The use of surface geophysical techniques to detect fractures in bedrock; an annotated bibliography

Prepared in cooperation with the U.S. Department of Energy

Dallas L. Peck, Director

Free on application to the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 CONTENTS

INTRODUCTION

The need for cost effective methods to locate individual fractures or fracture zones in bedrock has increased in recent years. Such methods are critical to a wide variety of hydrogeologic applications, one of the most important of which is hazardous-waste management. Bedrock fractures provide a conduit for the migration of waste from disposal areas to household or municipal wells. Information about fractures is needed to predict the movement and fate of contaminants. Surface geophysical techniques have been used successfully to locate fractures and to describe their geometry.

Another hydrologic application is the evaluation of the suitability of various geologic structures or sites for nuclear-waste repositories. Plutons in continental shield areas and salt domes have been considered for this use because of their typically lo.v primary permeability and porosity and because of their relative tectonic stability (Wright and others, 1980; Mair and Green, 1981; Soonawala and Dence, 1981; Green and Mair, 1983; Bazinet and Legault, 1985). Granitic plutons commonly contain fractures or fracture systems that must be studied to prevent the migration of radioactive waste.

Fractured rock is a potential source of ground water in many parts of the wor kl. Most bedrock wells depend on fractures for recharge and perme<iJility. Information on fracture configuntion can be used to help locate wells and to reduce the chances of drilling dry holes.

Many other activities require information on bedrock fractures. Fractures play a role in the stability of mines and tunnels (Stephansson and others, 1979), need to be considered in developing geothermal energy systems (Aamodt and others, 1977), and are an important factor in the in-place mining of uranium, copper, or other minerals by chemical leaching. Fractures within bedrock units typically are evalucted by surficial mapping of outcrops, by interpreting lineations on aerial or satellite photos, or by analyzing lithologic samples from boreholes. The usefulness of these methods for fracture-detection studies is limited. Fractures are not always continuous throughout a body of rock, and those present at depth may h£Ne no surface ~pres sion. In addition, unconsolidated sediment or artificial fill commonly covers the bedrock surface and limits the use of surficial-mapping techniques. Drilling is expensive and must be supplemented by other indirect methods.

The increased demand for information on fractures within a rock body, and the limited usefulness of traditional data-collection methods, has led to the development and use of new methods for gc:thering that information. Among the most promising developments in this area is the introduction of geophysical methods to locate and evaluate bedrock fractures.

ANNOTATED REFERENCES

p. 4011-4015. the fractures, (4) bulk modulus of the pore fluids, and (5) the direction of seismic-wave propagation. Seismic velocities are decreased in a direction normal to the plane of the cracks.

Bamford, D., and Nunn, K.R., 1979, In-situ seismic measurements of crack anisotropy in the Carboniferous limestone of northwest England: Geophysical Prospecting, v. 27, p. 322-338.

as the variation in seismic P -wave velocity with direction, was determined from P-wave velocities measured by using 12-channel seismic-refraction equipment in Carboniferous limestone at three sites in northwestern England. The limestone, which is partly overlain by a few meters of drift, contains three dominant vertical joint sets having known orientations.

files, fan shots, and offset shots. Directional variations in P-wave velocity were correlated with "previously mapped orientations of joints." The authors suggest that the use of seismic-reflection techniques might allow measurement of velocity anisotropies over a wider range of conditions than that provided by refraction techniques.

Bazinet, Robert, and Legault, Jean, 1985, Scalar audiomagnetotellurics: A tool in the evaluation of nuclear waste disposal sites: Society of Exploration Geophysicists Annual International Meeting, 55th, Washington, D.C., 1985, expanded abstracts of the technical program with author's biographies, p. 149-150.

be used to measure indirectly the water content of fractured or porous rock by measuring the rock's apparent resistivity. This technique is a simplified version of the conventional audiomagnetotelluric method. At any given frequency, the technique measures only a single pair of the electrical and magnetic components of a naturally occurring electromagnetic (EM) field. By doing this, the expression for apparent resistivity is reduced from a complex tensor to a scalar equation; this change simplifies the interpretation process. Because the depth of penetration varies as a function of frequency, a vertical profile can be constructed.

filled fractures within a pluton at a potential nuclear waste-disposal site on the Canadian Shield P-wave velocity anisotropy, which is defined

Crampin, Stuart, McGonigle, Robert, and Bamford,

Field observations consisted of linear pro-

The scalar audiomagnetotelluricmethod can

Denahan, B.J., and Smith, D.L., 1984, Electrical

This technique was used to locate waterin Ontario. Two rock layers were distinguished, on the basis of differing apparent resistivities. The top layer, which is relatively conductive, was interpreted to be a combination of glacial overburden and the fractured top of bedrock. This layer is underlain by highly resistive material, which was interpreted to be nonfractured bedrock. The article does not mention whether this model was checked against test-hole or surface-mapping data.

nique over other surface geophysical techniques for detecting fractures. (1) Only water-filled fractures will be detected; those filled with quartz or other minerals will be ignored. (2) Data from deep fractures can be obtained.

