Dipole-dipole Resistivity Research Articles

Irrigation channel seepage investigations

Most irrigation channels in Australia are earth lined and leak water to the surrounding terrain. The cost of lost water can be high in economic and environmental costs. The solution is to completely line the canals or replace them with pipes, both expensive options. An alternative is to focus on areas of highest seepage. In this study we trialed a dipole-dipole resistivity array towed behind a dinghy in the canal. The approach followed trials using electromagnetic techniques in previous years that showed conductivity changes below and around the canals in part reflected seepage. We inverted the resistivity results to sections and statistically analysed the data from various depths in the ground by correlating the resistivity against pondage seepage results. Our conclusion was that the resistivity worked best where there was a diffuse seepage from the canal. The greatest effect was around the watertable. Where seepage rates were low there was no correlation with resistivity. The correlations improved with increasing seepage particularly where the inverted sections indicated the seepage rates were uniform along the channel.

A Case Study of Dipole-Dipole Resistivity for Geotechnical Engineering from the Ras en Naqab Area, South Jordan

A data set utilizing the dipole-dipole resistivity technique covering an area of 5 excavation levels at a proposed highway site at the Ras en Naqab escarpment, south Jordan has been collected and interpreted to provide additional stratigraphic and subsurface structural information at the site. The interpretations obtained from 2D-modelling of the field data demonstrated a strong correlation between resistivity anomalies and mapped subsurface geology. Based upon the lateral and vertical resistivity changes with depth, the thickness and slope of the subsurface members was outlined. Analysis of the exposed section showed that estimated member thicknesses and quantitative assessment of resistivity inferred from dipole-dipole resistivity data were in good agreement with the thickness and lithology found in the section.

Geophysical Investigation of Vadose Zone Conductivity Anomalies at a Hydrocarbon Contaminated Site: Implications for the Assessment of Intrinsic Bioremediation

Electromagnetic (EM-31), dc resistivity, and ground penetrating radar (GPR) methods were used to investigate part of a former refinery site in Kalamazoo, Michigan, USA. The study area is underlain by about [Formula: see text] of heterogeneous glacial drift deposits overlying a sandy outwash unit. Three large above ground storage tanks (ASTs) formerly occupied the study area and were removed by the U.S. Environmental Protection Agency (USEPA) in 1997. EM-31 surveys revealed eight remnants of buried pipe ranging from [Formula: see text] to more than [Formula: see text] in length. Emanating from some of these abandoned pipe segments are extensive near surface conductivity anomalies, with values ranging from [Formula: see text] to greater than [Formula: see text] vs. background values of less than [Formula: see text]. GPR and dipole-dipole resistivity surveys performed along selected profiles with anomalous conductive zones revealed a channel form within the shallow subsurface. Shallow hand augured soil borings into these conductive zones encountered hydrocarbons perched on thin clay lenses, or trapped in low permeability sandy-silt layers, while borings in background conductivity areas showed only clean sand and silt. We also observed that areas underlain by clays in association with perched hydrocarbons exhibited a higher conductivity (greater than [Formula: see text] and up to [Formula: see text]) when compared to areas underlain by uncontaminated clays. Based on these observations and results of our prior work in similar settings, we infer that the conductivity anomalies may be due to the occurrence of intrinsic biodegradation of the hydrocarbons. Thus, the previous conductive model originally proposed by our research group for ‘aged’ hydrocarbon spills at the water table can be extended upward to include vadose zone occurrences of biodegradation of these contaminants. The findings from this study have significant implications for the use of geophysical techniques in the assessment of vadose zone biodegradation.

Geostatistical approach to bayesian inversion of geophysical data: Markov chain Monte Carlo method

This paper presents a practical and objective procedure for a Bayesian inversion of geophysical data. We have applied geostatistical techniques such as kriging and simulation algorithms to acquire a prior model information. Then the Markov chain Monte Carlo (MCMC) method is adopted to infer the characteristics of the marginal distributions of model parameters. Geostatistics which is based upon a variogram model provides a means to analyze and interpret the spatially distributed data. For Bayesian inversion of dipole-dipole resistivity data, we have used the indicator kriging and simulation techniques to generate cumulative density functions from Schlumberger and well logging data for obtaining a prior information by cokriging and simulations from covariogram models. Indicator approaches make it possible to incorporate non-parametric information into the probabilistic density function. We have also adopted the Markov chain Monte Carlo approach, based on Gibbs sampling, to examine the characteristics of a posterior probability density function and marginal distributions of each parameter. The MCMC technique provides a robust result from which information given by the indicator method, that is fundamentally non-parametric, is fully extracted. We have used the a prior information proposed by the geostatistical method as the full conditional distribution for Gibbs sampling. And to implement Gibbs sampler, we have applied the modified Simulated Annealing (SA) algorithm which effectively searched for global model space. This scheme provides a more effective and robust global sampling algorithm as compared to the previous study.

