Apparent Conductivity Measurements Research Articles

Design, Development and Application of a Modular Electromagnetic Induction (EMI) Sensor for Near-Surface Geophysical Surveys.

Low-frequency electromagnetic induction (EMI) is a non-invasive geophysical method that is based on the induction of electromagnetic (EM) waves into the subsurface to quantify changes in electrical conductivity. In this study, we present an open (design details and software are accessible) and modular system for the collection of EMI data. The instrument proposed allows for the separations between the transmitter to be adjusted and up to four receiving antennas as well as the acquisition frequency (in the range between 3 and 50 kHz) to permit measurements with variable depth of investigation. The sensor provides access to raw data and the software described in this study allows control of the signal processing chain. The design specifications permit apparent conductivity measurements in the range of between 1 mS/m and 1000 mS/m, with a resolution of 1.0 mS/m and with a sampling rate of up to 10 samples per second. The sensor allows for a synchronous acquisition of a time stamp and a location stamp for each data sample. The sensor has a mass of less than 5 kg, is portable and suitable for one-person operation, provides 4 h of operation time on one battery charge, and provides sufficient rigidity for practical field operations.

Frequency Domain Electromagnetic mapping for delineating subsurface structures related to the historical port of Emporiae

AbstractIn recent years, there has been a growing interest in the application of geophysical methods to reconstruct the palaeo‐landscapes of sites of special historical interest in support of the planning of archaeological researches. Given the extent of the surface to be investigated, electromagnetic methods have proven to be very suitable for their speed, resolution and versatility for this objective. In particular, coastal areas of the Mediterranean have undergone significant changes in the position of the coastline, because of changes in sea level and sediment inputs that have covered natural harbours used for the establishing colonies.In this paper, we present the results of a geophysical survey conducted using frequency‐domain electromagnetic (FDEM) method carried out to get the geometry of a coastal area near to Emporiae (NE, Spain), which was supposed to form a natural port that was used by Greeks and Romans for its first colonial settlements on the Iberian Peninsula. The results obtained from a dense network of apparent conductivity measurements, supported punctually by other geophysical data (VES and ERT) and boreholes, has allowed us to define the geometry of the basin and confirm the hypotheses of the existence of harbour buried under the coastal and alluvial sediments in the bay close to the remains from Greek and Roman times.

Determination of the conductivity and thickness of conductive layers on conductive base materials

An eddy current technique for the simultaneous determination of the layer thickness and conductivity of a uniform conductive layer on a known conductive base material is presented. Layer thickness and conductivity are determined by fitting apparent conductivity measurements taken on the surface of the part at a discrete set of frequencies to a calibration volume. The calibration volume is constructed from apparent conductivity measurements taken on a set of known layer materials, of various known thicknesses, as a function of frequency. Experimental results that support the technique are presented.

3D linear inversion of magnetic susceptibility data acquired by frequency domain EMI

Low induction number EMI instruments are able to simultaneously measure a soil's apparent magnetic susceptibility and electrical conductivity. This family of dual measurement instruments is highly useful for the analysis of soils and archeological sites. However, the electromagnetic properties of soils are found to vary over considerably different ranges: whereas their electrical conductivity varies from ≤0.1 to ≥100mS/m, their relative magnetic permeability remains within a very small range, between 1.0001 and 1.01 SI. Consequently, although apparent conductivity measurements need to be inverted using non-linear processes, the variations of the apparent magnetic susceptibility can be approximated through the use of linear processes, as in the case of the magnetic prospection technique.Our proposed 3D inversion algorithm starts from apparent susceptibility data sets, acquired using different instruments over a given area. A reference vertical profile is defined by considering the mode of the vertical distributions of both the electrical resistivity and of the magnetic susceptibility. At each point of the mapped area, the reference vertical profile response is subtracted to obtain the apparent susceptibility variation dataset. A 2D horizontal Fourier transform is applied to these variation datasets and to the dipole (impulse) response of each instrument, a (vertical) 1D inversion is performed at each point in the spectral domain, and finally the resulting dataset is inverse transformed to restore the apparent 3D susceptibility variations.It has been shown that when applied to synthetic results, this method is able to correct the apparent deformations of a buried object resulting from the geometry of the instrument, and to restore reliable quantitative susceptibility contrasts. It also allows the thin layer solution, similar to that used in magnetic prospection, to be implemented. When applied to field data it initially delivers a level of contrast comparable to that obtained with a non-linear 3D inversion. Over four different sites, this method is able to produce, following an acceptably short computation time, realistic values for the lateral and vertical variations in susceptibility, which are significantly different to those given by a point-by-point 1D inversion.