David, 1980, Estimating crack parameters from observations of P-wave velocity anisotropy: Geophysics, v. 45, no. 3, p. 345-360.

resistivity investigations ofpotential cavities underlying a proposed ash disposal area: Environmental Geology and Water Science, v. 6, no. 1, p. 45-49.

was performed to define solution cavities in fossiliferous Eocene limestone at a proposed ashdisposal site in Florida. The limestone, which is over lain by up to 5 ft of unconsolidated material, contains solution cavities having average diameters of 1 to 5 ft. The survey was performed on a 1,200- The authors list two advantages of this tech-

The authors mapped fractures in shallow

A direct current (D.C.) resistivity survey by 2, 600-ft grid by using a Wenner array having constant electrode spacing. At some locations, Wenner array depth soundings having systematically increased electrode spacings also were performed. Several zones of low apparent resistivity were detected, and they were attributed to fluidfilled solution cavities. Results of resistivity surveys were checked by drilling 54 test borings, 24 of which coincided with the resistivity stations. The authors reported an 86-percent agreement between data collected during drilling and interpretations of resistivity measurements.

Frolich, R K., 1986, Size estimates of buried fracture zones with geophysical methods [abs.]: Geological Society of America, Abstracts with Programs, v. 18, no. 1, p. 17.

locate fracture zones in crystalline rock in Rhode Island. The author suggests that fracture zones commonly are found beneath "valleys and topographic lows," because fractured rock is easily eroded. The gravity survey located gravity lows in the area of a lineament and in a river valley. These lows are interpreted as representing fracture zones that are 500 m wide and 1 to 4 km deep and that have porosities ranging from 2 to 4 percent. He cautions that these figures are approximations "indicating the order of magnitude rather than the precise geometry of fracture zones."

buried glacial stream channels. The surveys detected "negative anomalies which can be explained by non-magnetic gaps" associated with fracture zones. These anomalies were detectable by using station spacings of 100ft, in areas having "extremely low and irregular levels of magnetization." Information from the magnetic surveys was correlated with "outcrop evidence and magnetic susceptibility measurements on hand samples."

Goryunov, I. I., 1972, Ulelnoe elektricheskoe soporotivlenie treschinovatoi parody [Electrical resistivity of fractured rock]: U.S. Bureau of Mines Report TR-3-72, 15 p. [Translation of Prikladnaya Geofizika (USSR), no. 38, p. 173-179, 1964, Law, D. A, and Brown, J. W., translators.]

bra to derive a theoretical relation between the geometry of fractures within a rock and that rock's electrical resistance. The author made several Gravity and magnetic surveys were used to

Green, AG., and Mair, J.A, 1983, Subhorizontal

The magnetic surveys were performed over

This paper describes the use of matrix algesimplifying assumptions. (1) Three randomly oriented fracture systems are assumed to cut an otherwise uniform, isotropic body of rock. (2) Each fracture system was assumed to consist of parallel fractures that extended indefinitely in two dimensions and that had identical width and regular spacing. (3) The fractures were assumed to be filled with a material having a resistivity lower than that of the surrounding bedrock.

inverse direction. The resistance tensor of the fractured rock can be calculated by using the resistivity of the rock matrix and of the fluid filling the fractures, as well as by the geometry of the fractures. Conversely, the density and width of fractures can be determined if the resistance of fractured rock is known.

of approximately 2, 300 m within a borehole were measured by using the methods outlined above. Resulting values were found to correspond with measurements made by microscopic examination of thin sections of the borehole core. The article did not discuss field methods used to make these measurements.

fracture zones in a granitic pluton: Their detection and implications for radioactive waste disposal: Geophysics, v. 48, no. 11, p. 1428-1449.

reflection techniques to locate horizontal fracture zones at depths up to 800 m within a granitic pluton on the Canadian Shield in southeastern Manitoba. The pluton is overlain by a discontinuous layer of highly conductive till having a maximum thickness of about 25 m. Previously, the till had hindered electromagnetic and resistivity measurements at a nearby site (Soonawala and Dence, 1981).

4. 8-kmarea. Small construction-type earth tamp-

ers were used as a sound source for three of the profiles; explosives were used in the other two. The receiving setup consisted of 24 arrays of 9 geophones each. Each group of geophones was deployed along a 10- to 20-m line having a 10- to 20-m interval between arrays.

reflectors, which were interpreted as fracture zones. Holes drilled along the profiles intersected major fracture zones at depths of 300 to 600 m. This observation was in close agreement with the The method can be applied in a forward or

The width and density of fractures at a depth

This study used high-resolution, seismic-

Data were collected along five profiles over a

The surveys revealed two gently dipping depths predicted from the seismic profiles. Borehole-televiewer examination of one drill hole confirmed the shallow dip of the fractures.

