Low Induction Number Research Articles

Mapping soil water dynamics and a moving wetting front by spatiotemporal inversion of electromagnetic induction data

AbstractCharacterization of the spatiotemporal distribution of soil volumetric water content (θ) is fundamental to agriculture, ecology, and earth science. Given the labor intensive and inefficient nature of determining θ, apparent electrical conductivity (ECa) measured by electromagnetic induction has been used as a proxy. A number of previous studies have employed inversion algorithms to convert ECa data to depth‐specific electrical conductivity (σ) which could then be correlated to soil θ and other soil properties. The purpose of this study was to develop a spatiotemporal inversion algorithm which accounts for the temporal continuity of ECa. The algorithm was applied to a case study where time‐lapse ECa was collected on a 350 m transect on seven different days on an alfalfa farm in the USA. Results showed that the approach was able to map the location of moving wetting front along the transect. Results also showed that the spatiotemporal inversion algorithm was more precise (RMSE = 0.0457 cm3/cm3) and less biased (ME = −0.0023 cm3/cm3) as compared with the nonspatiotemporal inversion approach (0.0483 cm3/cm3 and ME = −0.0030 cm3/cm3, respectively). In addition, the spatiotemporal inversion algorithm allows for a reduced set of ECa surveys to be used when non abrupt changes of soil water content occur with time. To apply this spatiotemporal inversion algorithm beyond low induction number condition, full solution of the EM induction phenomena can be studied in the future.

Three-dimensional finite volume simulation of the response of azimuth electromagnetic wave resistivity while drilling in inhomogeneous anisotropic formation

The azimuth electromagnetic wave resistivity while drilling is a new type of well logging technique. It can real-time detect the formation boundary, realize geosteering and borehole imaging in order to keep the tool always drilling in the some meaning reservoir. For effectively optimizing tool parameters, proper explanation and evaluation of the data obtained by azimuth electromagnetic wave resistivity while drilling, the efficient numerical simulation algorithm is required. In this paper, we use the finite volume algorithm in the cylindrical coordinate to establish the corresponding numerical method so that we can effectively simulate the response of the tool in various complex environments and investigate the influences of the change in formation and tool parameters on the tool response. Therefore, according to the typical coil architecture of the instrument of azimuth electromagnetic wave resistivity while drilling, we first introduce the electrical and magnetic dyadic Green's functions in inhomogeneous anisotropic formation by the electrical current source in the cylindrical coordinate. Through superposition principle, we derive the integral formula to compute the electric field intensity excited by tilted transmitter coils and the induction electrical potential on tilted receiving coils both mounded on the drill collar. Then, we use the coupled electrical potentials of the dyadic Green's functions to overcome the low induction number problem during modeling the electrical fields in inhomogeneous anisotropic formation. Furthermore, we use Lebedev grid in both and z directions to reduce the number of grid nodes, and the standard method to compute the equivalent conductivity in heterogeneous units for enhancing the discrete precision. On the basis, by the three-dimensional finite volume method, we discrete the equations about the coupled electrical potentials in the cylindrical coordinates and obtain the large sparse algebraic equation sets about the coupled electrical potentials field on the Lebedev grid. A combination of incomplete LU decomposition with the bi-conjugate gradient stabilization is used to solve the numerical solution. Finally, we validate the algorithm by comparing the numerical results obtained by two different methods, study the effects of the drill collar, anisotropy, the tilted angles of both coil, and borehole on the instrument response in inhomogeneous anisotropic formation. The numerical results show that the tool response obtained by the three-dimensional finite volume algorithm in the cylindrical coordinate system in anisotropic formation accord with that those obtained by other algorithms. The drill collar, inhomogeneous anisotropic n the formation will lead to both the smaller amplitude ratio and the smaller phase difference. In addition, when the coils of both transmitting and receiving coils are tilted, the amplitude ratio and phase difference of the tool are more sensitive to the change in formation parameter.

Geophysical characterization of permafrost terrain at Iqaluit International Airport, Nunavut

Iqaluit International Airport presently suffers from instabilities and subsidence along its runway, taxiways and apron. In particular, asphalt surfaces are significantly impacted by settlement and cracking. These instabilities may be related to permafrost, permafrost degradation and associated drainage conditions. Low induction number electromagnetic measurements along with galvanic and capacitive electrical resistivity surveys were performed over selected areas within the airport boundary and in the near vicinity to assist with permafrost characterization and to investigate active permafrost processes. Electrical resistivity images suggest distinct electrical signatures for different terrain units and sediment types, and for ice-rich material including ice wedges. Anomalous regions are identified that are coincident with localized settlement problems. Repeated resistivity maps reveal seasonal changes indicative of high unfrozen water content and freeze/thaw of groundwater beneath airport infrastructure in distinct regions related to surficial geology. Even with continuous permafrost and cold permafrost temperatures, the resistivity models reveal anomalously conductive material at depth that is not obviously correlated to mapped surficial sediments and that may represent thaw susceptible sediments or significant unfrozen water content.

