Borehole Fracture Research Articles

Characterization of an unstable rock mass based on borehole logs and diverse borehole radar data

Unstable rocky slopes are major hazards to the growing number of people that live and travel though mountainous regions. To construct effective barriers to falling rock, it is necessary to know the positions, dimensions and shapes of structures along which failure may occur. To investigate an unstable mountain slope distinguished by numerous open fracture zones, we have taken advantage of three moderately deep (51.0–120.8 m) boreholes to acquire geophysical logs and record single-hole radar, vertical radar profiling (VRP) and crosshole radar data. We observed spallation zones, displacements and borehole radar velocity and amplitude anomalies at 16 of the 46 discontinuities identified in the borehole optical televiewer images. The results of the VRP and crosshole experiments were disappointing at our study site; the source of only one VRP reflection was determined and the crosshole velocity and amplitude tomograms were remarkably featureless. In contrast, much useful structural information was provided by the single-hole radar experiments. Radar reflections were recorded from many surface and borehole fracture zones, demonstrating that the strong electrical property contrasts of these features extended some distance into the adjacent rock mass. The single-hole radar data suggested possible connections between 6 surface and 4 borehole fractures and led to the discovery of 5 additional near-surface fracture zones. Of particular importance, they supplied key details on the subsurface geometries and minimum subsurface lengths of 8 of the 10 previously known surface fracture zones and all of the newly discovered ones. The vast majority of surface fracture zones extended at least 40–60 m into the subsurface, demonstrating that their depth and surface dimensions are comparable.

Numerical Study of Elastic-Brittle Failure of Notched Openings in Rocks

Borehole breakout is the process by which portions of borehole or tunnel wall fracture or spall when subjected to compressive stresses. The stress-strain characteristics of rock during loading and unloading confining pressure are studied firstly. To overcome the difficulties in analytical model studies, a numerical code, RFPA2D (Rock Failure Process Analysis), developed by CRISR, Northeastern University, China, is used to investigate the progressive failure of breakout around tunnel. The heterogeneity of rock was also taken into account in the software. The numerical simulation reproduces the formation notch in rocks by the growth, interaction and coalescence of randomly distributed macrocracks. It is illustrated from the numerical simulated results that breakout direction of tunnel is parallel with the minor stress tensor in the plane perpendicular to the borehole axis. Specifically due to the inclusion of heterogeneity, some peculiarities are studied both in the evolution of fracture and the influence of borehole on the peak intensity of specimen as well as the AE event patterns.

Seismoelectric and seismomagnetic measurements in fractured borehole models

Seismoelectric and seismomagnetic fields generated by seismic waves in fluid-saturated fractured borehole models are experimentally investigated with an electrode and a Hall-effect sensor. In a borehole with a horizontal fracture, the Stoneley and flexural waves induce seismoelectric and seismomagnetic fields on the borehole wall and an electromagnetic wave propagating at light speed at the horizontal fracture. In a borehole with a vertical fracture, the acoustic field generated by a monopole or dipole source is similar to that in a borehole without a vertical fracture. However, the acoustic wave propagating along the vertical fracture induces seismoelectric and seismomagnetic fields, whose apparent velocities are equal to that of a Stoneley wave. Experimental results show that two different kinds of electric and magnetic fields are generated by acoustic waves in borehole models with horizontal and/or vertical fractures. One is an electromagnetic wave propagating with light speed. The second is a stationary or localized seismoelectric and seismomagnetic field. Seismoelectric and seismomagnetic measurements might be a new logging technique for exploring fractures in a borehole.

In situ rock stress determinations in deep boreholes at the Underground Research Laboratory

Deep in situ rock stress determinations are typically conducted using hydraulic fracturing in an existing borehole or by applying specialized overcoring techniques that require the installation of a strain-monitoring instrument in a central pilot borehole that is subsequently overcored. The unusually high horizontal to vertical stress ratio at depth in Canada's Underground Research Laboratory (URL) results in problems in the application of either of these methods. At 420-m depth, the rock in the URL is massive granite with virtually no fractures. The maximum principal stress is approximately 60 MPa and is sub-horizontal. The minimum principal stress is only 11 MPa, resulting in a horizontal to vertical stress ratio of almost 6 to 1. In these conditions, hydraulic fracturing is unable to produce axial fractures in sub-vertical boreholes or is unable to fracture the rock at all. Overcoring methods requiring a pilot hole are not feasible because of persistent core discing. To resolve these problems, AECL has applied a remote data logger/signal conditioner to the design of a Deep Doorstopper Gauge System (DDGS) for use to 1000-m depth. The DDGS is much less susceptible to core discing than most other overcoring methods because it does not require a pilot hole. This paper describes the operating principle of the DDGS and the results of field testing at the URL, as well as issues related to its use and the interpretation of results.

