Continuous Resistivity Profiling

Basic Concept

Continuous resistivity profiling (CRP) is the waterborne application of the land-based electrical resistivity (ER) method. The surface ER method employs a stationary array of electrodes, each of which is driven into the ground. In contrast, CRP uses a floating electrode array that is coupled directly with the water and advanced (i.e. towed) along the water surface (see picture below) while continuous measurements are collected (Binley, 2015).

ER methods are based on the electrical potential difference (i.e., voltage) from current-carrying electrodes that are in contact with the earth or, as with CRP, water. The voltage distribution surrounding the current electrodes depends on the electrical resistivities of the subsurface materials and their spatial variations. Thus, CRP can be used to detect spatial variations in subsurface electrical resistivity, or its inverse, conductivity, in estuarine, riverine, lacustrine, and shallow ocean environments to map sub-bottom structures.

Theory

A single ER measurement requires a minimum of four electrodes coupled to an earth medium (e.g., water or earth). CRP involves injecting electrical current via a battery into two current electrodes and measuring the voltage across the other (i.e., potential-) electrode pair(s). CRP measures the ability of electrical current to flow through the surface water and into the underlying (i.e., sub-bottom) materials. Thus, CRP is sensitive to the electrical resistance of the materials (i.e., water and sub-bottom sediments) located between the potential electrodes.

ER surveys and CRP can employ multiple electrode pairs in various arrays (i.e., spatial geometries), which are selected based on equipment used, site parameters, and/or survey applications (Binley, 2015). Due to geometry, data-acquisition speed, and compatibility with a multichannel-resistivity meter (i.e., combination ammeter-voltmeter), dipole-dipole arrays (see figure below) are typically used for water-based applications (Ward, 1990). However, other arrays (e.g., modified Wenner) have been successfully used for CRP (Mansoor and Slater, 2007).

The transmitted direct current (DC) and subsequent voltage induced across the potential electrodes are measured and recorded by a resistivity meter. The ratio of measured voltage to induced current is calculated using Ohm’s Law to produce the resistance value of the material(s) being measured. However, because it varies with material type and shape, resistance is not a diagnostic material property. As such, resistivity methods customarily present data as apparent resistivity (p_a) values.

Apparent resistivity is the value of resistivity (in ohm-m) that an electrically homogeneous and isotropic half-space would yield given the particular electrode arrangement and spacing. Because CRP measures the subsurface, data represent a half-space, which excludes any space above the surface that, if added, would render a whole space. Apparent resistivity calculations involve a geometric factor, which depends on the electrode array and spacing, that corrects resistance measurements (in ohm) for electrode configuration (Mussett, 2000).

As described in Johnson and White (2007), the apparent resistivity of the water and subsurface is determined using the equation p_a = k ∆V/I. The computed apparent resistivity is denoted as p_a, ∆V is the measured potential difference, and I is the injected current. The geometric factor, k, depends on the electrode configuration. For dipole-dipole surveys, k= πn(n+1)(n+2)a, where a is the electrode spacing and n is an integer equal to the measurement number.

CRP data can be initially plotted in a pseudosection, which displays data relative to traverse position and depth below water surface. The horizontal position of a data point is equal to the electrode-pair midpoint. The vertical position is estimated to be where two electrode-crossing lines that are angled 45° away from the surface intersect below the measurement midpoint. Additionally, pseudo-depths can be calculated as the Frechet derivative for a homogeneous half space (Loke, 2001).

CRP measurements are most affected by the materials closest to the electrodes, and measurement sensitivity decreases with depth. As such, CRP measurements represent the weighted average of resistivity over a large volume of earth materials. Additionally, because current dissipates in water, only short electrode spacings can be used (Johnson and White, 2007). As a result, CRP data typically do not produce high resolution interpretations over large areas of investigation associated with long electrode spacings.

Generally, the CRP resolution is one-half the electrode spacing. Though electrode spacing can be reduced to improve resolution, this subsequently reduces the depth of investigation (DOI). Therefore, one of the limitations within a CRP survey is the potentially limited DOI (i.e., the depth below which inverted resistivity data are insensitive to subsurface variations). When using a dipole-dipole array, the DOI in uniform medium is approximately 25 percent of the total streamer length (i.e., electrode-array length).