Slingram EMI Devices for Characterizing Resistive Features Using Apparent Conductivity Measurements: check of the DualEM‐421S Instrument and Field Tests

AbstractThis article addresses the characterization of resistive archaeological targets and near surface structures by electromagnetic induction (EMI). It presents tests achieved with the DualEM‐421S instrument (Dualem Inc., Milton, Canada) in order to be able to quantitatively compare these measurements to the standard technique of direct‐current (d.c.) resistivity. The test was done over the Gallo‐roman site of Vieil‐Evreux in Normandy, France and one‐dimensional (1D) and three‐dimensional (3D) inversions were applied to the data set obtained.We have first investigated the signal‐to‐noise ratio of each of the six DualEM receiver coils both in a static mode and for a quad‐pulled system. The dependence on the roll angle was also measured and it is shown that rotation of DualEM must be taken into account if the roll angle is more than ±10°. Absolute calibration and in‐phase/quadrature (out of phase) component discrimination was checked by measuring the response of a small conductive and non‐magnetic sphere. Several electromagnetic soundings by measuring the instrument response at different heights were done in order to check the quadrature (out‐of‐phase) response of the instrument. Inversions of these electromagnetic soundings were compared to d.c. vertical electric soundings (VESs) over four locations and found in accordance. Several maps using different coil configurations (HCP, VCP, PERP) and different heights were performed and inverted, both for a wide mesh (5 m) and for a finer one (0.5 m). The wide mesh allows a global and rapid description of the surface geology context (continuous d.c. measurements cannot deliver equivalent depth of investigation). The fine mesh conductivity maps clearly show the walls of a fanum (temple) as well as other structures and prove that the DualEM‐421S is able to map correctly archaeological resistive targets. These maps and their interpretations were compared to previous results obtained by d.c. technique. Copyright © 2016 John Wiley & Sons, Ltd.

Soil apparent conductivity measurements for planning and analysis of agricultural experiments: A case study from Western-Thailand

In experimental trials, the success or failure of agricultural improvements is commonly evaluated on the agronomic response of crops, using proper experimental designs with sufficient statistical power. Since fine-scale variability of the experimental site can reduce statistical power, efficiency gains in the experimental design can be achieved if this variation is known and used to design blocking, or some proxy variable is used as a covariate. Near-surface geophysical techniques such as electromagnetic induction (EMI), which describes subsurface properties non-invasively by measuring soil apparent conductivity (ECa), may be one source of this information. The motivation of our study was to investigate the effectiveness of EMI-derived ECa measurements for planning and analysis of agricultural experiments. ECa and plant height measurements (the response variable) were taken from an agroforestry experiment in Western Thailand, and their variability was quantified to simulate multiple realizations of ECa and the residuals of the response variable from treatment means. These were combined to produce simulated data from different experimental designs and treatment effects. The simulated data were then used to evaluate the statistical power by detecting three orthogonal contrasts among the treatments in the original experiment. We considered three experimental designs, a simple random design (SR), a complete randomized block design (CRB), and a complete randomized block design with spatially adjusted blocks on plot means of ECa (CRBECa). Using analysis of variance (ANOVA), the smallest effect sizes could be detected with the CRBECa design, which indicates that ECa measurements could be used in the planning phase of an experiment to achieve efficiencies by improved blocking. In contrast, analysis of covariance (ANCOVA) demonstrated that substantial power improvements could be gained when ECa was considered as a covariate in the analysis. We therefore recommend that ECa measurements should be used to characterize subsurface variability of experimental sites and to support the statistical analysis of agricultural experiments.

전자기 유도 장비를 이용한 북극해 해빙의 두께측정

The ice trials of the first Korean icebreaking research vessel "ARAON" were performed at the Arctic Ocean in July-August 2010. The sea ice concentrations of Arctic Ocean were 4/10 to 10/10 and the range of sea ice thickness was roughly 1.0 to 3.5m. In this research, sea ice thickness characteristics at the old ice floes were determined from results of drill hole and apparent conductivity measurements. Especially we measured apparent conductivity using an electromagnetic induction instrument (EM31-MK2) and estimated the sea ice thickness through the empirical equation from Cold Regions Research & Engineering Laboratory, CRREL. The results of estimated sea ice thickness were compared to drill hole measurement results and then, we suggest the new empirical equation to estimate sea ice thickness of single layer type sea ice during the summer season of Arctic Ocean by curve fitting approach to these data.