Simulating Infiltration Tests in Fractured Basalt at the Box Canyon Site, Idaho

The results of a series of ponded infiltration tests in variably saturated fractured basalt at Box Canyon, Idaho were used to build confidence in conceptual and numerical modeling approaches used to simulate infiltration in fractured rock. Specifically, we constructed a dual-permeability model using TOUGH2 to represent both the matrix and fracture continua of the upper basalt flow at the Box Canyon Site. A consistent set of hydrogeological parameters was obtained by calibrating the model to infiltration front arrival times in the fracture continuum as inferred from Br− samples collected from fracture–borehole intersections observed during the infiltrating tests. These parameters included the permeability of the fracture and matrix continua, the interfacial area between the fracture and matrix continua, and the porosity of the fracture continuum. To calibrate the model, we multiplied the fracture–matrix interfacial area by a factor between 0.1 and 0.01 to reduce imbibition of water from the fracture continuum into the matrix continuum during the infiltration tests. Furthermore, the porosity of the fracture continuum, as calculated using the fracture aperture inferred from pneumatic-test permeabilities, was increased by a factor of 50 yielding porosity values for the upper basalt flow in the range of 0.01 to 0.02. The fracture-continuum porosity was a highly sensitive parameter controlling the arrival times of the simulated infiltration fronts. Porosity values are consistent with those determined during the Large-Scale Aquifer Pumping and Infiltration Test at the Idaho National Engineering and Environmental Laboratory.

Simulating Infiltration Tests in Fractured Basalt at the Box Canyon Site, Idaho

The results of a series of ponded infiltration tests in variably saturated fractured basalt at Box Canyon, Idaho were used to build confidence in conceptual and numerical modeling approaches used to simulate infiltration in fractured rock. Specifically, we constructed a dual‐permeability model using TOUGH2 to represent both the matrix and fracture continua of the upper basalt flow at the Box Canyon Site. A consistent set of hydrogeological parameters was obtained by calibrating the model to infiltration front arrival times in the fracture continuum as inferred from Br− samples collected from fracture–borehole intersections observed during the infiltrating tests. These parameters included the permeability of the fracture and matrix continua, the interfacial area between the fracture and matrix continua, and the porosity of the fracture continuum. To calibrate the model, we multiplied the fracture–matrix interfacial area by a factor between 0.1 and 0.01 to reduce imbibition of water from the fracture continuum into the matrix continuum during the infiltration tests. Furthermore, the porosity of the fracture continuum, as calculated using the fracture aperture inferred from pneumatic‐test permeabilities, was increased by a factor of 50 yielding porosity values for the upper basalt flow in the range of 0.01 to 0.02. The fracture‐continuum porosity was a highly sensitive parameter controlling the arrival times of the simulated infiltration fronts. Porosity values are consistent with those determined during the Large‐Scale Aquifer Pumping and Infiltration Test at the Idaho National Engineering and Environmental Laboratory.

A Further Study on the Mechanism of Airdecking

Airdecking is used in mining for two quite different applications. One is to enhance the fragmentation by amplifying the induced fracturing and the second is for pre-split blasting in which the borehole fracturing is reduced. This paper deals with the first of these effects. A forth coming paper will describe pre-splitting by airdecking. The use of air decks to enhance rock fragmentation and so to reduce explosive costs has been the practice for quite long time. Although a number of studies has been conducted to verify the advantages of blasting with air decks and to investigate the mechanisms involved, the proposed mechanisms still cannot explain clearly the phenomena observed in practice and the design approach adopted for this kind of blasting is still primary based on rules-of-thumb. In this paper, the theory of shock tubes is adopted to (a) investigate the processes of the expanding detonation products, (b) study the interactions between the explosion products and the stemming or bottom of blasthole, and (c) to decide the distribution of the changing pressure of explosion products along blasthole. Numerical simulation and theoretical analyses are then performed to study the physical process of blasting with air decks. Finally, a reasonable value for the airdecking ratio is decided theoretically. It is shown that the pressure-unloading process caused by the propagation of the rarefaction wave and the reflected rarefaction waves in the detonation products plays an important role in the enhanced fragmentation of rock when blasting with air decks. The unloading process can induce tensile stresses of rather high magnitude in the rock mass surrounding blasthole. This favors fracturing of the rock. The reflected shock wave with a magnitude of gas pressure higher than that of the average detonation pressure in a fully charged blasthole acts as the main energy source to break the rock in the air deck and stemming portions. The second and succeeding strain waves induced by the unloading or reloading of the pressurewithin the blasthole also contribute to form the initial fracture network in the rock around the blasthole. It is also revealed that there exists a reasonable range of values for the airdecking ratio. For ANFO, this value varies from 0.13-0.40.

