Shock recovery experiments were carried out on Westerly granite and Hospital Hill quartzite targets in the peak pressure range 8 to 25 GPa, preshock temperatures of 25 ø, 450 ø, and 750øC and pulse durations of 2 to 7 gs using internally heated momentum traps and explosive plane wave generators. Optical and transmission electron microscopy analyses of quartz and feldspar shocked at 25øC revealed the previously documented progression, with increasing pressure: (1) fracturing; (2) planar fractures and shock mosaicism; (3) shock mosaicism and planar deformation features (PDFs); and (4) isotropization. This same sequence is observed for experiments at elevated preshock temperature but with specific microstructures occurring at lower pressures than those in previous experiments at room temperature. At 750øC, strong shock mosaicism, partially thermally recovered, is characteristic of feldspar shocked to 8 GPa, whereas 15 GPa is required for its development in quartz and for the generation of PDFs in both minerals. The results suggest that threshold pressures for formation of the various microstructures and phases are expected to vary systematically as a function of the preshock temperature of the target material. We suggest that PDFs are generated in the shock transition by progressive, heterogeneous, phase transformation of the crystal structure to form dense glass or high pressure polymorphs. The onset pressures for PDFs in specific crystallographic orientations is not influenced strongly by temperature, but the character of the PDFs does change as preshock temperature is increased at the same peak shock stress. The change from multiple sets of thin PDFs at low temperature to thicker single sets of PDFs at moderate temperature, and finally to complete isotropization at high tempera tures, reflects a change in the phase transformation mechanism as a function of temperature. In contrast, development of shock mosaicism in quartz and feldspar occurs throughout the duration of shock loading and is better developed at elevated temperatures where the kinetics are enhanced by the additional thermal energy in the target. crostructures in known volcanic rocks, diatremes, and landslides [Carter et al., 1990; Officer and Carter, 1991] and the apparent differences between these features and classic impactinduced microstructures [Alexopoulos eta/., 1988] indicates that the endogenous microstructures, if of a legitimate shock origin, must be generated in an environment that is quite different from impact. Microstructural data from recent shock recovery experiments on Hospital Hill quartzite and Westerly granite [Reimold and Hiirz, 1986a, b; Huffman et al., 1990] and single crystal quartz [Langenhorst et al., 1992; Gratz et al., ] indicate that preshock temperature may be of fundamental importance to the character of high-strain-rate microstructures at specific peak shock pressures. These observations then raise the question as to what other parameters might influence microstructural development in the dynamic range. Temperature, st...
The art and science of pore pressure prediction has improved dramatically over the last 10 years with major advances in the areas of imaging and velocity analysis and in the theory of pore pressure in clastic basins. This paper discusses the current state of pore pressure theory for clastic basins and also addresses some of the developments that are occurring and will occur in the next ten years that will impact this critical technology area. Introduction Pre-drill pressure prediction using geophysical data and methods has historically been done using very simple models and overly simplistic estimates of the Earth's velocity field. The methods usually incorporate a locally calibrated set of curves for pressure that contained imbedded assumptions about the cause of pressure in the geological section sampled by the control wells. The advent of the effective stress concept and the pressure prediction methods that developed from that concept led to a much-needed inclusion of fundamental physics into the art of pressure prediction. The use of effective stress methods has become the standard for pressure prediction with many variants including the Eaton method, the Bowers method1, and the Sperry Sun method to name a few. The range of software available for pressure prediction has grown significantly in recent years, along with the sophistication of the parameters used. Still, weaknesses remain due to (1) the limitations of the seismic velocities themselves, (2) the lack of understanding of the basic causes of pressure and (3) the effects of pressure on physical properties, including velocity, density and porosity, of the rocks. The level of sophistication that is used in pressure prediction has improved steadily over the last few years, and the future looks even more promising. This paper will discuss some of critical challenges facing pore pressure prediction and some of the solutions that are on the horizon. Effective Stress And Loading Path Dependency Of Pressure One way to think about abnormal pressure is to recognize that the velocity of any rock in the subsurface is a direct function of its depositional and burial history. Fig. 1 shows a hypothetical loading path for a rock in a clastic basin in porosityvelocity-effective stress space. This diagram demonstrates the interplay between velocity, porosity and effective stress during the burial and loading process. The loading path starts at an effective stress of zero, and the velocity increases and porosity decreases until the material changes over from a Wood's Equation material to a frame-bearing clastic rock that can support an effective stress on the grains. The Wood's Equation portion of the loading path occurs as the material is initially deposited and compacted near the surface. Once the critical porosity is reached, the material follows the primary compaction curve, achieving either a compacted or undercompacted state. If allowed to compact normally with fluid draining out of the pore spaces, a rock will continue up the normal loading path and velocity will increase and porosity will decrease.
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