Harmon, E.J., 1984, Investigation of a previously unexplored basaltic aquifer using complementary geophysical methods, in Nielsen, D.M., and Curl, M., eds., National Water Well Association/D. S. Environmental Protection Agency Conference on Surface and Borehole Geophysical Methods in Groundwater Investigations, San Antonio, Texas, 1984, Proceedings: Worthington, Ohio, National Water Well Association, p. 273-287.

furmation -a series of Pliocene basalt flows in southern Colorado. The basalt flows, which range from 250 to 370 ft in thickness, are interbedded with thin layers of sand, gravel, clay, and minor ash flows. Major north-south-trending normal faults and associated fracture systems cut the Servilleta and produce characteristic horst and graben structures. Reworked sand, gravel, and clay deposits, which average 250 to 300ft in thickness, overlie the basalt.

formed in the area as part of an exploration program to find a source of water for a coal-slurry pipeline. The results of each survey were crosschecked against the results of the other surveys and against data obtained by test drilling, aquifer testing, and conventional geologic mapping.

refraction line were used to locate a large, previously mapped fault. This technique was less successful with smaller faults and with their associated fracture zones. The authors suggest that the refraction survey may have been "too limited" to detect the smaller scale faults and fracturing. Total-field ground magnetometer surveys, however, were more successful in locating these small-scale features. Magnetometer-survey results were confirmed by test drilling. Gradientarray and dipole-dipole resistivity surveys both were able to clearly delineate the smaller scale faults.

Huntley, D.J., and Misher, H.M., 1984, Relationship between permeability and electrical resistivity in granular and fractured rock aquifers, in Nielsen, D.M., and Curl, M., eds., National Water Well Association/D. S. Environmental Protection This investigation focused on the Servilleta

Various geophysical surveys were per-

Imse, J.P., and Levine, E.N., 1985, Conventional and

Data from a 3, 800-ft-long seismic-Agency Conference on Surface and Borehole Geophysical Methods in Groundwater Investigations, San Antonio, Texas, 1984, Proceedings: Worthington, Ohio, National Water Well Association, p. 18-36.

examine the relation between electrical conductivity and permeability in fractured rock and unconsolidated aquifers. Permeabilities of fracturedrock aquifers, determined by pumping tests, were compared with permeabilities determined by dipole-dipole resistivity surveys. Twelve sites were examined, and 24 true-resistivity cross sections were constructed. "Distinctive vertical, low resistivity anomalies through, or immediately adjacent to" well sites were interpreted as representing clay weathering within fracture zones . .At three well sites, clay weathering zones recorded in borehole logs were not detected by the resistivity survey. Fractures in the remaining four well sites intersect relatively unweathered rock. The authors suggest that the presence of such highly conductive clay minerals in the weathered zones "strongly affects the apparent formation factor in fractured rock." Formation factor is defined as the ratio of the resistivity of a fractured rock to that of the fluid that saturates the rock.

state of the art geophysical techniques for fracture detection: National Water Well Association Annual Eastern Regional Groundwater Conference, 2d, Portland, Maine, 1985, Proceedings, p. 18-36.

penetrating radar, and seismic-refraction techniques were used in an attempt to delineate fracture zones within flat-lying carbonate rocks at a site in northern New York. Bedrock in the area is characterized by extensive development of karst topography and is overlain by topsoil, weathered bedrock, and artificial fill from 0 to more than 30 ft in thickness.

radar were the most successful techniques used. Information from these methods was independently verified by subsequent mapping.

using a 24-channel seismograph having 250-ft spreads and a shotgun-type sound source. Some fracture zones were indicated on the seismic profiles as distinct low-velocity zones. In areas where individual fractures are separated by intact rock, This survey was part of a project designed to

Microgravity, electrical resistivity, ground-

The seismic surveys were conducted by the fractures were recognized as directional variations in seismic velocities.

fracture system, much of the seismic energy was conducted directly to the geophones through the intact rock between the fractures. Such a fracture system had relatively little effect on seismic velocities. If a line was perpendicular to the fracture trend, however, seismic energy had to travel across fracture openings to reach the geophones. This effect lowered seismic velocity in that direction.

lected by using a 300-MHz antenna. The data were reportedly of sufficient quality to allow interpretation 'Nith no further processing. Fracture zones were marked on the radar record as hyperbola-shaped reflections. Such features were produced when the radar signal was reflected from the sides of the fracture as the radar unit passed overhead.

Jammallo, J. M., 1984a, Delineation of bedrock fracture trace zones by remote sensing and magnetics and their hydrogeologic implications: Burlington, Vt., University of Vermont, unpublished master's thesis, 274 p.

tion with a surface geophysical survey to define fracture zones in northern Vermont. The study area, which is underlain by metamorphic rock, has a thin mantle of till at higher elevations and stratified drift and alluvial sand in valleys. Fractures, which initially were identified as linear features on aerial photographs, were located by a systematic magnetometer survey. To confirm the results of this work, joints were mapped at outcrops.

ing to their origin to separate those caused by fracture zones from those resulting from magnetic mineralization or cultural interference. Most of the photolineaments caused by fracture zones were found to have strong positive magnetic anomalies. The azimuth of each anomaly was defined in the field by performing a continuous circular survey about the point of greatest intensity. Orientations thus determined were found to correspond to the directions of the photolineaments within ± 10°. The joint outcrop mapping data were not analyzed statistically, although "slight positive correlation" seemed to occur between the areas of high joint density noted in the field and areas of "fracture If a refraction line paralleled the trend of a

Jammallo, J.M., 1984b, Use of magnetics to enhance

Ground-penetrating radar data were col-

Kerschner, D. R., 1980, The location of fractured

This study integrated aerial photointerpreta-

Kirk, K.G., and Rauch, H.W, 1977, Location of

Magnetic anomalies were classified accordtrace concentration" defined by magnetic and photographic methods.