Applications

Though useful, pseudosections have limited value and typically represent the measured apparent resistivity (i.e., not true resistivity). Thus, CRP data are often analyzed further using numerical inversion algorithms to determine depth-dependent resistivity variations in two-dimensional profiles (i.e., tomographs) and/or three-dimensional volumes. Such inverse modeling, in conjunction with advanced interpretation and incorporation of comprehensive site information, helps overcome issues of non-uniqueness.

Non-uniqueness, or equivalence, is used to convey the idea that no single inverse model will fit a given set of resistivity data. Rather, many inverse models may provide a suitable fit between observed and predicted data. Inclusion of all known site information is required for the most comprehensive survey results and can lead to the most geologically reasonable subsurface-structure interpretations. Thus, using CRP in conjunction with other waterborne geophysical methods and/or surface geophysical methods increases inverse-model validity.

Interpreting CRP inverse-model results requires consideration of the solid and liquid components of the earth as described by Ohm’s Law. That is, mineral grains that comprise soil and rock are essentially nonconductive (i.e., highly resistive). Resistivity tends to decrease with the presence of certain ore minerals, fine-grain materials (e.g., clay minerals), and high temperatures. However, subsurface resistivity predominantly depends on the amount of fluid within pores and/or fractures and the dissolved solids within the fluid.

Thus, resistivity surveys are typically used to map the variations of rocks and/or sediments that occur concurrently with changes in porosity and conductivity of pore-space occupying fluids.

For example, the figure below is a resistivity model incorporating bathymetric and water-column resistivity.

Typically, CRP is used for large-scale surveying and often collected concurrently with continuous seismic profiling or waterborne GPR for added data-interpretation value. In addition to those previously mentioned, possible limitations of CRP include poor data resolution and insufficient bulk-resistivity contrasts between fresh groundwater and saline surface water. Furthermore, measurement errors can often result from vegetation entrainment, boat speed, off-line array movement, or random noise. Regardless, CRP has been successfully used for the following:

• Classification of sediments
• Determination of depth to bedrock
• Identification of submarine groundwater discharge (SGD)
• Mapping of fresh/saline-water intrusion
• Mapping of conductive contaminant plumes
• Studying of groundwater-surface water interaction and hyporheic exchange
• Time-lapse monitoring of hydraulic properties (e.g., changes in submarine groundwater discharge, contaminant discharge, and groundwater-surface water interactions)

Examples/Case Studies

Ball, L.B., Kress, W.H., and Cannia, J.C., 2006, Determination of canal leakage potential using continuous resistivity profiling techniques in western Nebraska and eastern Wyoming, in Proceedings, 19th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems: Seattle, WA, European Association of Geoscientists and Engineers, doi:10.3997/2214-4609-pdb.181.90.

Abstract: In the North Platte River Basin, a ground-water model is being developed to evaluate the effectiveness of using leakage of water from selected irrigation canals to enhance ground-water recharge. The U.S. Geological Survey, in cooperation with the North Platte Natural Resources District, used land-based capacitively coupled and water-borne direct-current continuous resistivity profiling techniques to map the lithology of the upper 8 meters and interpret the relative canal leakage potential of 110 kilometers of the Interstate and Tri-State Canals in western Nebraska and eastern Wyoming. Lithologic descriptions from 25 test holes were used to evaluate the effectiveness of both techniques for indicating relative grain size. An interpretive color scale was developed that symbolizes contrasting resistivity features indicative of different grain-size categories. The color scale was applied to the vertically averaged resistivity and used to classify areas of the canal as having either high, moderate, or low canal leakage potential. When results were compared with the lithologic descriptions, both land-based and water-borne continuous resistivity profiling techniques were determined to be effective at differentiating coarse-grained from fine-grained sediments. Both techniques were useful for producing independent, similar interpretations of canal leakage potential.

Ball, L.B., and Teeple, A.P., 2013, Characterization of Major Lithologic Units Underlying the Lower American River Using Water-Borne Continuous Resistivity Profiling, Sacramento, California, June 2008: U.S. Geological Survey Open-File Report 2013-1050, 13 p.