Comparing one- and two-dimensional EMI conductivity inverse modeling procedures for characterizing a two-layered soil

Apparent electrical conductivity (σa) measured with electromagnetic induction (EMI) instruments is used widely as a proxy to map lateral variations in soil properties. To account for the vertical changes in soil properties, EMI inversion techniques have been developed. Improvements in EMI sensing instrumentation allow simultaneous recording of σa measurements with different depth responses, facilitating 1D and 2D inversions. In this study, different inversion procedures of EMI data were evaluated on their effectiveness to characterize the depth of the interface between two contrasting soil layers, as well as their respective conductivities. A 1D-laterally constrained inversion procedure was compared with a non-constrained, robust 1D-inversion procedure. Both procedures make use of low induction number (LIN)-approximated depth response curves and provided similar results, although calibration with soil data was essential to attribute absolute values to the inversion data. Aforementioned 1D procedures were then compared to a procedure wherein the full solution of Maxwell's equations, which describe the electromagnetic signal response into the soil, is applied. All approaches rendered similar results. This shows that despite their limitations, the cumulative approximations of the EMI signal response can be used as a valuable and effective alternative to the full solution of Maxwell equations within EMI inversion procedures, especially when the measurements are mainly situated within the LIN measurement range. In a final step, 2D inversions of the EMI data were compared to 2D-inverted electrical resistivity tomography (ERT) data. The general patterns of the resulting inverted soil models were largely comparable and consistent with the observed soil information. To conclude, the different inversion procedures revealed analogous results which were largely comparable and consistent with the soil information. However absolute values could impossibly be obtained without any prior knowledge about the vertical distribution of the soil model. Therefore, the implementation of a thorough calibration based on soil observations was essential to guide the inversion results to a realistic outcome.

Sensitivity of dipole magnetic tomography to magnetic nanoparticle injectates

Abstract We compare the sensitivity of magnetic field measurements to perturbations of magnetic permeability with the field sensitivity to conductivity alterations. The conductivity perturbations can be caused by the different conductivity of the injected and formation resident fluid and the magnetic permeability perturbations by injecting magnetic nanoparticles. We devise a simple method for calculating the magnetic permeability sensitivity required to solve parametric inverse problems in electromagnetic (EM) tomography involving magnetic permeability perturbations. We apply the method in 3D and 2D axisymmetric to both crosswell and single-well configurations in a homogeneous background. The emphasis of this paper is on measurements acquired at very low induction numbers (kr ≪ 1; k being the wave number and r the distance) where the wave length or the skin depth becomes an irrelevant scale. The sensitivity to magnetic permeability perturbations is significantly higher than conductivity perturbations at low frequencies. Furthermore, at the low-frequency regime, the magnetic permeability sensitivity is largely independent of frequency, unlike the conductivity sensitivity which improves with frequency, consistent with quasi-static dipole magnetics. The sensitivity patterns for conductivity and magnetic permeability are quite different. The four lobes with opposite signs surrounding the source and receiver in case of magnetic permeability provide opportunities for vertical discrimination of magnetic measurements. In the case of a single-well configuration, the sensitivity to magnetic permeability perturbation is larger close to the wellbore than conductivity. Also, the sign reversals of both real and imaginary parts of the magnetic permeability enhance the horizontal sensitivity of the magnetic measurements. The geometry of the problem also affects the sensitivities quite significantly. Both 3D and 2D axisymmetric are considered in this study. The geometrical spreading of the perturbed region in the axisymmetric case suppresses the sensitivity value for both conductivity and magnetic permeability. The magnetic permeability perturbations exhibit a higher sensitivity when the magnetic contrast agents are in the proximity of the source and receivers (or near the wellbore in case of single-well). On the other hand, EM measurements are more sensitive to conductivity perturbations in the interwell region in the case of a crosswell and off-wellbore in the case of a single-well system at moderate and high frequencies.