identification of bedrock fracture trace zones for well locations: National Water Well Association Annual Eastern Regional Groundwater Conference, 1st, Newton, Mass., 1984, Proceedings, p. 273-287.

greater detail by Jammallo (1984a).

zones in bedrock by earth resistance, seismic refraction, and soil temperature surveys [abs. ]: Geological Society of America, Abstracts with Programs, v. 12, no. 2, p. 45.

ity, and subsurface-temperature measurements to locate fractures in bedrock overlain by till. Fracture zones were denoted as areas of lowered seismic velocity and resistivity. In addition, the temperature 4ft below land surface, near the fracture zone, was more stable over the course of the experiment than that in the surrounding area. This effect was attributed to moist material within the fracture zone acting as a heat sink. The results from each of the three methods were in close agreement.

fracture zones by electrical resistivity surveying with the tri-potential resistivity technique [ abs.]: Eos, Transactions of the American Geophysical Union, v. 58, no. 6, p. 392.

grates data from three commonly used resistivity sounding arrays: CPPC (current-potential-potential-current electrode arrangement), CPCP (current-potential-current-potential electrode arrangement), and CCPP (current-current-potential-potential electrode arrangement). This method was applied to locate fractures in shalesandstone rocks at an underground coal gasification plant in West Virginia. Fracture zones and their corresponding resistivity anomalies were delineated by plotting apparent resistivities and percentage residual resistivities. Percentage residual resistivity is equal to (RcrPc-RccPP-RcPcP) X This talk summarized the results reported in

This study used seismic-refraction, resistiv-

The tri-potential resistivity technique inte- (100/RcPPc), where RcPPc is equal to apparent resistivity measured by the current-potential-potential-current electrode arrangement, RccP P is equal to apparent resistivity measured by current-current-potential-potential electrode arrangement, and ReP cp is equal to apparent resistivity measured by the current-potential-current-potential electrode arrangement.

Six fracture zones were located, four of which correspond to mapped photolineaments. The percentage-residual-resistivity measurement was reportedly able to detect the location, surface width, and minimum depth of the fracture zones.

Leonard-Mayer, P.J., 1984a, A surface resistivity method for measuring hydrologic characteristics of jointed formations: U.S. Bureau of Mines Report of Investigations 8901, 45 p.

used to determine the strike direction of the dominant set of fractures or joints within bedrock. In this technique, Wenner, Lee, and Wenner-Lee arrays are rotated about a fixed point, and current-potential readings are taken at fixed intervals of rotation (commonly 10° ). The resulting apparent resistivity readings, when plotted on polar coordinate axes, approximately define an ellipse. Strike direction of fractures and joints may be determined from the graph by noting directions of high apparent resistivity. This method assumes that joints or fractures may be treated as thin layers having resistivities that contrast with those of the surrounding rock.

at four sites in Minnesota and Wisconsin and in shale at a coal mine in Pennsylvania. Wenner and Wenner-Lee arrays were used in all surveys. Because structural patterns in the rocks at each of the sites are known from previous work, results of the surface geophysical surveys could be confirmed. The Wisconsin sites also were examined in a separate study by Taylor (1982, 1984), and the Minnesota site was the subject of a similar survey by Technos, Inc. (1985).

cessful in locating water-filled joints and fractures but could not locate those filled with air. Water within joints and fractures creates a zone of increased conductivity (decreased resistivity). In air-filled fractures, the direction of maximum apparent resistivity varies over time. This effect is attributed to differential drying of the fractures in the rock. At the Pennsylvania site, survey results Leonard-Mayer, P.J., 1984b, Development and use of

The azimuthal-resistivity method may be

Mair, J.A, and Green, AG., 1981, High resolution

This method was applied in carbonate rocks

Mallik, S.B., Bhattacharya, D.C., and Nag, S.K.,

The resistivity surveys were notably sucwere in good agreement with fracture orientation, whereas "the effect of joints was much less obvious." A horizontal-resistivity survey was run along the length of a valley at this site by using a Wenner-Lee array having a constant survey orientation. Zones of low apparent resistivity encountered along this line are thought to represent zones of increased fracturing.

Water flowing from a river through fractures and into the mine at the Pennsylvania site caused measurable streaming potential. Drops in the streaming potential were found to correspond well with fracture and joint strike determined by the azimuthal resistivity method.

azimuthal resistivity surveys for jointed formations, in Nielsen, D.M., and Curl, M., eds., National Water Well Association/D. S. Environmental Protection Agency Conference on Surface and Borehole Geophysical Methods in Groundwater Investigations, San Antonio, Texas, 1984, Proceedings: Worthington, Ohio, National Water Well Association, p. 52-91.

greater detail by Leonard-Mayer (1984a).

seismic reflection profiles reveal fractures within a homogeneous granite batholith: Nature, v. 294, p. 439-442.

later in greater detail by Green and Mair (1983 ).

