Acoustic Televiewer Research Articles

Logging the Chalk by Rory N. Mortimore. Whittles Publishing, Caithness, Scotland, 2014. No. of pages: x + 357. Price: UK£135.00. ISBN 978‐184995‐098‐5 (hardback)

When this hefty book fell on my doormat, my first thought was that it could not have been written by a better man. Rory Mortimore succeeds in presenting a splendid overview of decades of his research into the Chalk (mid- and Late Cretaceous), with the focus on the British Chalk (mostly onshore, but also offshore). However, there are comparisons with Upper Cretaceous strata across mainland Europe (mostly France, plus northern Germany and the Maastricht area in the Netherlands). During 30 years, Mortimore has described and interpreted over 3000 chalk cores and visited hundreds of temporary outcrops – ample experience, I would say! This is a comprehensive guide to the Chalk, describing this rock type in detail for the benefit of geologists, (geotechnical) engineers and, also, collectors of fossils. Chalk in the field is of prime importance – things can be seen out there and information gained will help in interpreting cores (best done in bright light, preferably sunlight!). Core loss (around flint levels), nodular chalks and fractures are all discussed. In fact, because of the sheer wealth of detail, some chapters may become very dense and difficult to read. Recent data, obtained in close co-operation with engineers, hydrogeologists and reservoir geologists, have resulted in a much better picture of Chalk. Throughout the book the standard lithostratigraphy (it took over 25 years to reach consensus over this) is used and, where appropriate, CIRIA (Construction Industry Research and Information Association) classification is supplied as well. There are seven chapters and two appendices (three more can be downloaded from the publisher's website). The first describes, classifies and interprets the Chalk, mainly for engineering purposes. Special attention is paid to sheet flints (often missed in cores) and trace fossils, in particular Zoophycos, as well as other features essential in core interpretation. Several examples of macrofossils (cephalopods, inoceramid bivalves, echinoderms) that can aid in logging are provided. Numerous full-colour pictures, with clear labelling (sedimentary features, lithostratigraphic interpretations) are supplied in all chapters. All logs are neat, with colour coding to assist in correlative schemes. Chapters 2–4 discuss the stratigraphic framework for the southern, transitional and northern Chalk ‘provinces’, respectively. In Chapter 5 we get our feet wet, entering the Central Graben and North Sea chalks. Correlative schemes with German, Danish and Dutch chalk sections are included. Chapter 6 focuses on weathering styles in cores and in the field (CIRIA grades; see also Appendix 1), with examples of groundwater staining, karst phenomena, dissolution pipes and relic pipes. The final chapter looks to the future, discussing the impact of optical and acoustic televiewers, and gamma and resistivity logs on Chalk studies – it concludes with several recommendations. Appendix 2 will be very useful in describing flints during logging. Strangely enough there is no reference here to work carried out by the late Felder brothers between 1970 and 2005 near Maastricht, nor to Hans Zijlstra's studies of the Upper Cretaceous–lowermost Paleocene of mainland Europe. Editorial control appears to have been tight, but some taxon names are spelt wrongly, both in the text and in figures; other misspellings refer to non-English names. In summary, this book deserves a wide readership. In fact, every Chalk worker should have a copy. I would certainly have benefitted from it when interpreting the >25 cores of the A2 motorway tunnel at Maastricht several years ago.

Stress Analysis in Boreholes Drag Bh and Leknes Bh, Nordland, North Norway

Nordland in northern Norway is characterized by enhanced seismicity and uplift that makes it the most tectonically active area in Norway. This study is part of a project entitled Neotectonics in Norway—Implications for Petroleum Exploration, which aims at enhancing the understanding of regional-scale stress and strain dynamics in Nordland, and to impact risk and hazard assessment and petroleum exploration. This paper attempts to constrain the orientation of in situ horizontal stress using high-resolution acoustic televiewer logging data. The Geological Survey of Norway has drilled two 0.8 km deep near-vertical boreholes on opposite sides of the Vestfjord in Nordland, the open bight of sea that separates the Lofoten archipelago from the Norwegian mainland. Both boreholes are drilled just North of 68°N, with borehole Leknes Bh located near the geographic center of the Lofoten archipelago, and borehole Drag Bh located on approximate equal distance from the shore, on the Norwegian mainland. The results of this study are in most practical aspects inconclusive, mainly due to poor data quality. The data analysis has revealed erroneously high-borehole diameter, and several artifacts such as eccentric logging tool, rugose borehole wall, spiral hole, tool sticking and missing data. Four intervals with passive in situ stress indicators (borehole breakout and drilling-induced fractures) were found in travel time and amplitude images of the Drag Bh, suggesting approximately N–S orientation of maximum horizontal stress. However, these intervals are not found in cross-plots. Either result yields the lowest World Stress Map ranking quality (E).

