Electromagnetic Instrumentation for Exploration and the Environment: A Retrospective Look by Canada's Leading Manufacture
Geonics Limited has a very rich and varied history. This talk will provide a historical perspective about how a
few key individuals shaped the development of some of the world's most useful electromagnetic (EM)
geophysical instrumentation. A brief review of these systems, including the science behind them, will
showcase the evolution of each to the market place and emphasize how a combination of business savvy and
a constant investment to research is what lead to a successful line of instrumentation. In 1950 a company
called Aeromagnetic Surveys Ltd. was established that was considered "the largest and most diversified air-
survey firm in the world" (FLIGHT, 1954), for its time. It employed Vaino Ronka and Alex Herz, young engineers,
who patented several new EM technologies including an in-phase and quadrature towed bird helicopter EM
system (the first commercial transistorized instrument). The two also set new standards for ground based
horizontal loop EM systems and won several mining Blue Ribbon Awards. By the end of 1958, Mr. Ronka
began offering independent design services for geophysical instruments and it became inevitable that one day
he would form his own company. Geonics Limited was incorporated in 1962 by Vaino Ronka and Alex Herz
and the EM-16 VLF receiver, first sold in 1965, became the first successful instrument. It's considered the best
selling electrical geophysical tool of all-time and is still sold today by the same model name 44 years later. In
1974, the company was purchased by James Duncan McNeill, the former chief engineering physicist of
Barringer Research Ltd. During his time as president of Geonics he was responsible for an explosion of new
instruments from the 70's, 80's and into the 90's that permanently placed Geonics instruments in virtually every
government environmental lab and consulting firm active in near-surface geophysics. His ability to foresee
new problem areas and to define new roles that geophysical methods could play in a solution helped
established several key areas of application for Geonics instruments outside of mineral exploration. These
included civil engineering, sea-ice thickness and permafrost mapping, environmental site assessment,
groundwater exploration, agriculture and archaeology. It is safe to say that without him Geonics would have far
fewer geophysical instruments (Kaufman, 1994). Mr. McNeill sold the company to Swedish Geological AB
(SGAB), a Swedish government company which also owned ABEM in the early 90's only to purchase it back a
year later with new partner Miro Bosnar the VP and chief engineer. Mr. McNeill retired soon after and Geonics
has been owned by Mr. Bosnar, the current president and chief engineer, ever since. Mr. Bosnar has been with
Geonics since 1968 and has arguably been the most influential person in the history of the company. He has
been involved in the design, research and development of every instrument manufactured since that time and
has established him self as the most experienced and respected designer of all types of EM systems in the
world. From fixed-wing and helicopter towed-bird EM systems to the PROTEM ground transient EM system,
ground conductivity meters and a wide variety of metal detectors that have revolutionized the search for
unexploded ordnance (UXO) and improvised explosive devices (IED's). EM equipment is generally recognized
as one of the most difficult to manufacture but Mr. Bosnar continues to maintain customer loyalty through a
strong commitment to research and development and high-quality instruments, including well over 50
commercial products over the last five decades, to an ever-broadening market place.
Investigating near-surface soil conditions using multiple ground conductivity meters attached to a single platform
Our research significantly adds to the ground conductivity knowledge base. It shows how to improve the results from use of electromagnetic induction ground conductivity meters, such as the Geonics EM-31, 34 and 38. These meters measure the weighted average of the conductivity of a volume beneath the instrument. It is well known that surveying an area with several conductivity meters, each of which has a different depth of penetration, provides valuable information on conductivity changes with depth. Our research produced four major findings which will increase the usefulness of the information provided. First, the research demonstrates that the time and cost of a ground conductivity survey can be dramatically reduced when using two or more instruments simultaneously. Second, our experiments indicate that selecting certain instrument orientations and distances between the instruments substantially decreases interference between the instruments. Third, using booms to position the instruments in front of or to the side of an all terrain vehicle (ATV) reduces interference from the ATV to low levels. Fourth, counter to conventional wisdom, placing the conductivity meters above the ground rather than on the ground increases the depth of penetration when the meter is in the horizontal dipole orientation. It also increases the depth of penetration when the meter is in the vertical dipole orientation as long as the meter height is greater than 0.58 times the separation between the transmitting and receiving coils.