Error Analysis in Measured Conductivity under Low Induction Number Approximation for Electromagnetic Methods

We present an analysis of the error involved in the so-called low induction number approximation in the electromagnetic methods. In particular, we focus on the EM34 equipment settings and field configurations, widely used for geophysical prospecting of laterally electrical conductivity anomalies and shallow targets. We show the theoretical error for the conductivity in both vertical and horizontal dipole coil configurations within the low induction number regime and up to the maximum measuring limit of the equipment. A linear relationship may be adjusted until slightly beyond the point where the conductivity limit for low induction number (B=1) is reached. The equations for the linear fit of the relative error in the low induction number regime are also given.

Frequency domain electromagnetic induction survey in the intertidal zone: Limitations of low-induction-number and depth of exploration

Subsurface investigation in the Belgian intertidal zone is severely complicated due to high heterogeneity and tides. Near-surface geophysical techniques can offer assistance since they allow fast surveying and collection of high spatial density data and frequency domain electromagnetic induction (EMI) was chosen for archaeological prospection on the Belgian shore. However, in the intertidal zone the effects of extreme salinity compromise validity of low-induction-number (LIN) approximated EMI data. In this paper, the effects of incursion of seawater on multi-receiver EMI data are investigated by means of survey results, field observations, cone penetration tests and in-situ electrical conductivity measurements. The consequences of LIN approximation breakdown were researched. Reduced depth of investigation of the quadrature-phase (Qu) response and a complex interpretation of the in-phase response were confirmed. Nonetheless, a high signal-to-noise ratio of the Qu response and viable data with regard to shallow subsurface investigation were also evidenced, allowing subsurface investigation in the intertidal zone.

Soil Moisture Assessment over an Alpine Hillslope with Significant Soil Heterogeneity

We strive to assess soil water content on a well‐studied slow‐moving hillslope in Austria. In doing so, we employ time lapse mapping of bulk electrical conductivity using a geophysical electromagnetic induction system operated at low induction numbers. This information is complemented by the acquisition of soil samples for gravimetric water content analysis during one survey campaign. Simple visual soil sample analysis reveals that the upper material in the survey area is a spatially highly variable mixture of predominately sandy, silty, clayey and organic materials. Due to this heterogeneity, classical approaches of mapping soil moisture on the basis of stationary mapping of electrical conductivity variations are not successful. Also the time‐lapse approach does not allow ruling out some of the ambiguity inherent to the linkage of bulk electrical conductivity to soil water content. However, indication is found that time‐lapse measurements may have supportive capabilities to identify regions of low precipitation infiltration due to high soil saturation. Furthermore, the relationship between the mean electrical conductivity averaged over a full vegetation period and an already available ecological moisture map produced by vegetation analysis is found to resemble closely the relationship observed between gravimetric soil water content and electrical conductivity during the time of sample collection except for highly organic soils. This leads us to the assumption that the relative soil moisture distribution is temporarily stable except for those areas characterized by highly organic soils.

Inversion of slingram electromagnetic induction data using a Born approximation

We present an efficient approximate inversion scheme for near-surface loop-loop EM induction data (slingram) that can be applied to obtain 2D or 3D models on a normal desktop computer. Our approach is derived from a volume integral equation formulation with an arbitrarily conductive homogeneous half-space as a background model. The measurements are not required to fulfill the low induction number condition (low frequency and conductivity). The high efficiency of the method is achieved by invoking the Born approximation around a half-space background. The Born approximation renders the forward operator linear. The choice of a homogeneous half-space yields closed form expressions for the required electromagnetic normal fields. It also yields a translationally invariant forward operator, i.e., a highly redundant Jacobian. In connection with the application of a matrix-free conjugate gradient method, this allows for very low memory requirements during the inversion, even in three dimensions. As a consequence of the Born approximation, strong conductive deviations from the background model are underestimated. Highly resistive anomalies are in principle overestimated, but at the same time difficult to resolve with induction methods. In the case of extreme contrasts, our forward model may fail in simultaneously explaining all the data collected. We applied the method to EM34 data from a profile that has been extensively studied with other electromagnetic methods and compare the results. Then, we invert three conductivity maps from the same area in a 3D inversion.