Enhanced detection of hydraulically active fractures by temperature profiling in lined heated bedrock boreholes

Summary The effectiveness of borehole profiling using a temperature probe for identifying hydraulically active fractures in rock has improved due to the combination of two advances: improved temperature sensors, with resolution on the order of 0.001 °C, and temperature profiling within water inflated flexible impermeable liners used to temporarily seal boreholes from hydraulic cross-connection. The open-hole cross-connection effects dissipate after inflation, so that both the groundwater flow regime and the temperature distribution return to the ambient (background) condition. This paper introduces a third advancement: the use of an electrical heating cable that quickly increases the temperature of the entire static water column within the lined hole and thus places the entire borehole and its immediate vicinity into thermal disequilibrium with the broader rock mass. After heating for 4–6 h, profiling is conducted several times over a 24 h period as the temperature returns to background conditions. This procedure, referred to as the Active Line Source (ALS) method, offers two key improvements over prior methods. First, there is no depth limit for detection of fractures with flow. Second, both identification and qualitative comparison of evidence for ambient groundwater flow in fractures is improved throughout the entire test interval. The benefits of the ALS method are demonstrated by comparing results from two boreholes tested to depths of 90 and 120 m in a dolostone aquifer used for municipal water supply and in which most groundwater flow occurs in fractures. Temperature logging in the lined holes shows many fractures in the heterothermic zone both with and without heating, but only the ALS method shows many hydraulically active fractures in the deeper homothermic portion of the hole. The identification of discrete groundwater flow at many depths is supported by additional evidence concerning fracture occurrence, including continuous core visual inspection, acoustic televiewer logs, and tests for hydraulic conductivity using straddle packers as well as rock core VOC data, where available, that show deep penetration and many migration pathways. Confidence in the use of temperature profiles and the conceptual model is provided by numerical simulation and the demonstrated reproducibility of the evolution of the temperature signal measured in the lined holes with and without heating. This approach for using temperature profiling in lined holes with heating is a practical advance in fractured rock hydrogeology because the liners are readily available, the equipment needed for heating is low cost and rugged, and the time needed to obtain the profiles is not excessive for most projects.

공기-브러쉬와 폭약 세척 방법에 의한 암반관정의 세척 효과 검증

The application of appropriate rehabilitation works can improve productivity and water quality of clogged wells, with extending the operation of the wells for a certain period. This study verified rehabilitation effect of the clogged wells by means of explosive method and air brush surging and by using hydraulic tests, water quality, and geophysical logs (optical and acoustic televiewer logs) before and after the cleaning works on the two wells drilled in bedrock in Mt. Geumjeong, Busan City. Air-brush surging method resulted in maximum 273% increase of the productivity with the concentration of , , and a decrease of turbidity and the concentration of , . The explosive method resulted in maximum 156% increase of the productivity with an increase of the concentration of , and a decrease of turbidity, , .

시추 및 물리검층자료의 상관해석을 통한 청주화강암체의 물성 정보 및 절리 분포

To clarify the distribution of joints and fracture zones in the Cheongju granitic mass, we analyzed drill-core and geophysical well-logging data obtained at two boreholes located 30 m from each other. Lithological properties were investigated from the drill-core data and the samples were classified based on the rock mass rating (RMR) and on rock quality designation (RQD). Subsurface discontinuities within soft and hard rocks were examined by geophysical well-logging and cross-hole seismic tomography. The velocity structures constructed from seismic tomography are well correlated with the profile of bedrock depth, previously mapped from a seismic refraction survey. Dynamic elastic moduli, obtained from full waveform sonic and logging, were interrelated with P-wave velocities to investigate the dynamic properties of the rock mass. Compared with the correlation graph between elastic moduli and velocities for hard rock at borehole BH-1, the correlation points for BH-2 data showed a wide scatter. These scattered points reflect the greater abundance of joints and fractures near borehole BH-2. This interpretation is supported by observations by acoustic televiewer (ATV) and optical televiewer (OTV) image loggings.

