Acoustic core logging in blast-damaged rock

Thill, R.E. and D' Andrea, D. V., 1975. Acoustic core logging in blast-damaged rock. Eng. Geol., 10: 13–36.The Bureau of Mines, in cooperation with the Duval Corp., conducted a blast-fragmentation experiment to determine the feasibility of preparing a porphyry copper-molybdenum deposit for in-situ leaching. The blast was designed with ten 9-inch-diameter blastholes to depths of 110 feet in an equilateral triangle configuration; spacings between blastholes were 15, 20, and 25 ft. One of the major problems in the experiment was in assessing blast damage. Acoustic core-logging equipment and methods were devised and used as one approach in solving this problem. Ultrasonic pulse travel-times were determined in four diametral directions at 2-ft intervals of depth to a final depth of 120 ft in three preblast and six postblast drill cores at the Duval test site. The acoustic logging program provided compressional wave travel-time at 0°, 45°, 90°, and 135° around the core circumference, maximum travel-time difference, mean compressional-wave velocity, and an anisotropy factor. Other acoustic parameters introduced in the analyses were stiffness modulus, seismic quality designation (SQD), and a compensated velocity to account for portions of the core that were nonrecoverable or too highly fractured to permit diametral travel-time measurements.The acoustic parameters all indicated the deterioration in structural quality from the preblast condition, in which the rock already was badly fractured and weathered, to the more highly fractured postblast condition. Because of the highly fragmented, poor structural condition of the rock after blasting, the rock was indicated to be suitable for in-situ leaching, at least at the 20- and 15-ft blasthole spacings, and even in some zones in the rock at the 25-ft blasthole spacing.     

Interpretation of single-well tracer tests using fractional-flow dimensions. Part 1: Theory and mathematical models

Generalized equations using fractional-flow dimensions were derived to estimate the Darcy and seepage velocities obtained from the point-dilution and the single-well injection-withdrawal field tests conducted in fractured-rock aquifers. Seepage velocities can only be estimated from single-well tests if the hydraulic conductivity and the hydraulic gradient are known a priori. However, if a radial-convergent test is also performed between two boreholes, the kinematic porosity can be estimated and be used to estimate the seepage velocity from the single-well test results. To apply the generalized equations, the flow dimension and the extent of the flow region must be known. Therefore, the generalized radial flow (GRF) model of Barker (1988; a generalized radial flow model for hydraulic tests in fractured rock. Water Resour Res 24(10):1796–1804) is used to estimate the flow dimension because of its wide range of applications. A pumping test performed on the boreholes will yield an estimate of the fractional-flow dimension by applying the GRF model. Electronic Publication     

The ups and downs of “Tectonic Quiescence”—recognizing differential subsidence in the epicontinental sea of the Oxfordian in the Swiss Jura Mountains

Recent studies in northern Switzerland have shown that epicontinental areas thought to have been tectonically stable during the Mesozoic were not necessarily as rigid as presumed. By comparing Oxfordian facies boundaries and depocenters in their palinspastic position with known faults in the basement, a direct relationship between the two can be demonstrated. Previously, the lack of obvious synsedimentary tectonic features has lulled scientists into believing that the realm of the Swiss Jura was tectonically stable during the Mesozoic. However, it can be shown that facies and sedimentary structures are largely influenced by tectonics. Subsurface data provide evidence for the presence of Paleozoic troughs in the basement which, apparently, were prone to reactivation during the Pan-European stress-field reorganization taking place in the Late Jurassic. This led to differential subsidence along pre-existing lineaments within the study area, which can be recognized in the distribution of Oxfordian epicontinental basins and their coeval shallow-water counterparts. Eustatic sea-level fluctuations played an important role in the development of shallow-water facies patterns, but a subordinate role in the control of accommodation space in basins.

While tectonic activity is often recorded in the sedimentary record in the form of platform break-ups and associated sedimentary debris, more subtle indicators may be overlooked or even misinterpreted. Sedimentary structures and isopach maps, as well as subsurface data in the study area suggest that subtle synsedimentary tectonic movements led to the formation of two shallow, diachronous epicontinental basins during the Late Jurassic. It becomes possible to recognize and differentiate the combined effects of local and regional tectonism, eustasy and sedimentation.     

Rates of deformation of an extensional growth fault/raft system (offshore Congo, West African margin) from combined accommodation measurements and 3-D restoration

The aim of this paper is to quantify the evolution in time and space of the accommodation (space available for sedimentation) in the case of a growth fault structure resulting from gravity‐induced extension comprising a listric fault/raft system located along the West African margin. To achieve this, use was made of an original approach combining two complementary techniques (accommodation variation measurements and 3‐D restoration) in order to quantify vertical and horizontal displacement related to deformation, using a data set made up of a 3‐D seismic survey and well logs. We applied sequence stratigraphic principles to (i) define a detailed stratigraphic framework for the Albo‐Cenomanian and (ii) measure subsidence rates from accommodation variations. 3‐D restoration was used to (iii) reconstruct the evolution of the 3‐D geometry of the fault system. The rates of horizontal displacement of structural units were measured and linked to successive stages in the growth of the fault system. Subsidence of the structural units exhibits three scales of variation: (1) long‐term variation (10 Ma) of c. 80 m Ma^?1 for a total subsidence of about 1400 m, compatible with the general subsidence of a passive margin, and (2) short‐term variations (1–5 Ma) corresponding to two periods of rapid subsidence (about 150–250 m Ma^?1) alternating with periods of moderate subsidence rate (around 30 m Ma^?1). These variations are linked to the development of the fault system during the Albian (with downbuilding of the raft and development of the initial basin located in between). During the Cenomanian, the development of the graben located between the lower raft and the initial basin did not seem to affect the vertical displacements. (3) High‐frequency variations (at the scale of genetic unit sets) range between ?50 and 250 m for periods of 0.2–2 Ma. Accommodation variations governed these cycles of progradation/retrogradation rather than sediment flux variations. In addition, the nine wells display a highly consistent pattern of variation in accommodation. This suggests that the genetic unit sets were controlled at a larger scale than the studied system (larger than 20 km in wavelength), for example, by eustatic variations. Translation rates are between 3 and 30 times higher than subsidence rates. Therefore, in terms of amplitude, the main parameter controlling the space available for sedimentation is the structural development of the fault system, that is to say, the seaward translation of the raft units, itself resulting from a regional gravity‐driven extension.     

Formation of silica oncoids around geysers and hot springs at El Tatio, northern Chile

Siliceous oncoids, up to 4 cm in diameter, are common on the laterally extensive sinter aprons that surround the spectacular geysers and hot springs at El Tatio in northern Chile. Many of these complex oncoids developed close to geyser and spring vents that discharge boiling water. Internally the oncoids, which are composed of precipitated amorphous silica, are formed of complex arrays of spicules and concentric laminae as well as detrital volcanic grains. Spicular growth is dominant in most examples. The formation and growth of the spicules and concentric laminae were mediated by a microbial community which included filamentous microbes, mucus, and possibly bacteria. The microbes and mucus were silicified by replacement and encrustation. In some laminae the filamentous microbes lay parallel to the growth surface; in other laminae most filaments forming the thin mats were suberect. Amorphous silica precipitated between the filaments occluded porosity and commonly disguised the microbial fabric. The oncoids grew on the proximal sinter aprons around the geyser vents and hot spring pools. Most growth took place subaerially with the silica delivered to the precipitation sites by splashing water from the geysers and/or periodic shallow flooding of the discharge aprons. Unlike silica oncoids at other geothermal sites, vertical growth of oncoids that formed in some rimstone pools was not limited by water depth.