Electrical resistivity imaging of the marginal faults of Eumsung Basin with a standard-deviation analysis

Electrical resistivity surveys were undertaken in the Eumsung Basin (Cretaceous), Korea to delineate basin geometry. Basin boundaries were effectively clarified in electrical dipole-dipole resistivity section as high resistivity contrast bands. A statistical analysis on the resistivity section was tested in terms of standard deviation and was applied to electrical dipole-dipole and magnetotelluric resistivity structures. The objective was to improve resistivity imaging of basin-margin fault and basin structure. This focusing strategy proved to be effective in the subsurface reconstruction of basin architecture as well as the surface demarcation of basin-margin faults and brittle fracture zones. On the basis of statistical analysis, the maximum depth of the basin varies from about 1km in the northern part to approximately 4 km in the middle part. The approach of image focusing technique can be an important tool in clarification of some major ambiguities that one is often faced with in interpretation of resistivity inversion results.

Electrical resistivity and MT imaging in the northern-middle part of Eumsung Basin (Cretaceous), Korea

A geophysical survey was undertaken in the Eumsung Basin to delineate the basin geometry. The basin boundaries are identified in electrical dipole-dipole resistivity section as high-resistivity contrast bands. The high resistivities most likely originate from the basement of Jurassic granite and Precambrian gneiss, contrasting with the lower resistivity of the sedimentary sequence (Cretaceous). The boundaries are also identified in the segments of magnetotelluric (MT) resistivity section, which are qualitatively correlated with the results of vertical electrical sounding, very-low-frequency electromagnetic survey, and magnetic field measurement. Three-dimensional geometry of the basin is delineated using the composite resistivity-structure information from three MT lines (two cross-basin E-W lines and an intersecting N-S line). The maximum depth of the basin varies from about 1 km in the northern part to 3 km or more in the middle part. This variation supports the view that the basin experienced pull-apart opening with rapid subsidence of the central blocks and asymmetric cross-basinal extension.

Dipole-dipole resistivity imaging of the Ahuachapán-Clpilapa Geothermal field, El Salvador

The application of a fast imaging technique to dipole-dipole resistivity data from the Ahuachapán-Chipilapa geothermal field produces resistivity sections comparable to those obtained by trial-and-error methods using relatively intensive forward computations. The imaging technique is an approximate inverse method based on low resistivity variations that produces reasonably good results for moderate and high resistivity contrasts. The method considers distributions of minimum structure for representing the earth and uses quadratic programming in the search for acceptable solutions. The images are obtained in a single iteration in the fashion of a fast transformation of apparent resistivity pseudo-sections into true sections of resistivity versus depth. The transformation was applied to four long-offset dipole-dipole lines. The same lines were also interpreted independently using exact forward modeling by different interpreters. In general, the images reproduce the main features of the forward models but also include subtle variations that are difficult to recognize in trial-and-error results. In some cases, the images combine features of the different individual models obtained by the interpreters. The application to Ahuachapán-Chipilapa illustrates the usefulness of the imaging technique in the interpretation of dipole-dipole resistivity survey data from geothermal fields.

Use of the EM-34 instrument in groundwater exploration in the Shepparton region

Since the late 1970's, the Rural Water Commission of Victoria (RWC) has been offering a groundwater exploratory drilling service to landholders in the irrigation areas of the Shepparton Region. The aim of this service has been to encourage exploration for shallow aquifers in the Shepparton Formation and in turn to encourage groundwater pumping for water-table control. The selection of drilling sites in early exploration programs was based on geomorphological expressions of prior stream activity. Later, the dipole-dipole resistivity survey method was used. While giving good resolution of targets in the sedimentary profile, this method was found to be too restrictive in covering large areas of a property. In 1986, the RWC acquired a Geonics EM-34 unit to become the main geophysical tool in groundwater exploration. Since this time, the unit has been used on over 60 individual properties, as well as being used for broader scale exploration surveys on the Riverine Plains. The EM-34 unit has proved to be a useful tool in defining areas where shallow sand aquifers occur in the Upper Shepparton Formation. The RWC uses the horizontal dipole technique with a constant coil separation of 20 m. This technique has allowed good line coverage of up to 7500 m in a day, while allowing fair resolution of the target within the top 15-20 m of the Shepparton Formation. Experience has shown that a number of non-rigorous categories can be applied to the various ranges of conductivity readings. Such categories aid in the interpretation and selection of drilling targets.