Investigation of Electrical Anisotropy in the Biscayne Aquifer Using a Square Array With a Multi-electrode Resistivity Imaging System
Electrical anisotropy plays a crucial role in estimating the orientation of porosity and fluid flow in porous media. Anisotropy is often measured by deploying a linear D.C. resistivity array along a range of directions and plotting the measured apparent resistivity as a function of azimuth to define an anisotropy eclipse. The square array is an alternative electrode configuration where the current and potential electrodes are deployed on opposite sides or on diagonal vertices of a square. For each square array, two measurements are made in the perpendicular (α, β) directions and one in the diagonal direction (γ). The square array has several advantages over liner arrays including faster set-up time, smaller area requirements, and greater sensitivity to anisotropy. In this study, a 28 electrode resistivity imaging system was used to investigate anisotropy in the Biscayne aquifer near Miami, FL. Electrodes were placed at equal distances on a circle forming 7 separate square array configurations at 12.86° intervals. For each circle, 42 separate measurements were recorded corresponding to the α, β, and γ configurations rotated through 180°. The radius of the circular array was expanded from 4 m to 32 m in order to investigate the variation of anisotropy with depth. Measurements at all radii resolved well defined anisotropy ellipses with a coefficient of anisotropy of around 1.01 and a direction of maximum apparent resistivity of around 141°. This anisotropy may be due to enhanced secondary porosity in the perpendicular (NE-SW) direction. This study demonstrates that even small values of electrical anisotropy may be effectively and efficiently measured with a multi-electrode resistivity system.
Remote Resistivity Contrast Mapping With Capacitive Sensors and an Inductive Source
The RMIT University Geophysics group has developed a fieldworthy prototype instrument for the remote measurement and mapping of resistivity contrasts. The receiver consists of several electrodes that are capacitively coupled to the ground electric field, but guarded in such a way as to remove any sensitivity to electric fields in the air above. The transmitter is a multiturn, closed-loop, inductive source that is resonated at a low enough frequency that the field seen at the receivers is in the resistive limit. Modelling shows that the instrument is sensitive only to lateral resistivity contrasts, and is fundamentally insensitive to horizontal layering. Further, the choice of base frequency at the resistive limit ensures that materials with the conductivity of typical soils including clays are transparent to the EM energy. We have tested two generations of the instruments over laboratory and field targets, and achieved measurements consistent with target expectations and numerical modelling. The system is particularly sensitive to buried resistors. It is also a very sensitive detector of disturbed ground, such as trenches, which even if in-filled tend to have a resistivity contrast with their surroundings. Tests on an archaeological site in Rome confirmed the consistency of our measurements with conventional dipole-dipole resistivity tomography. The advantage of this system using non-contacting measurements lies in the speed of data acquisition compared to that achieved with conventional resistivity arrays.
Characterization of a Former Uranium Mill Site Using Frequency-Domain Electromagnetic and Electrical Resistivity Surveys
Frequency-domain electromagnetic induction (FDEM) surveys were conducted at a former uranium mill site near Naturita, Colorado, to rapidly characterize site lithology and estimate variations in site hydrogeologic properties. The FDEM surveys were performed by towing a multi-frequency EM instrument behind an all-terrain vehicle permitting rapid high-density data collection over the 1.5 km x 0.5 km study site. Analysis of FDEM apparent conductivity maps was used to determine the placement of several electrical resistivity tomography (ERT) transects across areas of interest and adjacent to monitoring wells. Direct correlation between FDEM apparent electrical conductivity data and hydraulic conductivity measurements, recorded at discrete borehole intervals from slug tests, is poor due to the high electrical conductivity in this environment. FDEM apparent conductivity is an effective measurement over an integrated volume on the order of several cubic meters resulting in poor correlations with data recorded at discrete depth intervals. To fully capitalize on the information content of the FDEM data, we inverted the raw apparent electrical conductivity data using a code based on a one-dimensional (1D) forward model and two-dimensional (2D) regularization. This approach produces a three-dimensional (3D) volume of subsurface electrical conductivity. Correlation between measured hydraulic conductivity and inverted EM model conductivities, at depths of the well screens, appears promising and is the subject of ongoing research. The ERT data were also inverted to produce 2D cross sections. Where possible, ERT and FDEM conductivity models were correlated with water depths, lithologic logs, and hydraulic conductivity measurements from nearby monitoring wells. In general, ERT and FDEM results compare favorably, with the former providing information about deeper structures and the latter providing higher density spatial information.