Assessment of multi-frequency electromagnetic induction for determining soil moisture patterns at the hillslope scale

Hillslopes are fundamental landscape units, yet represent a difficult scale for measurements as they are well-beyond our traditional point-scale techniques. Here we present an assessment of electromagnetic induction (EM) as a potential rapid and non-invasive method to map soil moisture patterns at the hillslope scale. We test the new multi-frequency GEM-300 for spatially distributed soil moisture measurements at the well-instrumented Panola hillslope. EM-based apparent conductivity measurements were linearly related to soil moisture measured with the Aqua-pro capacitance sensor below a threshold conductivity and represented the temporal patterns in soil moisture well. During spring rainfall events that wetted only the surface soil layers the apparent conductivity measurements explained the soil moisture dynamics at depth better than the surface soil moisture dynamics. All four EM frequencies (7.290, 9.090, 11.250, and 14.010 kHz) were highly correlated and linearly related to each other and could be used to predict soil moisture. This limited our ability to use the four different EM frequencies to obtain a soil moisture profile with depth. The apparent conductivity patterns represented the observed spatial soil moisture patterns well when the individually fitted relationships between measured soil moisture and apparent conductivity were used for each measurement point. However, when the same (master) relationship was used for all measurement locations, the soil moisture patterns were smoothed and did not resemble the observed soil moisture patterns very well. In addition the range in calculated soil moisture values was reduced compared to observed soil moisture. Part of the smoothing was likely due to the much larger measurement area of the GEM-300 compared to the soil moisture measurements.

In‐situ apparent conductivity measurements and microbial population distribution at a hydrocarbon‐contaminated site

We investigated the bulk electrical conductivity and microbial population distribution in sediments at a site contaminated with light nonaqueous‐phase liquid (LNAPL). The bulk conductivity was measured using in‐situ vertical resistivity probes; the most probable number method was used to characterize the spatial distribution of aerobic heterotrophic and oil‐degrading microbial populations. The purpose of this study was to assess if high conductivity observed at aged LNAPL‐impacted sites may be related to microbial degradation of LNAPL. The results show higher bulk conductivity coincident with LNAPL‐impacted zones, in contrast to geoelectrical models that predict lower conductivity in such zones. The highest bulk conductivity was observed to be associated with zones impacted by residual and free LNAPL. Data from bacteria enumeration from sediments close to the resistivity probes show that oil‐degrading microbes make up a larger percentage (5–55%) of the heterotrophic microbial community at depths coincident with the higher conductivity compared to ∼5% at the uncontaminated location. The coincidence of a higher percentage of oil‐degrading microbial populations in zones of higher bulk conductivity suggests that the higher conductivity in these zones may result from increased fluid conductivity related to microbial degradation of LNAPL, consistent with geochemical studies that suggest that intrinsic biodegradation is occurring at the site. The findings from this study point to the fact that biogeochemical processes accompanying biodegradation of contaminants can potentially alter geoelectrical properties of the subsurface impacted media.

Nondestructive assessment of rangeland soil depth to petrocalcic horizon using electromagnetic induction

The distribution of soil depth to cemented subsoil horizons is important for land evaluation and management, particularly in arid and semiarid rangelands, where plant growth is limited by soil depth. Traditional methods of quantifying soil depth distribution are destructive, time‐consuming, and costly. Therefore nondestructive, efficient, and inexpensive methods for assessing soil depth are needed. We investigated the effectiveness of electromagnetic induction for nondestructively assessing the distribution of soil depth to petrocalcic horizon in a semiarid rangeland landscape in central Utah. Apparent conductivity was measured with the EM31 and EM38 electromagnetic induction meters in an 11.5‐ha survey area and in four 0.1‐ha plots. Soil depth was measured in hand‐excavated pits or in hand‐augered holes. Rangeland soil depth was positively and significantly correlated with apparent conductivity. Apparent conductivity measurements made by different operators were not significantly different, indicating that different operators should not affect precision. Apparent conductivity measurements made on different days about 3 weeks apart were significantly different, indicating that regressions of apparent conductivity and soil depth should be recalibrated if data are collected over extended time periods. Plant cover measured in 1‐m‐diameter subplots was not significantly correlated with apparent conductivity and therefore does not affect the ability of the EM38 to measure apparent conductivity. In a validation subset, apparent conductivity did not accurately predict actual soil depth, nor did it predict the corred distribution of soil depth classes, but it provided rudimentary information about soil depth distribution (<50 versus ≥50 cm) that could be useful to land managers.

A transmission‐line model for electrical logging through casing

Electrical logging in cased holes is a subject of strong current interest. In this paper we develop the conventional theory of physical responses of electrical logging through casing where the formation resistivity changes vertically and where the conductance of the casing varies. The theory is developed for a four‐electrode sonde with one current electrode, and three receiver electrodes that measure current leakage into the formation. All the theoretical developments in the paper are derived using approximate transmission‐line theory. An expression for the array coefficient is first derived and then used to obtain an expression for the apparent conductivity of the formation. Then a relation is established between the array coefficient, geometrical factor and conductance of the casing. This step led to a method of measuring casing conductance. Knowing casing conductance, the response of the array to vertical changes in formation conductivity is measurable. It was established, that when the array length is less than the thickness of the formation layer, the measured apparent conductivity is equivalent to the true conductivity of the formation, unlike the case of normal and induction logs. It was also established that measurement of apparent conductivity can be made accurately inside a casing of finite length when it essentially exceeds the length of the electrode array. The results contained in this paper show that electrical logging through casing is possible in wells with conventional casing strings that penetrate layered sedimentary formations.