Abstract: The levee system of the lower American River in Sacramento, California, is situated above a mixed lithology of alluvial deposits that range from clay to gravel. In addition, sand deposits related to hydraulic mining activities underlie the floodplain and are preferentially prone to scour during high-flow events. In contrast, sections of the American River channel have been observed to be scour resistant. In this study, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, explores the resistivity structure of the American River channel to characterize the extent and thickness of lithologic units that may impact the scour potential of the area. Likely lithologic structures are interpreted, but these interpretations are non-unique and cannot be directly related to scour potential. Additional geotechnical data would provide insightful data on the scour potential of certain lithologic units. Additional interpretation of the resistivity data with respect to these results may improve interpretations of lithology and scour potential throughout the American River channel and floodplain. Resistivity data were collected in three profiles along the American River using a water-borne continuous resistivity profiling technique. After processing and modeling these data, inverted resistivity profiles were used to make interpretations about the extent and thickness of possible lithologic units. In general, an intermittent high-resistivity layer likely indicative of sand or gravel deposits extends to a depth of around 30 feet (9 meters) and is underlain by a consistent low-resistivity layer that likely indicates a high-clay content unit that extends below the depth of investigation (60 feet or 18 meters). Immediately upstream of the Watt Avenue Bridge, the high-resistivity layer is absent, and the low-resistivity layer extends to the surface where a scour-resistant layer has been previously observed in the riverbed.

Cross, V.A., Bratton, J.F., Bergeron, E.M., Meunier, J.K., Crusius, J., and Koopmans, D., 2006,

Continuous resistivity profiling data from the upper Neuse River Estuary, North Carolina, 2004-2005: U.S. Geological Survey Open-File Report 2005-1306, doi:10.3133/ofr20051306.

Abstract: The Neuse River Estuary in North Carolina has suffered impacts of eutrophication in recent years. As part of a larger project to better constrain nutrient budgets in the estuary, field investigations were performed to study occurrence and discharge of fresh and brackish ground water and nutrients beneath the estuary itself (fig. 1). A Continuous Resistivity Profiling (CRP) system (Manheim and others, 2004) was used to map the depth of the freshwater-saltwater interface (FSI) in sub-estuarine groundwater. This study area serves as a typological representation of a submarine groundwater environment characteristic of a shallow estuary in a wide coastal plain that has not experienced glaciation. Similar settings extend from New Jersey to Georgia, and along the Gulf of Mexico in the U.S. This report archives 29 lines of data collected during 2004 and 2005 surveys representing almost 210 km of survey lines. These data are further explained in the Data Processing section of the report and previews available of the processed data are available.

Day-Lewis, F.D., White, E.A., Belaval, M., Johnson, C.D., and Lane Jr., J.W., 2006, Continuous resistivity profiling to delineate submarine ground-water discharge—Examples and limitations: The Leading Edge, v. 25, no. 6, p. 724–728, doi:10.1190/1.2210056.

Abstract: Aquifer-ocean interaction, saline intrusion, and submarine groundwater discharge (SGD) are emerging topics in hydrology and oceanography with important implications for water-resource management and estuarine ecology. Although the threat of saltwater intrusion has long been recognized in coastal areas, SGD has, until recently, received much less attention. It is clear that SGD constitutes a major nutrient flux to coastal waters, with implications for estuarine ecology, eutrophication, and loss of coral reefs; however, fundamental questions regarding SGD remain unanswered: What are the spatial and temporal distributions of SGD offshore? How do seasonal and storm-related variations in aquifer recharge affect SGD flux and nutrient loading? What controls do aquifer structure and heterogeneity impose? How are SGD and saline recirculation related? Geophysical methods can provide insights to help answer these questions and improve the understanding of this intriguing and environmentally relevant hydrologic phenomenon.

Dunbar, J.A., Amidu, S.A., and Allen, P.M., 2008, A Study of Seasonal Salinity Variation in Lake Whitney, Texas Using Continuous Resistivity Profiling, in Proceedings, 21st EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems: Philadelphia, PA, European Association of Geoscientists and Engineers, doi:10.3997/2214-4609-pdb.177.139.