Imaging of 3D electromagnetic data at low-induction numbers

We expressed electromagnetic measurements at low induction numbers as spatial averages of the subsurface electrical conductivity distribution and developed an algorithm for the recovery of the latter in terms of the former. The basis of our approach is an integral equation whose averaging kernel is independent of the conductivity distribution. That is, the recovery of conductivity from the measurements leads to a linear inverse problem. Previous work in one and two dimensions demonstrated that using a kernel independent of conductivity leads to reasonably good results in quantitative interpretations. This study extended the approach to 3D models and to data taken along several profiles over a given area. The algorithm handles vertical and horizontal magnetic dipoles with multiple separations for appropriate depth discrimination. The approximation also handles issues like negative conductivity measurements, which commonly appear when crossing near-surface conductors. This happens particularly when using vertical magnetic dipoles; whose averaging kernel has significant negative weights in the space between the dipoles, something that does not happen for the horizontal dipoles. In general, the more complex the kernel, the more complicated the signature of any given anomaly. This makes qualitative interpretations of pseudosections somewhat difficult when dealing with more than one conductive or resistive body. The algorithm was validated using synthetic data for imaging data from horizontal or vertical coils or from a combination of them. Imaging of field data from a mine tailings site recovered a shallow 3D conductive anomaly associated with the tailings.

Erratum to “Low induction number, ground conductivity meters: A correction procedure in the absence of magnetic effects” [Journal of Applied Geophysics, 75 (2011) 244–253

Erratum to “Low induction number, ground conductivity meters: A correction procedure in the absence of magnetic effects” [Journal of Applied Geophysics, 75 (2011) 244–253

Inversion of low-induction number conductivity meter data to predict seasonal saturation variation

Our research introduced a method to monitor saturation in the near surface. In agricultural settings, methods measuring electrical conductivity can provide useful information about soil type, moisture content, and salinity extent. Electrical conductivity meters have been used in a number of studies to determine soil properties in a qualitative sense. We examined the range of structures in which the use of low-induction number instruments can be used successfully to determine layered-earth electrical conductivity. We used an inversion routine which employs a Bayesian modification to the ridge-regression technique with a priori conductivity assumptions typical of agricultural areas. We performed joint inversion of horizontal and vertical dipole configurations at two coil separations for layer over half-space models with electrical properties of silt, loam, clay, and saline waters. Generally, the inversion code resolved layer thickness to better than 25% and electrical conductivity to better than 20% if the layer is less than 3-m thick. We then inverted field measurements acquired in salt-scalded areas in the Yass River Valley, New South Wales, Australia, to determine a layer over a half-space. With Kennedy’s formulation concerning the relationship between porosity, water saturation and electrical conductivity, we used the field results to predict autumn water saturation for the top layer to be 13% and the bottom layer to be 15%. In the spring, we used the field results to predict saturation of 50% for the top layer and 51% for the bottom layer, leading to a seasonal variation in soil saturation of approximately 36%. Predicted saturation was spatially consistent across the traverse line, suggesting that the developed methodology was successful.

Low induction number, ground conductivity meters: A correction procedure in the absence of magnetic effects

Abstract Ground conductivity meters, comprising a variety of coil–coil configurations, are intended to operate within the limits provided by a low induction number (LIN), electromagnetic condition. They are now routinely used across a wide range of application areas and the measured apparent conductivity data may be spatially assembled and examined/correlated alongside information obtained from many other earth science, environmental, soil and land use assessments. The theoretical behaviour of the common systems is examined in relation to both the prevailing level of subsurface conductivity and the instrument elevation. It is demonstrated that, given the inherent high level of accuracy of modern instruments, the prevailing LIN condition may require operation in environments restricted to very low (

Study of riverine deposits using electromagnetic methods at a low induction number

We conducted electromagnetic (EM) profiles along the Po River in Turin, Italy. The aim of this activity was to verify the applicability of low-induction-number EM multifrequency soundings carried out from a boat in riverine surveys and to determine whether this technique, which is cheaper than air-carried surveys, could be used effectively to define the typology of sediments and to estimate the stratigraphy below a riverbed. We used a GEM-2 handheld broadband EM sensor operating with six frequencies to survey the investigated area. Ground-penetrating radar (GPR), a conductivity meter, and a time-domain reflectometer were used to estimate the bathymetry and to measure the EM properties of the water. A global positioning system, working in real-time kinematic mode, tracked the route of the boat with centimetric accuracy. We analyzed the induction number, the depth of investigation (DOI), and the sensitivity of our experimental setup by forward modeling — varying the water depth, frequency, and bottom-sediment resistivity. The simulations optimized the choice of the frequencies that could be used reliably for the interpretation. The [Formula: see text] signal had a DOI in the Po River water [Formula: see text] of [Formula: see text] and provided sediment resistivities higher than [Formula: see text]. We applied a bathymetric correction to the conductivity data using the water depths obtained from the GPR data. We plotted a map of the river bottom resistivity and compared this map to the results of a direct sediment sampling campaign. The resistivity values [Formula: see text] were compatible with the saturated gravel and pebbles in a sandy matrix, which resulted from direct sampling and with the known geology.