Crosshole seismoelectric measurements in borehole models with fractures

Laboratory experiments are performed in cross‐borehole models with fractures to investigate seismoelectric conversions in the fractures. Using an electrode in a borehole, we measure the electric field induced by an acoustic wave, and we investigate the relationship between the electric signal and the fracture aperture using ultrasonic fracture borehole models. The experimental results confirm that a radiating electromagnetic (EM) wave is induced by a guide wave at a fracture between an acoustic source and electric receiver boreholes. The position of a vertical or inclined fracture between two boreholes can be determined using the arrival times of the EM wave and the formation velocity by placing the acoustic source at a different depth and recording electric signals with real or synthetic arrays of acoustic and electric receivers in the second borehole. Crosshole seismoelectric measurements might be a technique for investigating fractures between boreholes more directly than traditional acoustic measurements.

Sampling bias of discontinuity orientation caused by linear sampling technique

Orientation data of discontinuities are of paramount importance for rock slope stability studies because they control the possibility of unstable conditions or excessive deformation. Most orientation data are collected using linear sampling techniques, such as borehole fracture mapping and the detailed scanline method (outcrop mapping). However, data acquired by such linear sampling techniques are subject to bias, owing to the orientation of the sampling line. Even if a weighting factor is applied to orientation data to reduce this problem, the bias will not be significantly reduced when certain sampling orientations are involved. If the linear sampling orientation nearly parallels the discontinuity orientation, most of these parallel discontinuities will be excluded from the survey results. This phenomenon can cause serious misinterpretation of discontinuity orientation data because critical information is omitted. In the case study, orientation data collected using the borehole fracture mapping method (vertical) were compared to those based on orientation data from the detailed scanline method (horizontal scanline). Differences in results for the two procedures revealed a concern that a representative orientation of discontinuities was not accomplished. Equal area, polar stereo nets were used to determine the distribution of dip angles and to compare the data distribution for the borehole method with that for the outcrop scanline.

Reverse-slip offsets and axial fractures in road-cut boreholes from the Caledonides in Finnmark, northern Norway: neotectonic stress orientation indicators

Reverse-slip offsets of road-cut drillholes, and axial fractures developed in the walls of many such holes, have been recorded from areas in the Porsangerfjord and Laksefjord districts of the Caledonides of Finnmark, northern Norway. Displacement vectors of the offset boreholes indicate movement towards 120–136° in phyllites of the Laksefjord Nappe. At the base of the Kalak Nappe and in the Gaissa Nappe in western Porsanger, reverse-slip offset is directed almost due east. Observations of the extensional axial fractures, which are considered to have developed normal to the least principal in situ stress, σ 3, at the time of blasting, support a c. ESE–WNW orientation of S Hmax at or close to the surface in these parts of Finnmark, possibly with a slightly more E–W trend in western areas. The consistency of orientation of these structural features is considered to reflect a regional stress pattern and is not merely a manifestation of the local geology or topography.

Interpretation of structural domains in discontinuity data from Nirex deep boreholes at Sellafield