Application of borehole imagery in iron ore: examples from Mount Whaleback, Western Australia

The borehole televiewer is a down-hole geophysical logging device that produces continuous, oriented, 360° images of the drill hole wall. Two types of borehole images are currently being generated at the Mount Whaleback iron ore deposit in Western Australia. The optical televiewer (OTV) tool captures a 'real' image of the borehole wall whereas an acoustic televiewer (ATV) generates an image through variations in physical borehole size, scattering the energy from an acoustic beam. Orientation of the resulting 'virtual core' is derived through magnetometers and accelerometers, providing values within one degree for both inclination and azimuth. Down-hole structures such as fractures, joints, foliations and bedding planes are selected directly off the borehole image, recorded and displayed in a three-dimensional modelling package alongside adjacent drill holes. Measurements are corrected from apparent to true dip and magnetic to true north. Strata-bound magnetic anomalies are identified and corresponding measurements excluded. The importance of borehole imaging increases substantially with its ability to integrate the OTV and/or ATV data with other forms of lithological and structural data. At Mount Whaleback, the borehole televiewer has been implemented to provide information for geotechnical exploration including structural analysis, lithological characterisation and rock-mass conditions. Within the large area in and around Mount Whaleback, drilling may intersect a number of different geological formations with various rock types and conditions. With the application of the borehole televiewer, reverse circulation drilling can provide information traditionally only obtained by diamond drilling.

An empirical model for estimating hydraulic conductivity of highly disturbed clastic sedimentary rocks in Taiwan

This paper presents the establishment of an empirical HC model for estimating rock mass hydraulic conductivity of highly disturbed clastic sedimentary rocks in Taiwan using high-resolution borehole acoustic televiewer and double packer hydraulic tests. Four geological parameters including rock quality designation (RQD), depth index (DI), gouge content designation (GCD), and lithology permeability index (LPI) were adopted for establishing the empirical HC model. To verify rationality of the proposed HC model, 22 in-situ hydraulic tests were carried out to measure the hydraulic conductivity of the highly disturbed clastic sedimentary rocks in three boreholes at two different locations in Taiwan. Besides, the model verification using another borehole data with four additional in-situ hydraulic tests from similar clastic sedimentary rocks was also conducted to further verify the feasibility of the proposed empirical HC model. The field results indicated that the rock mass in the study area has a conductivity between the order of 10 − 10 m/s and 10 − 6 m/s at the depth between 34 m and 275 m below ground surface. Results demonstrate that the empirical HC model may provide a useful tool to predict hydraulic conductivity of the highly disturbed clastic sedimentary rocks in Taiwan based on measured HC-values.

Damage assessment for concrete structure using image processing techniques on acoustic borehole imagery

The deterioration of concrete structures is a growing problem worldwide. Drilled cores are usually collected from large dams for testing the concrete and rock foundation underneath to assess their safety. However, the borehole itself can be used to assess cracks and other damage and collect additional information on the surrounding materials. This paper evaluates various edge-detection algorithms, as well as transform and statistical-based methods, for their effectiveness in assessing damage in a concrete dam from digital borehole imagery obtained using an acoustic televiewer. The statistical-based approach was found to be the most efficient technique for damage assessment from acoustic imagery. A clustering technique was used to quantify damage from the imagery, such as vertical cracks, horizontal cracks, voids, stains, and foundation damage. Results were verified using log data. Further damage analysis consisted of determining minimum, maximum and mean crack-width openings.

Shaft sinking risk analysis through the integration of borehole radar and acoustic televiewer data in deep geotechnical boreholes