1983, Behaviour of fractures in hard rocks-A study by surface geology and radial VES method: Geoexploration, v. 21, p. 181-189.

ing) method was employed to detect fractures within amphibolites, metabasics, and granites at five sites in West Bengal, India. This study employed a Schlumberger array, which was rotated about each sounding station. Apparentresistivity measurements were made at 45° intervals and were plotted on polar diagrams. In a perfectly isotropic medium, the resulting plot would be circular. Apparent resistivities from these sites plotted as ellipses; the ellipsoid plots This talk summarized the results reported in

This paper summarizes the results reported

The radial VES (vertical electrical soundindicate anisotropic conditions and rock fractures. The orientation of the major axis of an ellipse indicates the strike direction of the corresponding fracture.

To confirm the results of the VES survey, fracture orientations were mapped at about 100 outcrops. The results were plotted on an equalarea net and were found to be in good qualitative agreement with the VES data. Both methods detected fracture systems striking in similar directions. The paper does not discuss the specific quantitative accuracy of the VES technique.

Moore, D.L., and Stewart, M. T., 1980, Geophysical signatures to fracture traces in west-central Florida [ abs.]: Geological Society of America, Abstracts with Programs, v. 12, no. 4, p. 202.

were used to locate fracture traces in carbonate rocks in west-central Florida. The fracture zones are approximately 1 to 3 km long and 200 to 300 m wide. The traces were successfully located in the field by using seismic-refraction and electricalresistivity techniques. Magnetic surveys were unsuccessful, and highly accurate gravity surveys were found to require" further investigations ... to determine their practical value" in this application.

Ogden, AE., and Eddy, P.S., Jr., 1984, The use of tri-potential resistivity to locate fractures, faults and caves for siting high yield water wells, in Nielsen, D.M., and Curl, M., eds., National Water Well Association/U.S. Environmental Protection Agency Conference on Surface and Borehole Geophysical Methods in Groundwater Investigations, San Antonio, Texas, 1984, Proceedings: Worthington, Ohio, National Water Well Association, p. 130-149.

different current-potential readings are taken at each survey station. In addition to the standard CPPC array, the CCPP and CPCP arrays are moved in a lateral traverse having a constant electrode spacing throughout the traverse.

solution cavities, and faults in carbonate rock in Arkansas. Fractures and solution cavities located by resistivity surveys were confirmed either directly by well logs or by observation of outcrops or indirectly by noting unusually productive wells in the vicinity of the fractures. The following Palmer, S.P., 1982, Fracture detection in crystalline

Infrared aerial and satellite photographs

In the tri-potential resistivity method, three

This method was applied to locate fractures, general relations were observed in this study: (1) Water-filled fractures and caves caused apparent resistivity to decrease in the CPPC and CPCP configurations and to increase in the CCPP configuration. (2) Air-filled fractures caused apparent resistivity to increase in the CPPC and CPCP configuration and to decrease in the CCPP configuration. (3) Air-filled caves caused an increase in apparent resistivity in all three configurations. The authors note that this technique may be used to detect fracture zones not mappable from aerial photographs.

rock using ultrasonic reflection techniques: Berkeley, Calif., University of California, Berkeley, PhD thesis, 338 p.

high-frequency, seismic-reflection techniques to detect fractures within granite in field and laboratory settings. Measurements were made by using both P- (compressional) and SH- (horizontally polarized shear) waves.

piezoelectric plates as ultrasonic sources and receivers. These plates were attached to the surface of experimental specimens by epoxy. The sound sources were excited with a step-voltage pulse. The resulting seismic waves were transmitted through the granite slab to the receivers. Signals from the receivers were processed by using a variety of techniques, including deconvolution and direct-array synthesis.

detected as hyperbola-shaped SH-wave reflections. Dip angles of the fractures were calculated from SH-wave velocity, source-receiver separation distance, and SH-wave arrival times at two separate receivers. Interference from Rayleigh waves made detecting fractures with unprocessed P-wave signals difficult. The author suggests that both Pand SH-wave reflection measurements are needed to reliably determine whether fractures are filled with air or water.

quarry in Madera County, Calif. Only SH waves were measured. The piezoelectric source and receivers were attached to the rock surface by 1/2-inch bolts and a piece of lead foil coated on both sides with a sticky resin. The rock surface in the contact area was ground smooth to provide better acoustical coupling. Because of its bulk, the signal processing equipment could not be brought This article describes the use of

The laboratory experiments used small

Fractures present within the rock were

Field work was performed in a granite into the field. Instead, receiver signals were recorded on tape in analog form and were later played back and processed in the laboratory.

by fractures. This phenomenon prevented the detection of fractures beyond the one nearest to the SH-wave source. In addition, water saturation increased attenuation of SH waves. These effects were confirmed by further laboratory experiments.

Park, Stephen, and Simmons, Gene, 1982, Crack induced velocity anisotropy in the White Mountains, New Hampshire: Journal of Geophysical Research, v. 87, no. 84, p. 2977-2983.

velocity can be used to detect fractures in rock. Such variations may be observed by using a series of rotated seismic lines that radiate from a common endpoint. This technique was tested at four sites in the White Mountains of New Hampshire. The sites are underlain by granite, quartz syenite, and volcanic rocks having discontinuous overburden up to 3 m in thickness. Measurements were made by using a 12-channel seismograph. Geophone spacing was 6 m, and rotation between lines was 20°. A 70-kg dropped weight and a sledgehammer served as sound sources.

method were within 30° of the actual orientations measured directly. The authors suggest that this relatively large discrepancy may have been caused by microcracks having a preferred orientation different from that of the main fracture system. This method was unable to distinguish the effects produced by the fractures from those related to the much smaller microcracks. The authors suggest that better results might be obtained by combining measurements of S-wave velocity anistropy with measurements of P-wave velocity anisotropy.