SUMMARY Knowledge of the fracture network is fundamental to the fluid transport and geotechnical modelling necessary to assess the suitability of a rock mass to host a potential underground repository for the disposal of radioactive waste. However, in the early stages of a site investigation programme, site-specific fracture data are frequently available only from boreholes. This was the case at Sellafield, where characterization of borehole fracture data presented a number of inter-related problems including the volume of data, two data sets for each borehole and borehole sampling effects. A solution to the presentation and interpretation of such data was found in simple graphical display which allows rapid appraisal of a range of fracture attributes in relation to depth, lithostratigraphy and structure or in any other context. The succession sampled by the boreholes in the immediate vicinity of the proposed repository at Sellafield comprises three major units. The Borrowdale Volcanic Group, of Ordovician age, is unconformably overlain by a Permo-Triassic cover sequence comprising up to 100 m of sedimentary breccia (Brockram) succeeded by c. 450 m of the fluvial St Bees Sandstone to rockhead. Fracture data from these are available from logging of the borehole core and from interpretation of resistivity and acoustic images of the borehole wall, providing accurate frequency and orientation information respectively. These show that the discontinuity character of each of the major units is distinctive, indicating overall lithological control on the fracture network. Fracture frequency in the sedimentary cover rocks is relatively low, especially in the Brockram, and orientation patterns are dominated by bedding. The fracture frequency in the volcanic rocks is generally high and orientation typically very variable in the volumetrically dominant welded ignimbrites. However, interbedded units of non-welded lapilli tuff and breccia have lower fracture frequency and simpler orientation patterns. In all units, variations in fracture frequency and orientation character on a scale of a few metres to 100 m can be correlated with fault intersections indicating a structural overprint. Borehole sampling effects are illustrated by comparison of data from differently orientated boreholes and the underlying causes reviewed. Orientation bias is a well known problem usually addressed by weighting according to the probability of the borehole intersecting a planar feature in a particular orientation. However, data from borehole wall imagery are subject to additional sampling effects which reduce the effectiveness of such weighting. A different approach is suggested, using the probability density function of the population from non-censored parts of the sample.

The interaction of tube waves with borehole fractures, Part I: Numerical models

We solve, numerically, the equations of elastodynamics that govern the propagation of waves in a fluid‐filled borehole intersected by one or more fluid‐filled fractures, thus extending earlier work on formations presumed to be rigid. The model is axis symmetric, and it allows for arbitrary radial and vertical variations in the elastic properties of the formation. We have developed a novel gridding scheme that takes advantage of the assumed thinness of the fracture compared to any relevant wavelength; this technique obviates the necessity for a mesh within the fracture itself and thereby saves computational time and memory. We illustrate our technique by means of examples with single and double fractures with or without washouts, as well as examples with variable width fractures. We compute the frequency‐dependent reflection and transmission coefficients of tube waves by fractures from the time waveforms generated by the present method. Major conclusions of this work are (1) reflections of tube waves by fractures can exceed the rigid frame prediction by as much as 50% even for “hard” formations, (2) a washout creates its own distinct signature on tube‐wave reflectivity, and (3) reflectivity spectra alone cannot distinguish closely spaced multiple fractures, unless the separation is comparable to the wavelength of the tube wave.

The interaction of tube waves with borehole fractures, Part II: Analytical models

We develop a series of analytical models that can be used to interpret Stoneley‐wave reflections from fractures intersecting a borehole, thus facilitating the determination of effective fracture apertures from logs. The first model considers the combined effects of borehole enlargements (e.g., washouts) and fractures on the reflection coefficient of Stoneley waves. The result is expressed in terms of the washout volume, which can be obtained from a caliper log, as well as the fracture width. The predictions of this model are in excellent agreement with finite‐difference calculations. Next we develop an analytical elastic model that generalizes the rigid formation model. It also agrees with finite‐difference calculations. Finally, we establish the equivalence between a closely spaced multiple fracture and a permeable medium. In all the examples, the generalization of the result for the single fracture in a rigid formation can be accomplished with standard logging measurements.

Resolving seismic anisotropy: Sparse matrix methods for geophysical inverse problems

Two techniques for the singular value decomposition (SVD) of large sparse matrices are directly applicable to geophysical inverse problems: subspace iteration and the Lanczos method. Both methods require very little in‐core storage and efficiently compute a set of singular values and singular vectors. A comparison of the singular value and vector estimates of these iterative approaches with the results of a conventional in‐core SVD algorithm demonstrates their accuracy. Hence, it is possible to conduct an assessment of inversion results for larger sparse inverse problems such as those arising in seismic tomography. As an example, we examine the resolution matrix associated with a crosswell seismic inversion of first arrival times for lateral variations in anisotropy. The application to a set of first arrival times from a crosswell survey at the Grimsel Laboratory emphasizes the utility of including anisotropy in a traveltime inversion. The isotropic component of the estimated velocity structure appears to be well constrained even when anisotropy is included in the inversion. In the case of the Grimsel experiment, we are able to resolve a fracture zone, in agreement with borehole fracture intersections. Elements of the resolution matrix reveal moderate averaging among anisotropy coefficients as well as between anisotropy coefficients and source‐receiver static terms. The information on anisotropy, such as the directions of maximum velocity, appears sensitive to lateral variations in velocity and must be interpreted with some caution.