A basic program to transform continuous polar dipole-dipole resistivity soundings to half-Schlumberger soundings

An interactive HP 9845B BASIC program transforms continuous polar dipole-dipole resistivity soundings to half-Schlumberger soundings. The program features graphic presentation of the field dipole-dipole data as well as the transformed half-Schlumberger data. An example of the transformation and its effectiveness in smoothing “high-frequency” noise is given.

Geoelectrical response of cavities in limestones: an experimental field study from Kurnool District, Andhra Pradesh, India

Geoelectrical field studies on cavernous Precambrian limestones of the Cuddapah Basin were conducted on an experimental basis in parts of Kurnool district, Andhra Pradesh, India. The objective was to study the effectiveness of the technique in delineating potential ground-water zones in them. The studies were confined to two sites viz., a bore hole tapping a cavity in Koilkuntla limestones at Illurukothapeta near the township of Banganapalli and over dry and water-filled parts of the Bilum caves in Narji limestones near the village of Kolimiguntla. These represent small- and large-dimension cavities, respectively. The field studies at these sites comprised a number of radial Schlumberger resistivity soundings, Wenner and Lee resistivity profiling, Lee resistivity soundings, buried current source dipole-dipole resistivity mapping, mise-à-la masse potential measurements and self-potential measurements. It was possible to delineate the orientation and lateral extent of the cavity encountered at 13 m depth in the borehole and the western extension of the water filled part of Bilam cave occurring at 35 m depth. Results of radial soundings in demarcating the shallow cavity encountered in borehole and mise-à-la-masse potential measurements for demarcating the extension of the water filled Bilam cave were found significant and useful. Self-potential measurements revealed the direction of ground water flow near the borehole.

Smoothing noise-degraded electrical soundings in shallow and deep investigation studies

A method of smoothing noise-degraded Schlumberger and axial dipole-dipole electrical founding curves is developed. It is based on two expressions relating the Schlumberger and the dipole-dipole apparent resistivity to the pole-pole apparent resistivity. The analysis in the frequency-domain reveals that the pole-pole spectrum is related both to the Schlumberger and to the dipole-dipole spectrum as in linear systems, with transfer functions typical of low-pass filters (smoothers). The calculation of the impulse responses allows to obtain in the spacing-domain relationships that are used to transform original Schlumberger and dipole-dipole curves into equivalent pole-pole curves. Smoothing is therefore an intrinsic property of the transformation. The goodness of the filtering process is demonstrated also by examples.

Interpretation of dipole-dipole resistivity monitoring data at Cerro Prieto

Repetitive dipole-dipole resistivity data have been taken on a yearly basis by LBL at Cerro Prieto since 1978. Stations along a single profile line extending from the Cucapa Mountains to the center of the Mexicali Valley and passing over the present production zone have been remeasured with sufficient accuracy to detect subsurface changes in resistivity, some of which are probably related to fluid production. The precision of the most recent measurements (November, 1981) averages about 1%. Results from 2.5 years of monitoring indicate a 5% annual increase in apparent resistivity over the present production area and decreases in apparent resistivity of the same magnitude in the regions immediately eastward and westward from the production zone. The increase in resistivity in the production zone is most likely due to dilution of reservoir fluids with fresher water, as evidenced by a drop in chloride content of produced waters. An attempt was made to determine whether specific lithologic zones in wellbores show resistivity changes with time by comparing well logs from newly drilled wells with logs from older nearby wells. Results show that lateral resistivity variations within stratigraphic units between closely-spaced wells are sufficient to obscure possible temporal changes. The area of decreasing resistivity in the eastern part of the field is associated with a steeply dipping conductive body, a zone of higher thermal gradients and an increase in shale thickness in the section. Well log analysis shows that the low resistivity is mostly due to higher temperatures. Decreasing resistivity in this area may be caused by an influx of hotter and more saline brines from depth. Recent measurements also show dramatic increase in near-surface resistivity at the western end of the monitored line. This is most likely due to recent changes in local irrigation practices which resulted in a general improvement in groundwater quality. To investigate the phenomenon of resistivity changes caused by groundwater movement and chemical reactions, we propose the establishment of an additional resistivity line crossing both the new eastern production zone and the present survey line at an angle of 60°. This line would permit the acquisition of baseline data over the future production zone (Cerro Prieto 11 and 111 areas), in conjunction with the present line, and would establish a grid of stations which could be used to map subsurface groundwater fronts.