The 2.06 Ga Bushveld Igneous Complex of South Africa is the world?s largest layered igneous intrusion. Situated to the north of the Johannesburg, it extends ~450 km from east to west and ~300 km north to south (Simmat et al., 2006), and has a maximum thickness of ~8 km. It is estimated to contain over half the world?s Platinum Group Element (PGE) resources (Cawthorn, 1999). The two PGE-bearing chromitite layers in the Bushveld Complex, the Merensky Reef and the UG2, occur within pyroxenites of the Critical Zone, show high lateral continuity and commonly dip at relatively shallow angles (2°-20°) towards the centre of the complex. These are overlain by a succession of norite and gabbronorite formations of the Main Zone, representing a more mafic phase of the intrusion. The project area is the location of a planned new Anglo Platinum mine on the western limb of the Bushveld, south of the Pilanesberg Alkaline Complex, a later volcanic event. Major capital investment is required to sink the twin shafts, accessing the underground platinum reefs from ~700 m depth. Due to the proximity to the Pilanesberg, dykes and faults are pervasive throughout the deposit. Several dyke varieties occur and these differ in composition, texture and orientation, and consequently represent different geotechnical hazards. Dolerite dykes tend to be hard and competent with similar rock strength properties to that of the surrounding Bushveld rocks, and are not expected to represent significant geotechnical problems. Lamprophyre dykes, however, are often soft, weather easily when exposed and demonstrate significant contrast in rock strength properties to the Bushveld rocks. Shaft geotechnical boreholes were drilled along the two proposed shaft axes and were logged with a suite of geophysical tools to detect potentially hazardous structures that may influence shaft sinking operations. This paper examines the integration of borehole radar (BHR) and acoustic televiewer (ATV) data down the proposed main- and ventilation shaft positions.

Integration of borehole geophysical data in 2D and 3D to develop a hazard index

A borehole hazard index is the integration of interpreted risk indexes with an existing geological and structural 3D mine or exploration model. Individual risk indexes are produced and combined with the purpose of providing a clear visual and quantitative method for determining varying degrees of risk associated with development through a particular geological rock volume. Disparate data sets are used to characterize separate risk indexes established from user defined criteria. Input data sets include geological and structural logs and mine layouts, complemented by a borehole geophysical suite including borehole radar, optical & acoustic televiewers, density, neutron, flowmeter and full wave form sonic. The user defined criteria are established for individual project requirements and can include factors such as the intersection of structures, the presence of water ingress, proximity to structures with specific orientations and the presence of lithological units prone to failure. The requirements of the integration environment vary; certain criteria can be adequately assessed in a 2D environment while other hazard indexes require data to exist in a true topological 3D environment where spatial queries can be performed. The applications of a borehole hazard index include shaft site evaluations, and shaft sinking development planning. Additionally a hazard index can serve as a mine production tool, evaluating hazards in front of the face which will affect both safety and production rates. Successful deployment requires regular and timely update of the local structural model which can be achieved by automating the hazard index generation once the starting model has been defined.

Lithological and structural characteristics of the Lake Bosumtwi impact crater, Ghana: Interpretation of acoustic televiewer images

Abstract— Bosumtwi is a very well‐preserved 1.07 Myr old, complex terrestrial impact crater located in south‐central Ghana, West Africa. The impact structure has a diameter of about 10.5 km and was formed in 2.1–2.2 Gyr Precambrian metasedimentary and metavolcanic rocks. Drilling and logging was carried out during the Lake Bosumtwi Drilling Project (BCDP) which was supported by the International Continental Scientific Drilling Program (ICDP). One of the aims of this project is to achieve detailed information on the subsurface structure and crater fill of one of the best preserved large young impact structures.We interpreted the wireline logs and televiewer images. The physical properties including shallow resistivity, p‐wave velocity, magnetic susceptibility, and borehole diameter of the breccia differ significantly from those of the meta‐graywackes and slate/phyllites. Fractures observed in the televiewer images are interpreted to determine their characteristic structural features. The fracture dip angles are steep (50–70°) and the two main dip directions are southeast and southwest. Most fractures observed in the borehole are open. The indicated main stress direction is north‐south.

Relationship between hydraulic conductivity and fracture properties estimated from packer tests and borehole data in a fractured granite

Hydraulic conductivity is closely related to fracture characteristics like fracture aperture and frequency, fracture length, fracture orientation and angle, fracture interconnectivity, filling materials, and fracture plane features. In this study, water injection tests were conducted at six boreholes of different depths drilled in fractured granite in the Mt. Geumjeong area, Korea. Hydraulic conductivity was calculated using the USBR (United States Bureau of Reclamation) water injection test method. Hydraulic conductivity was related to fracture frequency, squared fracture aperture, and the squared aperture of major fracture orientation obtained from acoustic televiewer and core log data. Fracture aperture had a stronger relationship to hydraulic conductivity than fracture frequency did. In addition, the correlation between transmissivity and cubed fracture aperture was higher than the correlation between hydraulic conductivity and the squared fracture aperture. The squared aperture with respect to major fracture orientation had a weaker correlation with hydraulic conductivity than the squared aperture using all fracture orientations. This suggests greater complexity for groundwater flow at the borehole scale compared to the regional scale. Meanwhile, the correlation coefficient between hydraulic conductivity and fracture frequency obtained from acoustic televiewer data was higher than that from core log data.