Visualization of the El Berrocal granite: application to rock engineering

This paper outlines the visualization of the El Berrocal granite using a computer-based geological modelling system, EarthVision, and discusses the application of this visualization to engineering aspects of waste disposal in crystalline rocks. The El Berrocal Project was an international study with the aim of understanding and modelling the migration processes which have controlled the distribution of naturally occurring radionuclides in a fractured granitic environment. EarthVision was used to provide three-dimensional geological models of the site structure and properties. Modelling of the site structure concentrated on the development of visualizations of the main discontinuities in the granite. These included a model of the main mineralized structures, a model of the regional fracture network, models of local fracture networks between borehole clusters and a visualization of the mineralogy of the fractures in individual boreholes. These fracture models were visualized with the boreholes and access gallery to the mine. In addition, the fracture network in the region of a large scale tracer test was visualized with the injection and extraction zones for the tracer test. Three-dimensional interpolations of the rock and fluid structure were undertaken. A model of the hydraulic conductivity illustrated large-scale variations in hydraulic conductivity and channelling effects in the tracer test zone. A model of the sulphate concentrations in the groundwater illustrated the interpolation of spatial data based on structural domains. The visualizations of the geology of the El Berrocal granite illustrate that, despite limitations, geological modelling can be a powerful and graphic tool in rock engineering. The use of computer visualizations can be provide the three-dimensional structural framework for computations, can aid decision making during the construction phase of waste repositories and can be useful in understanding and analysing the results of numerical calculations.

Borehole sampling of fracture populations-compensating for borehole sampling bias in crystalline bedrock aquifers, Mirror Lake, Grafton County, New Hampshire

Abstract The clustering of orientations of hydraulically conductive fractures in bedrock at the Mirror Lake, New Hampshire fractured rock study site was investigated by comparing the orientations of fracture populations in two subvertical borehole arrays with those mapped on four adjacent subvertical roadcuts. In the boreholes and the roadcuts, the orientation of fracture populations appears very similar after borehole data are compensated for undersampling of steeply dipping fractures. Compensated borehole and pavement fracture data indicate a northeast-striking population of fractures with varying dips concentrated near that of the local foliation in the adjacent rock. The data show no correlation between fracture density (fractures/linear meter) and distance from lithologic contacts in both the boreholes and the roadcuts. The population of water-producing borehole fractures is too small (28 out of 610 fractures) to roadcuts. The population comparisons. However, the orientation of large aperture fractures (which contains all the producing fractures) contains two or three subsidiary clusters in orientation frequency that are not evident in stereographic projections of the entire population containing all aperture sizes. Further, these subsidiary orientation clusters do not coincide with the dominant (subhorizontal and subvertical) regional fracture orientations.

A computer-based system for supporting decisions for tunneling in rock under conditions of uncertainty

The clustering of orientations of hydraulically conductive fractures in bedrock at the Mirror Lake, New Hampshire fractured rock study site was investigated by comparing the orientations of fracture populations in two subvertical borehole arrays with those mapped on four adjacent subvertical roadcuts. In the boreholes and the roadcuts, the orientation of fracture populations appears very similar after borehole data are compensated for undersampling of steeply dipping fractures. Compensated borehole and pavement fracture data indicate a northeast-striking population of fractures with varying dips concentrated near that of the local foliation in the adjacent rock. The data show no correlation between fracture density (fractures/linear meter) and distance from lithologic contacts in both the boreholes and the roadcuts. The population of water-producing borehole fractures is too small (28 out of 610 fractures) to roadcuts. The population comparisons. However, the orientation of large aperture fractures (which contains all the producing fractures) contains two or three subsidiary clusters in orientation frequency that are not evident in stereographic projections of the entire population containing all aperture sizes. Further, these subsidiary orientation clusters do not coincide with the dominant (subhorizontal and subvertical) regional fracture orientations.