Soonawala, N. M., and Dence, M. R., 1981, Geophysics in the Canadian nuclear waste program -A case history: Society of Exploration Geophysicists Annual International Meeting, 51st, Los Angeles, Calif., 1981, Proceedings, p. 83-98.

borehole-geophysical surveys was performed at the planned site of an underground laboratory in a granitic pluton in the Canadian Shield in Canada. The objectives of the surveys were to define the SH waves were found to be totally reflected

Azimuthal variations in seismic P-wave

Stephansson, Ove, Lande, G., and Bodare, A, 1979,

Fracture orientations measured by this

A variety of airborne, ground-based, and size of the pluton, to identify and locate fractures and other structural discontinuities within it, and to predict the pluton's long-term tectonic stability. A separate, high-resolution, seismic-reflection survey also was performed at a nearby site within the pluton (Mair and Green, 1981; Green and Mair, 1983).

km2 area were mapped. A strong correlation was noted between the mapped fracture traces and results of the ground-based gradient resistivity survey. Areas having low-fracture density were found to coincide with areas of high resistivity (greater than 10,000 ohm-m). Areas having highfracture density coincided with resistivity lows (less than 2,000 ohm-m).

dered very low frequency (VLF) surveys, some trends on the VLF map may indicate major vertical fracture zones. These trends agree "reasonably well" with mapped fracture traces and results of resistivity surveys.

A seismic study of shallow jointed rocks: International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, v. 16, p. 319-327.

wave propagation at shallow depths in jointed rocks. According to the theory, point-source elastic waves will propagate around the tip of an open joint, where the waves will be refracted according to Huygen' s principle. This propagation causes the velocity curve to be displaced along the time axis of a traveltime diagram. The resulting intercept time is used with wave velocity and joint spacing to calculate depth to the joint.

depth of artificial joints in concrete-block laboratory models and of natural joints at two sites in Sweden. The concrete blocks, each of which contained from 2 to 20 joints, were tested by using a pulse generator having a swinging metal head as a sound source. According to the authors, "the calculated depth of jointing is in good agreement'' with actual joint depth.

explosive-filled, hollow-cylinder energy source.

Experimentally determined depths of joints within a granitic gneiss were less than the actual depths measured directly on the face of a pit within the gneiss. This effect was attributed to deformation of Fractures visible in outcrops within a 3. 8-

Although highly conductive overburden hin-

This paper develops a theory for seismic-

The theory was applied to determine the

Field tests were performed by using an the joints by a horizontal stress field within the rock body. At the other field site-an underground magnetite mine-the theoretically determined joint depth averaged 0.5 m, which is within the 0.3- to 0.8-m depth range where the majority of joints were actually found.

Taylor, R W., 1982, Evaluation of geophysical surface methods for measuring hydrological variables in fractured rock units: U.S. Bureau of Mines Research Contract Report, contract H0318044, 147 p.

in southeastern Wisconsin by using the azimuthal resistivity technique, together with seismicvelocity and attenuation measurements. The sites are underlain by a Silurian dolomite, which unconformably overlies an Ordovician sandstone. At one of the sites, an oolitic iron formation is present between the shale and the dolomite. A thin (0- to 5-ft thick) layer of till overlies the bedrock. These sites were studied also by Leonard-Mayer (1984a, b) and by Taylor (1984).

formed by using Wenner and Lee arrays rotated about a fixed point. Apparent resistivity readings were taken at 10° intervals, and constant a spacings were maintained. The results were plotted as apparent resistivity ellipses. The major axes of the ellipses correspond with the direction of primary joint strike within approximately so, whereas the secondary joint set had no effect on the resistivity ellipse.

ground was detected by continuously monitoring a depth-sounding array. This apparatus incorporated a microcomputer and permanently emplaced electrodes.

channel seismograph and a sledgehammer sound source. P-wave first-arrival times and attenuation of P-wave first breaks and Rayleigh wave peaks were recorded as a function of azimuth. These measurements did not provide any useful information.

electrical resistivity measurements, they were able to determine the primary strike of joints without further field work. The seismic measurements were judged to be of no value for this application.

Taylor, R W., 1984, The determination of joint orientation and porosity from azimuthal resistivity mea-Fractured bedrock was examined at two sites

The azimuthal resistivity surveys were per-Technos, Inc., 1985, Hydrogeologic investigation of

Vertical movement of rainwater through the

Seismic data were collected by using a 6-

The authors concluded that, by analyzing the surements, in Nielsen, D.M., and Curl, M., eds., National Water Well Association/U.S. Environmental Protection Agency Conference on Surface and Borehole Geophysical Methods in Groundwater Investigations, San Antonio, Texas, 1984, Proceedings: Worthington, Ohio, National Water Well Association, p. 37-49.