Vertical Spatial Sensitivity and Exploration Depth of Low‐Induction‐Number Electromagnetic‐Induction Instruments

Vertical spatial sensitivity and effective depth of exploration (de) of low‐induction‐number (LIN) instruments over a layered soil were evaluated using a complete numerical solution to Maxwell's equations. Previous studies using approximate mathematical solutions predicted a vertical spatial sensitivity for instruments operating under LIN conditions that, for a given transmitter–receiver coil separation (s), coil orientation, and transmitter frequency, should depend solely on depth below the land surface. When not operating under LIN conditions, vertical spatial sensitivity and de also depend on apparent soil electrical conductivity (σa) and therefore the induction number (β). In this new evaluation, we determined the range of σa and β values for which the LIN conditions hold and how de changes when they do not. Two‐layer soil models were simulated with both horizontal (HCP) and vertical (VCP) coplanar coil orientations. Soil layers were given electrical conductivity values ranging from 0.1 to 200 mS m−1 As expected, de decreased as σa increased. Only the least electrically conductive soil produced the de expected when operating under LIN conditions. For the VCP orientation, this was 1.6s, decreasing to 0.8s in the most electrically conductive soil. For the HCP orientation, de decreased from 0.76s to 0.51s Differences between this and previous studies are attributed to inadequate representation of skin‐depth effect and scattering at interfaces between layers. When using LIN instruments to identify depth to water tables, interfaces between soil layers, and variations in salt or moisture content, it is important to consider the dependence of de on σa

Fundamentals of the capacitive resistivity technique

Capacitive resistivity (CR) is an emerging geophysical technique designed to extend the scope of the conventional methodology of dc resistivity to environments where galvanic coupling is notoriously difficult to achieve — for example, across engineered structures (roads, pavements), hard rock, dry soil, or frozen ground. Conceptually, CR is based on a four-point array capacitively coupled to the ground. Under certain conditions, capacitive measurements of resistivity are equivalent to those obtained with the dc technique, thus making dc interpretation schemes applicable to CR data. The coupling properties of practical sensor realizations are shown to be a function of their geometrical arrangement. Separate bodies of theory are associated with two complementary but distinct sensor types: the capacitive-line antenna and the plate-wire combination. The use of plate-wire combinations results in localized coupling, which, in conjunction with a quasi-static (low-frequency) formulation of the transfer impedance, provides a valid emulation of the dc measurement with point electrodes. A parametric study of the complex, quasi-static transfer impedance reveals the existence of a restricted range of practical parameters that permits successful operation of CR instruments at low induction numbers. Theory predicts that emulating the dc measurement is compromised if low-induction-number operation is not maintained throughout a survey area, as in a zone of high conductivity. Validation of the theory is achieved using specially designed field-scale experiments carried out with a CR instrument capable of measuring the full complex transfer impedance. At intermediate dipole separations, the results are consistent with the predictions of quasi-static theory. Deviations observed in the near and far separation zones can be explained by geometric effects or the breakdown of quasi-static conditions, respectively. Finally, under suitable conditions the phase-sensitive expression for apparent resistivity as a function of the complex transfer impedance is shown to reduce to the classical dc formula for the in-phase component. CR, while capable of emulating dc resistivity, also can be regarded as a physical complement of the inductively coupled ground-conductivity method.

Rapid non-contacting resistivity logging of core

Abstract We demonstrate a non-contact approach to whole-core and split-core resistivity measurements, imaging a 15 mm-thick, dipping, conductive layer, producing a continuous log of the whole core and enabling the development of a framework to allow representative plugs to be taken, for example. Applications include mapping subtle changes in grain fabric (e.g. grain shape) caused by variable sedimentation rates, for example, as well as the well-known dependencies on porosity and water saturation. The method operates at relatively low frequencies (i.e. low induction numbers), needing highly sensitive coil pairs to provide resistivity measurements at the desired resolution. A four-coil arrangement of two pairs of transmitter and receiver coils is used to stabilize the measurement. One ‘coil pair’ acts as a control, enabling the effects of local environmental variations, which can be considerable, to be removed from the measurement at source. Comparing our non-contact approach and independent traditional ‘galvanic’ resistivity measurements indicates that the non-contact measurements are directly proportional to the reciprocal of the sample resistivity (i.e. conductivity). The depth of investigation is discussed in terms of both theory and practical measurements, and the response of the technique to a variety of synthetic ‘structures’ is presented. We demonstrate the potential of the technique for rapid electrical imaging of core and present a whole-core image of a dipping layer with azimuthal discrimination at a resolution of the order of 10 mm. Consequently, the technique could be used to investigate different depths within the core, in agreement with theoretical predictions.