Inverting fluid conductivity logs for fracture inflow parameters

Fluid electrical conductivity logging experiments introduced by Tsang et al. [1990] represent a new technology for studying the hydraulic properties of discrete fractures in open boreholes. These experiments consist of replacing water in the well bore with deionized water and then pumping the well to induce formation water to return to the well bore. During and after the fluid exchange the entire well is logged to measure fluid electrical conductivity (FEC). At locations where formation water enters the well there are abrupt increases in the borehole FEC, thereby revealing the location of hydraulically conductive fractures. As pumping continues, high FEC water moves up the well bore at a rate proportional to the total inflow rate below the observation point. Inflow rates and FECs of formation water associated with each fracture can be inferred by modeling the FEC profiles with the one‐dimensional advective dispersion equation. In this paper I present an inverse model which estimates the inflow rates and formation FEC values that optimally fit observed FEC logs acquired during fluid exchange experiments described by Tsang et al. [1990]. With this inverse model, inflow rates are constrained by the rate at which the well is pumped during the experiment. The forward model is solved numerically using a control volume finite difference scheme with power law upstream weighting and source term linearization. The inverse problem is solved using the Gauss‐Newton iterative method. The rows of the Jacobian matrix, or the sensitivity coefficients, are calculated numerically with the same algorithm that solves the forward problem. Both constrained and unconstrained inverse models are used to interpret fluid logging experiments performed in research wells penetrating Piedmont rocks of North Carolina.

The Production of Chemical Reaction-Induced High-Pressure Gas Pulses for Rock Fracturing

Past research has shown that gas generating reactions that produce high borehole pressures by ramping up to the pressures slowly can be useful in producing fracturing around a borehole without damaging it. Most chemical reactions used for high-pressure gas fracturing have produced the gas by using a single component reaction, such as the ignition of nitrocellulose. Two-component solid-liquid and liquid-liquid reactions are examined in this report to determine if a gas-producing reaction suitable for use in borehole fracturing can be developed. Surface passivation effects make it difficult to produce useful solid-liquid reactions. Laboratory and field tests using a liquid-liquid system (triethylaluminum and water) demonstrated that liquid phase reactions will proceed with the speed and peak pressure needed to produce useful fracturing. The field testing also showed that with suitable precautions triethylaluminum could be safely handled in a borehole. Liquid-liquid chemical reaction systems (such as triethylaluminum and water) offer advantages, in that the pressure pulse can be regulated by modifying the reactants or designing the downhole container to control the mixing of the reactants. In the case of triethylaluminum, there are additional advantages, in that the reactive compound is pyrophorlc, not explosive, and the products from the water-triethylaluminum reaction (ethane and aluminum oxide) are not toxic.

Hydrogeologic Evaluation Of Fracture Permeability In Crystalline Basement Aquifers Of Uganda

Throughout Africa, many communities rely on fractured crystalline basement rocks of Precambrian age as the primary source of water supply. The hydrogeology of these aquifers is often poorly understood, and boreholes are frequently sited and designed with little appreciation of the local fracture system. This is especially true in Uganda, where the role of fractures in determining well yield and relative importance of deep and shallow fracture zones have been a continous source of debate since a major national well drilling program was initiated over ten years ago. Recent studies in Uganda have attempted to resolve outstanding hydrogeological questions through the implementation of one of the most comprehensive borehole fracture studies ever undertaken in continental Africa. During the study, 233 packer tests were carried out at 22 sites. Six of the sites, together with a further 16, were subjected to constant yield recovery tests. One well was tested before and after the deployment of yield-enhancing hydraulic fracturing techniques to investigate its effectiveness. Study results reveal the presence of an extremely poor aquifer with little transmissive capability. Across the study area, the hydraulic conductivity of the bedrock average <10-7 m/s; the majority of transmissivity values are less than 1 m2/d. Packer tests show that fractures occur throughout the depth of well, but are usually very few in number and often exhibit a relatively low permeability. Most wells relied on a single permeable zone for most of their water. In approximately half of the boreholes tested, the most permeable zone was found to occur at the top of the uncased section, just below the regolith. No relationship was found between the distribution of boreholes with shallow, intermediate or deep, high yielding fracture zones and physiographic features. Neither was any relationship observed between hydraulic conductivity and depth within the upper 60 meters of rock tested. Notably, however, no fracture zones more permeable than 10-5 m/s were found below a depth 57m, and it is suggested that a depth of approximately 60m below ground level may represent the effective base of aquifer. Packer tests performed in a hydraulically fractured borehole confirmed increases in borehole transmissivity by a factor of about 3. The tests were also able to show where improvement in hydraulic conductivity was being achieved.