tivity surveys were used to determine strike and porosity of joints in a Silurian dolomite over lain by approximately 2m of till. These sites were examined also by Taylor (1982) and by Leonard-Mayer (1984a, b). The major axis of the resistivity ellipse defined in this survey corresponded with the primary strike direction of the joints, as previously determined by other means. Joint porosities determined from the resistivity survey were within approximately 2 percent of the porosities as determined by gravity and seismic methods. The author concluded that the azimuthal resistivity method can be used successfully to determine the geometry and porosity of joint patterns within bedrock.

the Ironwood Landfill, Spring Valley, Minnesota: Miami, Fla., Technos, Inc., 88 p.

resistivity-sounding surveys were used to evaluate fluid flow through a fractured rock aquifer at a hazardous-waste landfill near Spring Valley, Minn. This landfill was the site of similar work by Leonard-Mayer (1984a, b). The study area is underlain by Ordovician and Devonian shales and carbonates that are covered by discontinuous unconsolidated sediments up to 15 ft in thickness. The upper 1 to 10 ft of bedrock is weathered to sand- and silt-sized particles, whereas the deeper units are cut by fractures and solution features. In addition, extensive karst topography was observed near the site.

linear miles and penetrated to depths of 20, 50, and 100ft. Nine resistivity soundings were made by using a Wenner array. Fractures were recognized as linear areas of increased conductivity (decreased resistivity) in both the EM and resistivity-sounding surveys. Water within the fractures caused the increased conductivity. This effect was further enhanced by the presence of landfill leachate within the water. Many of the linear anomalies observed in this survey were correlated with previously observed photolinea-At two sites in Wisconsin, azimuthal resis-

EM (electromagnetic conductivity) and

The EM surveys traversed a total of 15 ments or with other features that indicate fracture zones.

Ulriksen, C.P.F., 1982, Application of impulse radar to civil engineering: Hudson, N.H., Geophysical Survey Systems, Inc., 179 p.

and geophysicists for applications as widely varied as detecting salt damage in highway concrete and detecting fractures in bedrock. This type of radar transmits energy over a wide spectrum of frequencies rather than over a single frequency. Subsurface targets are located by their contrasting electrical properties. Highly resistive materials, such as most types of rock, are relatively transparent to radar waves and allow deep penetration. Radar waves will not penetrate highly conductive materials such as clay, shale, metallic ores, or rock having conductive ground water.

detect fractures in a granodiorite in Sweden. Two fracture zones were detected at a depth of 10 m, and their locations were confirmed by borehole observations. Impulse radar has been used also to detect a horizontal bedding plane approximately 3 m beneath the surface at a quarry in Sweden. The lithology of the rock is not specified. The radar record from this site is strikingly similar to a photograph of the quarry face.

Wire, J.C., Hofer, J.K., and Moser, D.J., 1984, Ground magnetometer and gamma ray spectrometer surveys for groundwater investigations in bedrock, in Nielsen, D.M., and Curl, M., eds., National Water Well Association/U.S. Environmental Protection Agency Conference on Surface and Borehole Geophysical Methods in Groundwater Investigations, San Antonio, Texas, 1984, Proceedings: National Water Well Association, p. 288-315.

tometer and gamma-ray spectrometer surveys were used to locate faults, contacts, weathering zones, and fracture zones within bedrock at several sites in California. Water migrating through bedrock fractures oxidizes magnetic minerals and decomposes rock-forming minerals. The effects of these processes are revealed in magnetometer surveys as magnetic lows. The alteration zone also becomes depleted in some radioactive elements; Impulse radar has been used by engineers

Wright, C., Lam, C. P., and Johnston, M., 1980,

This technique was successfully applied to

This paper describes how ground magnethis depletion causes anomalous total gamma-ray counts. Surveys of this type are reportedly most useful in felsic igneous and metamorphic rock and may be useful also in fractured sandstone. Results in fractured shale, or in more complex sedimentary environments such as dipping beds, are "more difficult. .. to evaluate." Surveys performed at a site in the Sierra Nevada batholith revealed magnetic and gamma-ray lows that suggest the presence of a fracture zone. This information was supplemented by observations at outcrops and by geological maps. A VLF survey was used with a resistivity sounding survey to locate a productive well within the fracture zone.

Seismic wave velocities in a rock body at Chalk River, Ontario, and their relationship to fractures [abs.]: Eos, Transactions of the American Geophysical Union, v. 61, no. 17, p. 361.

This survey detected fractures within crystalline rock at Chalk River, Ontario, by measuring seismic P- and S-wave velocities within the rock. Sound sources included a mechanical hammer and a shear wave gun. Surface and borehole geophones were used for recording. The surveys consisted of three shallow reflection lines up to 1. 3 km long. Different seismic velocities noted were attributed to wave propagation through fractured gneiss and quartz monzonite and thin sheets of gabbro.

The follo.ving papers were published in a foreign language, and fully translated versions of them were not available. Foreign language versions are available through the inter library loan system.