Interpretation of Well Logs at Cerro Prieto Geothermal Field: ABSTRACT

The Cerro Prieto geothermal field is located 35 mi (56 km) south of Mexicali, Baja California, Mexico. The subsurface geology of Cerro Prieto is typical of a deltaic depositional environment. Logging data from more than 70 wells have been analyzed. A comparison of the data derived from these logs with surface geophysics, mineralogic, and stratigraphic information leads to a model of subsurface fluid flow. Cross sections have been constructed using the values obtained from resistivity and density logs. The resistivity cross sections are consistent with the model derived from surface dipole-dipole resistivity measurements, lithology, production, and mineralogic characteristics. The density cross sections show good agreement with the location of a self-potential anomaly and the location of micro-earthquake hypocenters. Interpretation of these well logs leads to estimates of porosity, salinity, temperature, and permeability. High porosities are probably due to secondary dissolution. The salinity distribution shows that relatively fresh water extends from near the surface to depths of 4,000 to 5,000 ft (1,220 to 1,525 m) in the eastern part of the field and to shallower depths in the western part. Water of higher salinity underlies the fresh water. Permeability has been hard to estimate because of conflicting evidence from the well logs. In the proposed model of hydrothermal flow, hot brines enter End_Page 575------------------------------ the field at 5,000 ft (1,525 m) or more at the eastern part of the field and flow upward and westward through the field. End_of_Article - Last_Page 576------------

Roosevelt Hot Springs, Utah Geothermal Resource--Integrated Case Study: ABSTRACT

The Roosevelt Hot Springs geothermal resource is located along the western margin of the Mineral Mountains, approximately 19 km northeast of Milford, in southwestern Utah. To date, seven producing wells have been drilled by Phillips Petroleum Co. and Thermal Power Co. Construction will soon begin on the first stage of a 120-megawatt power plant. Detailed geologic mapping and the study of well logs and drill cuttings indicate that the geothermal reservoir is a fracture-controlled, liquid-dominated system. The host rocks of the reservoir are Precambrian metamorphic rocks and various Tertiary intrusives. The reservoir is mainly localized between the range front and an alluvial covered horst block, along which fluids have migrated to the surface forming an elongate north-trending dome of siliceous sinter. The reservoir is an area of high heat flow (over 1,000 mW/sq mi) and low near-surface electrical resistivity (less than 10 ohm-m). Aeromagnetic, gravity, and reflection seismic data help define the geologic structure within and around the alluvium covered reservoir. Trace element geochemistry shows that arsenic, lithium, and mer ury are enriched along fluid pathways of the geothermal system. Mercury concentrations greater than 20 ppb occur only at temperatures less than 225°C and reflect the present thermal configuration of the field. The system was efficiently explored using detailed geologic mapping in combination with thermal gradient studies and dipole-dipole resistivity. End_of_Article - Last_Page 982------------