Geophysical Monitoring of a Large-Scale Infiltration in a Heterogeneous Urban Fill
The goal of this study was to assess the contribution of electrical resistivity tomography coupled with flow modeling to understand the hydrodynamics of highly heterogeneous anthropogenic soils, namely urban fills, in the context of groundwater protection. Urban fills are generally made of a heterogeneous mixture of various wastes and soil and they are usually considered as contaminated land in most legislations. Moreover, the heterogeneous structure of these fills, and hence its influence on water flow, is difficult to characterize using conventional methods based on borehole drilling. In contrast, geophysical methods can sample a larger volume while producing a high density of data, and bridge the data gaps left by conventional methods. To characterize the influence of structure heterogeneity on flow dynamics, a large-scale controlled infiltration experiment was conducted and monitored using electrical resistivity tomography. The apparatus used was a Syscal Pro (IRIS instruments). The study area was 5x4 m2 and the thickness of the fill at the site was 2.5 to 3m. A controlled irrigation of the test surface was performed with a rainfall simulator at an intensity of 31 mm/h. Resistivity measurements were made using a grid 9x7 electrodes with a 1m spacing. Thirty others electrodes were installed in five boreholes drilled at the corners and the center of the test area. Water content was measured at 4 locations and at different depths to confirm the observed variations in electrical resistivity. Following the infiltration, the test area was excavated to confirm the nature of geophysical anomalies. The inversion of the data was performed with the RES3DINV software. The results show the usefulness of the method to characterize the infiltration in these very heterogeneous environments. The resistivity maps show that the flow is controlled by the distribution of the different materials in the fill allowing the identification of zones of preferential flow. Fill samples were taken to determine the water retention characteristics of the fill's constitutive materials using the Arya and Paris water retention model. Geophysical, excavation and water retention data were used to construct a water flow model using HYDRUS. Flow modeling has allowed the comparison of the distribution of moisture contents and the values of resistivity. The conversion of resistivity maps in soil moisture content maps was performed using the Friedman and Mualem's model which was modified by adding a term for surface resistivity. The calculated and measured resistivities were different. However, the distributions of calculated and measured resistivities were in reasonable agreement.
Relating Engineering and Petrophysical Properties of Unconsolidated sediments to Electrical parameters: Laboratory Measurements
Predicting the engineering properties of soils using non-invasive and cost-effective geophysical measurements is of interest to civil engineers involved in hte assessment of strength and stability of subsurface earth materials.. The effective use of geophysical techniques however demands adequate knowledge of how the fundamental petrophysical properties of a soil, which affect its strength and stability also influence its geophysical response. We investigate via laboratory experiments, how the composition and textural properties of soils which affect their stability and strength conditions also influence their electrical responses. Laboratory measurements of the spectral electrical response(0.01Hz to 10 kHz) of field samples with wide variability in physical and engineering properties are performed. Textural properties that are easily obtained from grain size distribution analysis, for example, fractal dimension, pore size parameter and specific surface area of the soil per unit mass and amount of fines are given attention. Electrical parameters which describe the electrical response of the model are extracted and their variations with the soil properties are analyzed. This work serves as a contribution toward the understanding of how engineering properties of unconsolidated geomaterials affect their electrical response. Such an understanding is valuable in the quest of using geoelectrical methods to solve engineering and environmental problems.
Joint application of GPR and electrostatic resistivity to assess mixed pavement condition
In planning maintenance and rehabilitation of paved streets it is of first importance to gather internal structure information to establish a diagnostic. We investigate the potential of the GPR and of the capacitively-coupled resistivity array profiling techniques to map the geometry and the defects present at various depths in streets with mixed pavement. GPR is excellent at delineating boundaries of material with contrasting electrical properties whereas the resistivity array is needed to determine the nature and quality of the imaged material. Thicknesses of asphalt and concrete can be continuously determined. Defects such as cracks, delaminations, voids and former repairs can be mapped. Quality of the concrete slab can be assessed by resistivity. The performances of the two techniques are demonstrated first using numerical modeling and imaging of typical pavement defects. Resistivity and GPR data were collected along a few streets in Montreal using a 1GHz GPR smart cart and a compact 2-receiver dipole resistivity system. Streets were selected to demonstrate the responses to different pavement defects. The results allow to show the performance and limitations of present systems. In particular, it is shown that multiple configuration arrays and real-time imaging for the resistivity pulled array are needed. For the arrays we investigate some designs and for the real-time imaging a technique based on Kalman filtering was developed.