Structure and stress state of Hawaiian island basalts penetrated by the Hawaii Scientific Drilling Project deep core hole

As part of the Hawaii Scientific Drilling Project (HSDP), an exploratory hole was drilled in 1993 to a depth of 1056 meters below sea level (mbsl) and a deeper hole was drilled to 3098 mbsl in 1999. A set of geophysical well logs was obtained in the deeper hole that provides fundamental information regarding the structure and the state of stress that exist within a volcanic shield. The acoustic televiewer generates digital, magnetically oriented images of the borehole wall, and inspection of this log yields a continuous record of fracture orientation with depth and also with age to 540 ka. The data depict a clockwise rotation in fracture strike through the surficial Mauna Loa basalts that settles to a constant heading in the underlying Mauna Kea rocks. This behavior reflects the depositional slope directions of lavas and the locations of volcanic sources relative to the drill site. The deviation log delineates the trajectory of the well bore in three‐dimensional space. This path closely follows changes in fracture orientation with depth as the drill bit is generally prodded perpendicular to fracture strike during the drilling process. Stress‐induced breakouts observed in the televiewer log identify the orientations of the maximum and minimum horizontal principal stresses to be north‐south and east‐west, respectively. This stress state is attributed to the combination of a sharp break in onshore‐offshore slope that reduces stress east‐west and the emergence of Kilauea that increases stress north‐south. Breakouts are extensive and appear over approximately 30% of the open hole.

New developments in real-time processing of full waveform acoustic televiewer data

The new full wave Acoustic Borehole Imager tool (ABI) is a multi-echo (amplitude and traveltime) system that gives optimum performance under a wide range of borehole conditions and is an improvement over existing single echo acoustic televiewer tools. The principle of the multi-echo system is the digital recording of each reflected acoustic wave train. Then, real-time processing of the acoustic data is made by a downhole Digital Signal Processor (DSP) to extract all valuable information, which is further compressed before transmission to the surface such that no special requirements are imposed on data transmission rates or on logging speed. Also, recording of the full acoustic waveform enables major improvements to the dynamic range of the system. Information about echoes from the tool's acoustic window provides the possibility to predict tool generated coherent noise and allows detection of echoes from the borehole wall, which are much smaller than coherent noise signals. When used in a PVC cased borehole, the system can be automatically adapted to record both the reflection at the casing and at the borehole wall. When used inside steel casing, the tool can detect echoes from the inner wall and the outer wall of the steel casing and the resulting data can used to calculate amount inner corrosion, outer corrosion, and remaining casing thickness. If the borehole casing was grouted with cement, information can be gathered about the presence and absence of cement and the quality of cement bonding.

Topographic stress perturbations in southern Davis Mountains, west Texas 1. Polarity reversal of principal stresses

We have applied a previously developed analytical stress model to interpret subsurface stress conditions inferred from acoustic televiewer logs obtained in two municipal water wells located in a valley in the southern Davis Mountains near Alpine, Texas. The appearance of stress‐induced breakouts with orientations that shift by 90° at two different depths in one of the wells is explained by results from exact solutions for the effects of valleys on gravity and tectonically induced subsurface stresses. The theoretical results demonstrate that above a reference depth termed the hinge point, a location that is dependent on Poisson's ratio, valley shape, and magnitude of the maximum horizontal tectonic stress normal to the long axis of the valley, horizontal stresses parallel to the valley axis are greater than those normal to it. At depths below this hinge point the situation reverses and horizontal stresses normal to the valley axis are greater than those parallel to it. Application of the theoretical model at Alpine is accommodated by the fact that nearby earthquake focal mechanisms establish an extensional stress regime with the regional maximum horizontal principal stress aligned perpendicular to the valley axis. We conclude that the localized stress field associated with a valley setting can be highly variable and that breakouts need to be examined in this context when estimating the orientations and magnitudes of regional principal stresses.