LBL resistivity studies at Cerro Prieto

In a joint venture, the LBL and CFE have undertaken a series of de resistivity studies at the Cerro Prieto geothermal field. In the present study dipole-dipole resistivity data have been collected and two-dimensional modeling studies have been performed using these and existing Schlumberger data taken by the CFE. The goals of the present research are: (1) to determine whether the reservoir region or controlling geologic structure can be delineated by careful de resistivity measurements and proper interpretation; (2) to establish permanent electrode stations so that measurements may be repeated at regular intervals to determine whether temporal changes in resistivity occur within the reservoir region; and (3) to perform the first set of replicate measurements. During 1978, two long dipole-dipole resistivity lines were surveyed, each with dipole lengths of 1 and 2 km, N = 1 to N = 5. The northern line D-D′ is 18 km long and extends eastward from the Cucapa Mountains into the Mexicali Valley passing just south of the Cerro Prieto volcano. This line follows CFE Schlumberger resistivity line 1. Line E-E′ is 16 km long and was measured several kilometers to the south of D-D′. This line trends subparallel to D-D′ and crosses the central portion of the present production zone. Data were collected with the LBL 25-kW motor-generator and signal-averaging receivers. The error in these measurements has been calculated to be between 5 and 15% at all stations. Two-dimensional modeling studies on both Schlumberger and dipole-dipole data sets yield a compatible model, consistent with known geological features. Several important observations are made from the models. (1) The reservoir region within the production zone is not clearly delineated with de resistivity measurements. This is an expected result, however, since the background resistivity is very low, between 1 and 2 ohm-m. (2) The Imperial and Cerro Prieto faults are clearly delineated in the resistivity models; each fault is characterized by a change in the bulk-section resistivity suggesting that they act as aquacludes.

General overview of geophysical studies at Cerro Prieto

Geophysical investigations by the CFE in the Mexicali Valley near the Cerro Prieto volcano began nearly 20 years ago. Initially, gravity and seismic refraction methods were used for structural information related to faults and basement configuration. Geophysical emphasis shifted to d.c. resistivity surveys in the early 1970's after it was determined that a large area of low resistivity coincided with the thermal manifestations south of the volcano and the known high temperature zone at depth. Supplemented by ground magnetic and gravity measurements, the resistivity data are being interpreted to yield a detailed picture of the structure concealed by valley fill and to identify promising areas for future exploration. Under the DOE/CFE agreement, the LBL has been conducting various types of electrical, passive seismic and precision gravity surveys over the field. This work is less concerned with geothermal exploration than the CFE geophysical program. It involves applying exploration methods to reservoir-related questions; namely, can the reservoir area be delineated and can changes in reservoir characteristics be detected and monitored by means of surface measurements? As the second question involves a long time scale, repeated measurements and correlations to production data, only baseline information is presently being obtained. Dipole-dipole resistivity, magnetotelluric (MT) and self-potential (SP) surveys have been completed and results will be discussed. The large dipolar SP anomaly found over the field has encouraged further use of the method in the Mexicali Valley. Careful interpretation of dipole-dipole and Schlumberger data set yields a subsurface resistivity model which provides a surprising amount of useful detail. Permanent current electrodes have been emplaced and repeat measurements will be made at later times to determine whether changes in resistivity occurred within the reservoir area. Reference magnetic MT technique, developed recently at U.C. Berkeley, was applied at Cerro Prieto. Earlier tests have shown that in the presence of electro-magnetic noise, such as cultural noise, this technique yields more accurate subsurface information than conventional MT. Regional seismicity has been studied for many years. Recently, the CICESE has maintained a four station network in the Northern Mexicali Valley. The LBL passive seismic results show epicenters within the reservoir boundary associated with right-lateral strike-slip movement. V P/V S data from these events indicate a Poisson's ratio of 0.4 for the upper 2–3 km of the field. Although limited, P-wave velocity and attenuation data imply a local low near the western edge of the field superimposed upon a slight high for the entire production region. Beginning in the fall of 1978, continuous monitoring by the LBL and CICESE will provide more complete data on field characteristics. The first phase of precision gravity surveys at 60 monuments is nearly complete. Repeated gravity measurements coupled with repeated first-order leveling by the CFE will be needed to distinguish subsidence or tectonic induced gravity changes from the small changes (80–100 μgal) that might be associated with mass depletion due to fluid withdrawal. A five-station seismic array is being installed in shallow holes so that the long-term changes in seismic activity and relationship to subsidence and geothermal fluid production can be studied.