Post-Processing Calibration of Frequency-Domain Electromagnetic Data for Sea-Ice Thickness Measurements

Sea-ice thickness measurements using electromagnetic (EM) instruments require accurate data. Calibration of sea-ice thickness data acquired using a low induction number (LIN) EM sensor can be performed by conducting a geometric sounding at a range of heights over level sea ice of known thickness, and by comparing the observed data with the expected layered-earth response. Calibration corrections for scaling, phase-mixing, and zero-offset errors can be derived using least-squares inversion to minimise the misfit between the observed data and the theoretical response, and can be incorporated in modelling algorithms used to determine sea-ice thickness.This paper presents a case history illustrating identification and correction of calibration errors in frequency-domain EM data for Antarctic sea-ice thickness measurements. Comparison of coincident EM measurements made using three EM31 instruments showed that measured apparent conductivities disagreed by up to around 100 mS/m, resulting in errors in the estimated sea-ice thickness of up to 60%. Separate calibration corrections were determined for each instrument by analysis of geometrical sounding data acquired over level sea ice. Sea-ice thickness at the calibration site was determined by making a large number of drilled thickness measurements over the footprint of the EM instrument, and seawater and sea-ice conductivities were determined using independent measurements. Best-fit scaling, phase-mixing, and zero-offset errors were determined via inversion of the geometrical sounding data, with the sea-ice thickness and seawater conductivity fixed at their known values. After application of the calibration corrections, sea-ice thicknesses derived from the three instruments agreed closely with each other and with drilling results. Calibration corrections derived in this manner have been shown to be valid over a period of at least several weeks.

Calibration of a Low Induction Number Electromagnetic Instrument for Sea Ice Thickness Measurements

Sea ice thickness measurements using low induction number electromagnetic instruments require accurate electromagnetic data. Calibration of EM sea ice thickness data acquired using a shipborne low induction number electromagnetic sensor can be performed by making measurements at a range of heights over level sea ice of known thickness, and by comparing the observed data with the expected layered-earth response. Calibration corrections can be derived using least-squares inversion to minimise the misfit between the observed data and the theoretical response, and can be applied to ship-borne sea ice thickness data during post-processing. This paper presents a case history illustrating identification and correction of calibration errors in low induction number electromagnetic data for shipborne Antarctic sea ice thickness measurements. The method described could also be applied to calibration of low induction number electromagnetic instruments for conventional environmental and engineering geophysical surveys.

Ground conductivity survey of a septic system during and after failure

Editor's note: The use of trade names is for descriptive purposes only, and does not imply particular endorsement by the authors or SEG. Many people (e.g., 25% of the U.S. population) use septic systems for wastewater disposal. In the United States, county health departments typically are responsible for overseeing the installation and operation of these systems. Because most systems rely on the soil to absorb the wastewater, special attention is required where the hydraulic permeability of soils may allow contamination to reach either the surface or a nearby aquifer. Electromagnetic induction (EMI) provides an efficient means for observing septic system performance and for guiding costlier onsite investigations. In 2001, the Ft. Wayne–Allen County Health Department identified several failing septic systems in the suburbs of Ft. Wayne, Indiana, U.S., through field observation and water-quality monitoring of nearby streams. One such system had been installed in a fine-textured (35–50% clay) soil of low permeability, and serviced a residence on a two-acre lot. The system was mapped in December 2001 and again in July 2002 with the DUALEM-2 and several other EMI instruments. The DUALEM-2 is designed to measure ground conductivity at low-induction-number (LIN). The instrument contains coils that operate in both the horizontal coplanar (HCP) and perpendicular (PRP) geometries. Transmitter-receiver separation is about 2 m. At LIN, the PRP geometry is sensitive to conductivity fluctuations to a depth of about 1.2 m beneath the instrument, and the HCP geometry is sensitive to a depth of about 3 m. To evaluate the location of the septic system absorption field and contaminant distribution, the instrument was carried at low ground-clearance aligned with north-south traverses spaced at 2-m intervals (Figure 1). Although the surveys progressed along serpentine paths, the transmitter-receiver orientation of the instrument was kept consistent. Continuous measurements were recorded at walking speed (about …