Groundwater resources evaluation case study via discrete fracture flow modeling

The Arizona Department of Transportation (ADOT) is preparing to upgrade State Route 260 between Payson and Heber. It is estimated that a total of about one million cubic meters of water will be required for embankment construction during a period of about 84 months to upgrade the first 33.8 km of the highway. ADOT is investigating various sources of construction water for use in the highway improvement project, including groundwater resources along the highway corridor. A region known as the RV site, underlain by fractured granite, is located 12.9 km east of Payson. The site includes three springs, a creek and several wells. Several boreholes and observation wells were made to a maximum depth of 157 m to obtain fracture data and to conduct pumping tests with monitoring. Fracture data recorded by acoustic televiewer logs were used to build a fracture network model for the rock mass. Results of a 24-hour and a 7-day pumping tests were used to calibrate hydraulic parameters of a finite element discrete fracture fluid flow model considering the region as a heterogeneous, anisotropic, fractured medium. A 38-day multi-well pumping test was used to validate the calibrated numerical model. The calibrated model showed the capability to provide reasonably accurate predictions for new pumping tests conducted in the same well field. The validated model was then used to simulate pumping exceeding a 7 year period under different scenarios incorporating different sets of boundary conditions and different pumping rates at multi-wells, with and without recharge, to evaluate the yield of the aquifer and to assess the effect of long-term pumping on the environment. The results indicated that (a) the combined yield of the wells in the RV site is sufficient to meet the water demand for the ADOT highway project and (b) the water levels in the well field would decline between 3.0 and 7.6 m after one year of pumping and by 12.2 to over 30.5 m during the life of the project.

Fractured‐Aquifer Hydrogeology from Geophysical Logs: Brunswick Group and Lockatong Formation, Pennsylvania

AbstractThe Brunswick Group and the underlying Lockatong Formation are composed of lithified Mesozoic sediments that constitute part of the Newark Basin in southeastern Pennsylvania. These fractured rocks form an important regional aquifer that consists of gradational sequences of shale, siltstone, and sandstone, with fluid transport occurring primarily in fractures. An extensive suite of geophysical logs was obtained in seven wells located at the borough of Lansdale, Pennsylvania, in order to better characterize the areal hydrogeologic system and provide guidelines for the refinement of numerical ground water models. Six of the seven wells are approximately 120 m deep and the seventh extends to a depth of 335 m. Temperature, fluid conductivity, and flowmeter logs are used to locate zones of fluid exchange and to quantify transmissivities. Electrical resistivity and natural gamma logs together yield detailed stratigraphic information, and digital acoustic televiewer data provide magnetically oriented images of the borehole wall from which almost 900 fractures are identified.Analyses of the geophysical data indicate that the aquifer penetrated by the deep well can be separated into two distinct structural domains, which may, in turn, reflect different mechanical responses to basin extension by different sedimentary units:1. In the shallow zone (above 125 m), the dominant fracture population consists of gently dipping bedding plane partings that strike N46°E and dip to the northwest at about 11 degrees. Fluid flow is concentrated in the upper 80 m along these subhorizontal fractures, with transmissivities rapidly diminishing in magnitude with depth.2. The zone below 125 m marks the appearance of numerous high‐angle fractures that are orthogonal to the bedding planes, striking parallel but dipping steeply southeast at 77 degrees.This secondary set of fractures is associated with a fairly thick (approximately 60 m) high‐resistivity, low‐transmissivity sandstone unit that is abruptly terminated by a thin shale bed at a depth of 190 m. This lower contact effectively delineates the aquifer's vertical extent at this location because no detectable evidence of ground water movement is found below it. Thus, fluid flow is controlled by fractures, but fracture type and orientation are related to lithology. Finally, a transient thermal‐conduction model is successfully applied to simulate observed temperature logs, thereby confirming the effects of ground‐surface warming that occurred in the area as a result of urbanization at the turn of the century. The systematic warming of the upper 120 m has increased the transmissivity of this aquifer by almost 10%, simply due to changes in fluid viscosity and density.

Acoustic televiewer logging in glacier boreholes

AbstractThe acoustic televiewer is a geophysical logging instrument that is deployed in a water-filled borehole and operated while trolling. It generates a digital, magnetically oriented image of the borehole wall that is developed from the amplitudes and transit times of acoustic waves emitted from the tool and reflected at the water–wall interface. The transit-time data are also converted to radial distances, from which cross-sectional views of the borehole shape can be constructed. Because the televiewer is equipped with both a three-component magnetometer and a two-component inclinometer, the borehole’s trajectory in space is continuously recorded as well. This instrument is routinely used in mining and hydrogeologic applications, but in this investigation it was deployed in two boreholes drilled into Upper Fremont Glacier, Wyoming, U.S.A. The acoustic images recorded in this glacial setting are not as clear as those typically obtained in rocks, due to a lower reflection coefficient for water and ice than for water and rock. Results indicate that the depth and orientation of features intersecting the boreholes can be determined, but that interpreting their physical nature is problematic and requires corroborating information from inspection of cores. Nevertheless, these data can provide some insight into englacial structural characteristics. Additional information derived from the cross-sectional geometry of the borehole, as well as from its trajectory, may also be useful in studies concerned with stress patterns and deformation processes.