Interpretation of gravity surveys in grass and buena vista valleys, nevada

The complete Bouguer gravity data from surveys in Grass and Buena Vista Valleys were interpreted by means of an iterative two-layer inversion process to obtain the depth to “basement” beneath the valley-fill material. Basement in this sense is an interface which, for a density contrast that must be determined by trial and error, has zero depth at the range fronts and the correct value at points where there is subsurface information, e.g. drill results. Supported by evidence from a deep U.S. Geological Survey drill hole in Grass Valley, a density contrast of 0–06 g/cm 3 was found to give a reasonable fit to the Paleozoic rock subcrop and the Paleozoic rock outcrop at the Sonoma Range. The depths to Paleozoic rocks were also found to agree very well with the seismic interpretation obtained by means of a finite element model of seismic reflection and refraction data along part of line E-E, Grass Valley. Gravity and seismic interpretations also give very close agreement on the location and dip of the Hot Springs fault. The gravity interpretation was compared to resistivity models obtained from dipole-dipole measurements on two lines. In general, the gravity interpretation agrees reasonably well with the configuration of the 150Ω m basement, although the electrical basement is usually 200–400m deeper than the density and velocity interface. This difference has not been explained, but could be due in part to resolution and nonuniqueness problems in the resistivity interpretation. One might modify the resistivity model to determine whether a better fit to the density-velocity model is possible. However, this has not been done. The Grass Valley gravity inversion reveals a complex basement picture which was interpreted in terms of numerous inferred normal faults, some of which have ssurface expressions. Inferred faults trend mainly north-south and northwest-southeast, but there is also a set of northeast-southwest trending faults. The latter are interesting because they not only conform to the regional trend of hot springs in northwestern Nevada, but two of them are associated with local thermal anomalies. The Hot Springs fault passes through Leach Hot Springs and there is an unexplained 6 HFU thermal anomaly near the Panther Canyon fault, the heat flow anomaly paralleling the Tobin Range. We can speculate that the intersection of older northeasttrending faults and the younger Basin and Range faults may provide the fracture permeability for ascending thermal waters. Two local gravity highs, both associated with heat flow anomalies were analyzed. Excess mass calculations for a 5 mgal residual anomaly centered at Leach Hot Springs gave an excess mass of 2.5 × 10 8 tonnes, believed due mainly to precipitated silica and an excess density of about 0.17 g/cm 8. This is equivalent to a silicified pipe 1 km in diameter and 1–9 km in depth extent. This vertical dimension seems sufficient to explain the 100 ms P-wave advance at Leach Hot Springs, but joint modeling of gravity and seismic delay data has not been done. The smaller Section 14 gravity anomaly was not studied in detail as it was believed to be caused by a shallow basement high, possibly fault controlled, extending northward from the Goldbanks Hills. The coincident 4 HFU anomaly could be quantitatively explained by high thermal conductivity of the shallow basement rocks. Gravity data from Buena Vista Valley were processed in a similar manner; a density contrast of 0.06 g/cm 3 gave a zero depth-to-basement that conforms closely with outcropping Mesozoic-Paleozoic rocks of the East Range. Over the valley, the depth to basement agrees well with 50 Ωm electrical basement determined from a model study of dipole-dipole resistivity data. A bulge in the gravity contours 2 km south of Kyle Hot Springs suggested a densification from precipitating CaCO 3, the principal deposit found at Kyle Hot Springs. A careful subtraction of the background gravity produced a broad 2 mgal local residual anomaly. The anomaly shape suggests it may be localized by northeast and northwest-trending faults, similar to the fault directions at Leach Hot Springs. The excess mass calculated for the anomaly is approximately 7.0 × 10 8 tonnes, nearly three times larger than the excess mass for the Leach anomaly. On the other hand, the age of the Kyle system, determined from radioelement abundances (Wollenberg et al., 1977), is approximately 78,000 yr, considerably less than the age determined for the Leach system. This implies that the average flow rate within the Kyle system must be considerably greater. Part of the difference in mass can be attributed to the fact that the total dissolved solids in the Kyle discharge waters is five times greater than at Leach (Wollenberg et al., 1977). Nevertheless, the evidence suggests that the Kyle system may be more active and more promising as a geothermal resource than Leach.

A Computer Program for Four Probe Resistivity Measurements in a Horizontally Layered Earth

A computer code to plot the response of a dipole-dipole resistivity array over a many-layered earth is described.

Electrical properties in the deep crust

Resistivities in the earth's crust have been evaluated by direct observation using dipole-dipole resistivity soundings and magnetotelluric resistivity methods, supplemented by laboratory measurements of the electrical properties of appropriate rock types at elevated temperatures. Assuming a three-layer crustal model, the field observations indicate an attenuation of one to ten decibels per kilometer in the resistant middle layer. It is pointed out, however, that the resistivities in this zone are biased toward the lower side. Higher resistivities are expected on the basis of the laboratory experiments and if they occur to any significant extent, lower transmission losses would be obtained than are predicted by the field measurements.