the cross-strike joints parallel to eƒ were controlled by the stress field causing the fossil distortion. Placing these structural data in a regional context allowed us to construct a dynamic and kinematic model for the structural evolution of the New York plateau. The model indicates variation in both boundry conditions and material behavior through a series of four distinct deformational events, which begin prior to lithification and end in the Recent. Thus our analysis suggests that the structural features we have used represent a set of highly sensitive tools for investigating the deformational history of the Appalachian foreland.

Volume-loss strain accompanying pressure solution of calcite occurred within both the Umbria-Marches Apennines of Italy and the Appalachian Mountains of western New York. Data from strain markers show that volume-loss strain was greater within the shallow portions of the Apennines than within the Appalachians. Within the deeper portions of both fold and thrust belts, strain was nearly volume-constant. Calcite solubility data suggest that downward circulation of meteoric water is necessary for the 35% volume-loss strain of the limestones within the Apennines. Strain at a depth of about 1 km was volume-constant and is interpretated as indicative of restricted pore fluid circulation. In the Appalachians, calcite comprises less than 1% of the clastic rocks, and a 10% volume-loss of this calcite may occur during circulation of connate or dehydration water derived from dewatering of the shales but in an environment that restricts the circulation of meteoric water. Here, the volume of calcite removed (0.1% of the total rock) is so small that circulation of meteoric water is not necessary for strain by pressure solution.