Analysis of stress‐induced oval fractures in a borehole at Deep Sea Drilling Project Site 504, eastern equatorial Pacific

Deep Sea Drilling Project (DSDP) Hole 504B is located in the eastern equatorial Pacific Ocean and extends to a total depth of 2111 m beneath the seafloor (mbsf). Several acoustic televiewer logs have been obtained in this well during successive stages of drilling, and the resulting digital images have revealed numerous oval‐shaped fractures seemingly etched into the borehole wall. A theoretical examination of these stress‐induced features identifies a unique and ephemeral set of stress distributions and magnitudes that are necessary for their production. Consequently, the ovals provide a basis for quantifying the magnitudes and orientations of the maximum and minimum horizontal principal stresses, SH and Sh at this site. Vertical, truncated breakouts and horizontal tensile fractures define the spatial boundaries of the ovals. Explicit criteria for their occurrence are combined with estimates for various physical properties of the rock to yield a range of possible values for the horizontal principal stresses. The conspicuous oval geometry is completed by a curved fracture that joins the vertical and horizontal components. Its degree of curvature is delineated by the modified Griffith failure criterion and is directly related to the principal stress difference (SH − Sh). Matching a series of type curves corresponding to specific values for (SH − Sh) with the actual undistorted well bore images allows the magnitude of the stress difference to be further constrained. With a value for (SH − Sh) of 45 ± 5 MPa the individual magnitudes of SH and Sh are determined more precisely. Final estimates for the horizontal principal stresses in DSDP Hole 504B at a depth of 1200 mbsf are 141 MPa ≤ SH ≤ 149 MPa and 91 MPa ≤ Sh ≤ 109 MPa. Stress magnitudes derived from this approach rely heavily upon the values of a variety of physical properties, and complementary laboratory measurements performed on relevant rock samples provide critical information. Uncertainties in estimating these properties translate into less precise predictions of principal stresses.

Fractured‐Aquifer Hydrogeology from Geophysical Logs; The Passaic Formation, New Jersey

AbstractThe Passaic Formation consists of gradational sequences of mudstone, siltstone, and sandstone, and is a principal aquifer in central New Jersey. Ground‐water flow is primarily controlled by fractures interspersed throughout these sedimentary rocks and characterizing these fractures in terms of type, orientation, spatial distribution, frequency, and transmissivity is fundamental towards understanding local fluid‐transport processes. To obtain this information, a comprehensive suite of geophysical logs was collected in 10 wells roughly 46 m in depth and located within a .05 km2 area in Hopewell Township, New Jersey. A seemingly complex, heterogeneous network of fractures identified with an acoustic televiewer was statistically reduced to two principal subsets corresponding to two distinct fracture types: (1) bedding‐plane partings and (2) high‐angle fractures. Bedding‐plane partings are the most numerous and have an average strike of N84°W and dip of 20° N. The high‐angle fractures are oriented subparallel to these features, with an average strike of N79° E and dip of 71° S, making the two fracture types roughly orthogonal. Their intersections form linear features that also retain this approximately east‐west strike. Inspection of fluid temperature and conductance logs in conjunction with flow meter measurements obtained during pumping allows the transmissive fractures to be distinguished from the general fracture population. These results show that, within the resolution capabilities of the logging tools, approximately 51 (or 18 percent) of the 280 total fractures are water producing. The bedding‐plane partings exhibit transmissivities that average roughly 5 m2/day and that generally diminish in magnitude and frequency with depth. The high‐angle fractures have average transmissivities that are about half those of the bedding‐plane partings and show no apparent dependence upon depth. The geophysical logging results allow us to infer a distinct hydrogeologic structure within this aquifer that is defined by fracture type and orientation. Fluid flow near the surface is controlled primarily by the highly transmissive, subhorizontal bedding‐plane partings. As depth increases, the high‐angle fractures apparently become more dominant hydrologically.