Abstract: Lake Whitney, Texas is an example of many reservoirs whose water quality is controlled by highly variable sodium chloride loads originating from near surface evaporate deposits within the upper reaches of the contributing watershed. We investigate seasonal changes in salinity within Lake Whitney using the continuous resistivity profiling (CRP) method. From December, 2006 to October, 2007 we repeated a 30-km long profile along the axis of the reservoir six times. A 135-m long, 11-electrode, marine resistivity array was used to collect eight dipole-dipole readings at offsets from 5-120 m, at intervals of approximately 1 m along the profile. Each profile was collected in an 8 to 10 hour period to produce a “snapshot” of the salinity distribution. Starting from a well mixed and highly saline (3 ohmm/2,300 mg/L) state in early winter 2006, the reservoir became progressively fresher, due to the inflow of fresher water (4 ohm-m /1,700 mg/L) over the winter months. Spring rains, caused resistivity to rise and salinity to drop as layers of freshwater flowed out along the surface from north to south after each storm. The freshwater in these layers progressively mixed with deeper, more saline water as it moved south, raising the average resistivity of the reservoir. A second layer of cold freshwater flowed along the reservoir bottom in the north and out into mid-water depths as it moved south. The greatest contrast within the water column always occurred near the main tributary inlet and was least near the dam, suggesting progressive downstream mixing. We conclude that the CPR method can be used to delineate the depth and lateral extent of abnormally fresh and saline regions within water reservoirs and can resolve shallow and deep zones of contrasting salinity within the water column.

Henderson, R.D., Day-Lewis, F.D., Abarca, E., Harvey, C.F., Karam, H.N., Liu, L., and Lane Jr., J.W., 2010, Marine electrical resistivity imaging of submarine groundwater discharge: Sensitivity analysis and application in Waquoit Bay, Massachusetts, USA: Hydrogeology Journal, v. 18, p. 173-185, doi: 10.1007/s10040-009-0498-z.

Abstract: Electrical resistivity imaging has been used in coastal settings to characterize fresh submarine groundwater discharge and the position of the freshwater/salt-water interface because of the relation of bulk electrical conductivity to pore-fluid conductivity, which in turn is a function of salinity. Interpretation of tomograms for hydrologic processes is complicated by inversion artifacts, uncertainty associated with survey geometry limitations, measurement errors, and choice of regularization method. Variation of seawater over tidal cycles poses unique challenges for inversion. The capabilities and limitations of resistivity imaging are presented for characterizing the distribution of freshwater and saltwater beneath a beach. The experimental results provide new insight into fresh submarine groundwater discharge at Waquoit Bay National Estuarine Research Reserve, East Falmouth, Massachusetts (USA). Tomograms from the experimental data indicate that fresh submarine groundwater discharge may shut down at high tide, whereas temperature data indicate that the discharge continues throughout the tidal cycle. Sensitivity analysis and synthetic modeling provide insight into resolving power in the presence of a time-varying saline water layer. In general, vertical electrodes and cross- hole measurements improve the inversion results regardless of the tidal level, whereas the resolution of surface arrays is more sensitive to time-varying saline water layer.

Okyar, M., Yilmaz, S., Tezcan, D., and Çavaş, H., 2013, Continuous resistivity profiling survey in Mersin Harbour, Northeastern Mediterranean Sea: Marine Geophysical Research, v. 34, p. 127-136, doi: 10.1007/s11001-013-9177-5.

Abstract: No detailed information has previously been available on the geological and geophysical characteristics of the sea floor and the underlying strata of Mersin Harbour, Northeastern Mediterranean Sea (Turkey). Continuous resistivity profiling (CRP) and borehole data from Mersin Harbour were used to interpret geoelectric stratigraphy of Neogene-Quaternary sediments in the area. This represents one of few such detailed case studies that have applied these valuable CRP techniques for the purpose of marine stratigraphic imaging. It was found that the Neogene-Quaternary sedimentary succession in the area consists of three geoelectric units (GU1, GU2, and GU3 from base to top). The lowest unit, GU1, has a resistivity value of greater than 20.0 ohm-m and consists of Miocene aged limestone and marl. The middle unit, GU2, is characterized by resistivity values ranging from 3.0 to 20.0 ohm-m. Its thickness is greater than 90 m, with the upper section being composed of stiff clay sequences which are Plio-Pleistocene in age. The uppermost unit, GU3, has resistivity values varying from 1.0 to 3.0 ohm-m. This unit displays a maximum thickness of 15 m, and is composed of Holocene muds together with gravel, sand, silt and clay (sometimes incorporating shells) materials of the Plio-Pleistocene age and their various mixtures, silty/clay limestone, and conglomerate sandstone. Comparisons of the geoelectric units with the depositional sequences interpreted from the available seismic data outwith, but close to, Mersin Harbour reveal that the geoelectric unit GU3 corresponds to the depositional sequences C (mainly Holocene) and B (mainly Plio-Pleistocene). The geoelectric unit GU2 partly correlates with the depositional sequence B which appears to be Plio-Pleistocene in age. The geoelectric unit GU1, which has not been encountered in previous seismic surveys, is a new discovery within Mersin Harbour. Limited correlation between the seismic and resistivity structures in the study area is attributed to differences in the acoustic impedance and resistivity contrasts of sub-bottom layers, as well as the penetration versus resolution performance of the systems.