Aleksandrov, B. L., 1973, Eksperimental' nyye issledovaniya vliyaniya fil' trtsii rastvora v treschinakh na soprotivleniye karbonatnoy parody [Experimental studies of the influence of movement of solutions through fractures on [electrical] resistivity of carbonate rocks]: Vyssdh. Uchebn. Zaved., Izv., Neft' Gaz N. 6, p. 21-25. [In Russian.] Bro, Maurice, and Hubert, Christian, 1980, Recherche des aqiferes de fracture dans les series schistoso-greseuses au Maili a 1' aide des techniques geophysiques [Exploration for aquifers in the fractured schistose sandstone formations of Mali using geophysical techniques]: Int. Geol. Cong. Abstr. -Congr. Geol. Int., Resumes 26, v. 3, p. 1092. [In French.] Faillat, Jean-Pierre, Leblond, Pierre, and Prevot, Jean, 1980, &sai d'adaptation de la methode des resistivities aux recherches d' eau dans les terrains cristallins fissures et fortement alteres de Cote d' Ivoire [Application of the resistivity method to water exploration in the highly altered fissured crystalline terranes of the Ivory Coast]: Int. Geol. Cong. Abstr. -Congr. Geol. Int., Resumes 26, v. 3, p. 1109. [In French.] Gershanovich, I.M., 1975, Razvedka mestorozhdeniy podzemnykh vod v tresch inovatykh porodakh geofizicheskiimi metod ami [Geophysical surveys of ground water in fractured rocks]: U.S. S. R, Izd. Nedra. [In Russian.] Lachaud, J.C., 1983, Role de la geophysique par methode electrique appliquee ala prospection des eaux souterraines en milieu fissure [Role of geophysics (electrical method) applied to prospection of ground water in fractured media]: Eau l'Industrie, Les Nuisances, v. 71, p. 37-40. [In French.] Lokin, Petar, and Jevremovic, Dragutin, 1977, Metodologija ispitivanja ispucalosti neotkrivenih stenkskih masa [Methods for the investigation of fractures in covered rock masses]: Zb. Rad., Rud-Geol. Fak., Univ. Beogradu 20, p. 311-324. [In Russian, with English summary.] Stephansson, Ove, Lande, G., and Bodare, A, 1978, Seismik for ytuppsprucket berg [A seismic study of shallow jointed rocks]: fuerdrag och diskussioner vid Bergmekaniskt diskussionsmote: Stiftelsen Bergteeknisk Forskning-BePo, p. 217-234. [In Swedish, with English Summary.] Vaubourg, P., 1979, Developpement des recherches d' eau souterraine en milieu fissure; resultats obtenus en Bretagne (France) [Development of methods of investigating ground water in fractured rocks; results in Brittany, France] [and] Methodes d' evaluation des ressources en eau souterraine [Methods for evaluation of ground-water resources]: Int. Assoc. Hydrogeol. Mem., v. 15, no. 1, p. 188-191. [In French with Russian Summary.]

The following references could not be obtained through the interlibrary loan system.

Bjelm, L., rollin, S., and Svensson, C., 1982, Geo radar som undersokningsmetod [Georadarlike investigation method] BFR project no. 800141-9, Coden: LUTVDG/(TVTG-3002)/1-71!1982. [In Swedish.] Brion, M., and Lachaud, J. C., 1979, Experimentations de methodes a Ia recherche d' eau dans les roches cristallines fracturees [Experimental application of geophysical methods to water exploration in fractured crystalline rocks]: Interafrican Comm. Hydraul. Stud., Liaison Bulletin, v. 37-38, p. 14-23. [In French.] Glushko, V.T., Vinogradov, V.V., and Yalans'kiy, AO., 1973, Doslidzhennya masivu girs'kikh porid u zoni nepruzhnikh deformatsiy na osnovi ul' trazvukovogo metodu [Ultrasonic surveys of the brittle deformation zone of a rock massif]: Akad. Nauk Ukr. RSR, Dopov., Ser. B, no. 7, p. 613-616. [In Ukrainian, with English and Russian summary.] Ikeda, K., 1980, Property and seismic wave velocity of fractured zones: Buts uri-Tanko, v. 33, no. 3, p. 34-41. [In Japanese, with English summary.]

During the past decade, the number of published reports on the application of surface geophysical techniques to detect fractures in rock has increased. This bibliography contains only 8 references published from 1972 through 1978, but 10 are listed in 1984. No references before 1972 were located. Many of the early articles listed were published in foreign languages, notably Russian, whereas the majority of the more recent articles are in English.

The most widely used techniques for fracture detection have been the various D. C. resistivity, seismic-reflection, and seismic-refraction methods. These methods have been used in a wide variety of geologic settings, in straight line and in rotating-array surveys. In the straight line surveys, measurements are made along a line having a constant azimuth. Fractures or fracture zones are detected as lateral anomalies in the geophysical property being measured. In rotating-array surveys, the arrays are rotated about a fixed point, and geophysical measurements are made at fixed intervals of rotation. Fractures or fracture zones are detected as directional variations in the geophysical parameter being measured. The most common rotating-array method has been the azimuthal-resistivity survey.

A number of other surface-geophysical techniques have been used to detect and characterize fractures, including magnetometer and gravity surveys, VLF surveys, ground-penetrating radar, gamma-ray spectrometry, and subsurface temperature measurements. None of these methods have been as widely applied as the various D. C. resistivity and seismic techniques.

Although the use of surface geophysical methods to detect bedrock fractures has increased, most of the reported studies are experimental in nature. No one has reported widespread use of these methods in a routine production mode. Most of the surveys currently being performed are still designed to improve understanding of a particular method or to prove that the method works, rather than to acquire data to help solve a real problem in the field.

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