References

Ball, L.B., Kress, W.H., and Cannia, J.C., 2006, Determination of canal leakage potential using continuous resistivity profiling techniques in western Nebraska and eastern Wyoming, in Proceedings, 19th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems: Seattle, WA, European Association of Geoscientists and Engineers, doi:10.3997/2214-4609-pdb.181.90.

Ball, L.B., and Teeple, A.P., 2013, Characterization of Major Lithologic Units Underlying the Lower American River Using Water-Borne Continuous Resistivity Profiling, Sacramento, California, June 2008: U.S. Geological Survey Open-File Report 2013-1050, 13 p.

Binley, A., 2015, Tools and Techniques: Electrical Methods, in Schubert, G., ed., Treatise on Geophysics: Cambridge, MA, Elsevier Science, v. 11, p. 233–259, doi:10.1016/B978-0-444-53802-4.00192-5.

Cross, V.A., Bratton, J.F., Bergeron, E.M., Meunier, J.K., Crusius, J., and Koopmans, D., 2006, Continuous resistivity profiling data from the upper Neuse River Estuary, North Carolina, 2004-2005: U.S. Geological Survey Open-File Report 2005-1306, doi:10.3133/ofr20051306.

Day-Lewis, F.D., White, E.A., Belaval, M., Johnson, C.D., and Lane Jr., J.W., 2006, Continuous resistivity profiling to delineate submarine ground-water discharge—Examples and limitations: The Leading Edge, v. 25, no. 6, p. 724–728, doi:10.1190/1.2210056.

Dunbar, J.A., Amidu, S.A., and Allen, P.M., 2008, A Study of Seasonal Salinity Variation in Lake Whitney, Texas Using Continuous Resistivity Profiling, in Proceedings, 21st EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems: Philadelphia, PA, European Association of Geoscientists and Engineers, doi:10.3997/2214-4609-pdb.177.139.

Henderson, R.D., Day-Lewis, F.D., Abarca, E., Harvey, C.F., Karam, H.N., Liu, L., and Lane Jr., J.W., 2010, Marine electrical resistivity imaging of submarine groundwater discharge: Sensitivity analysis and application in Waquoit Bay, Massachusetts, USA: Hydrogeology Journal, v. 18, p. 173-185, doi: 10.1007/s10040-009-0498-z.

Johnson, C.D., and White, E.A., 2007, Marine geophysical investigation of selected sites in Bridgeport, Harbor, Connecticut, 2006: U. S. Geological Survey Scientific Investigations Report 2007–5119, 32 p., doi: 10.3133/SIR20075119.

Loke, M.H., 2001, Tutorial: 2-D and 3-D electrical imaging surveys: 136 p.

Mansoor, N., and Slater, L., 2007, Aquatic electrical resistivity imaging of shallow-water wetlands: Geophysics, v. 72, no. 5, p. F211–F221, doi: 10.1190/1.2750667.

Mussett, A.E., and Khan, M.A., 2000, Looking Into The Earth: An Introduction to Geological Geophysics: New York, Cambridge University Press, 470 p.

Okyar, M., Yilmaz, S., Tezcan, D., and Çavaş, H., 2013, Continuous resistivity profiling survey in Mersin Harbour, Northeastern Mediterranean Sea: Marine Geophysical Research, v. 34, p. 127-136, doi: 10.1007/s11001-013-9177-5.

Ward, S.H., 1990, Resistivity and induced polarization methods, in Geotechnical and Environmental Geophysics: Tulsa, OK, Society of Exploration Geophysicists, v. 1, p. 147–190, doi: 10.1190/1.9781560802785.ch6.