Introduction
The earliest practitioners of geophysical EM varied
the transmitter-receiver geometry to change the portion of the earth sampled by
their systems. The first practitioner
to define the sampling volume of a system was Doll (1949), with the
introduction of the induction logger.
Geometric-sounding EM (G-SEM) systems, such as the
induction logger, remain unique among geophysical EM systems for the constancy
of their sampling volumes and the ease of interpretation of their measurements.
Following theoretical development by Wait (1962),
Howell (1966) constructed a G-SEM instrument for use at the surface. Further development by McNeill (1980)
facilitated the adoption of G-SEM systems for a variety of near-surface
applications.
An enhanced G-SEM system, which simultaneously
samples two volumes-of-earth, is presented herein, along with a classification
of G-SEM applications.
Surface G-SEM
Instruments
In contrast to systems for which the sampling volume
is related to operating frequency or time-interval of measurement, the
separation and orientation of the transmitter and receiver determine the
sampling volume of G-SEM systems.
For efficiency of operation, G-SEM instruments
transmit a time-varying sinusoidal magnetic field. The sampling volume remains solely dependent on geometry as long
as the frequency of the field is consistent with the low-frequency-approximation,
as defined by Wait (ibid.):
where i is
the square-root of –1, s0 is the conductivity and m is the permeability,
respectively, of the material in the volume-of-exploration, w is the angular frequency of
the transmitted field, and r is the spacing between the transmitter and receiver.
Wait (ibid.) analyzed the response of a
perpendicular system, i.e. where the windings of a transmitting coil are
horizontal, and the windings of a receiving coil are vertical and perpendicular
to the location of the transmitter.
Figure 1 is a schematic profile of a perpendicular system over the
earth.
Figure 1: Perpendicular system.
For instruments used above the surface, the
cumulative response of the earth to a given depth is convenient both as a
measure of the sampling volume and as a guide to interpretation. After Wait (ibid., 1982), the cumulative
response of the perpendicular system, in relative terms, is:
where h is
the depth in the earth, in units of the separation between the transmitter and
receiver.
Howell’s (ibid.) instrument incorporated a
perpendicular system but, due in part to design tolerances, this system became
much less popular than the horizontal co-planar system, in which the windings
of the transmitter and receiver are horizontal and co-planar.
Figure 2: Horizontal co-planar
system.
Figure 2 is a schematic profile of the horizontal
co-planar system; its cumulative response (after McNeill (ibid.)), in the terms
used previously, is:
The cumulative responses for the perpendicular
system and horizontal co-planar system are plotted in Figure 3.
A depth-of-exploration (DOE), beyond which a system
has little sensitivity, may be inferred from its cumulative response. McNeill (ibid.) has proposed 1.5
transmitter-receiver separations as the DOE for the horizontal co-planar
system, and this value has become widely accepted. A corresponding DOE for the perpendicular system is 0.6
separations.
Figure 3: Accumulation of
response with depth.
Enhanced G-SEM
System
Users of G-SEM frequently need more than a single DOE to assess the subsurface feature-of-interest. A common technique to address this need is to rotate a horizontal co-planar system about its axis-of-separation to orient its coils vertically. After McNeill (ibid.), the DOE of this vertical configuration is 0.75 separations.
G-SEM instruments require no contact with the ground. This enables G-SEM surveys to be conducted continuously at walking speed. To obtain dual depths-of-exploration with a horizontal co-planar system, however, requires halting the survey, taking a measurement, rotating the coils and taking a second measurement. This makes the survey slow and laborious, and discourages full use of the technique. The need for efficient dual-depth surveys motivated the development of an enhanced G-SEM system.
In recent years, much greater progress has occurred
in electronic-signal processing than with portable power-supplies. As a result, EM receivers have advanced much
further than EM transmitters, and the addition of a receiver adds little to the
weight or power-requirement of a system.
Figure 4 shows a system with one transmitter and two
receivers that is equivalent to a perpendicular system combined with a
horizontal co-planar system. This dual
system can sound two volumes-of-earth simultaneously.
Figure 4: Dual-geometry EM
system.
The independence of the simultaneous soundings given
by the dual system, as indicated by DOE, is greater that of horizontal/vertical
co-planar soundings. Thus, in addition
to surveying convenience, dual-system soundings yield slightly better
resolution of the geo-electric section.
Single-receiver G-SEM instruments have encountered
increasing competition from frequency- and transient-EM devices that
incorporate advances in electronic design.
The dual system incorporates such advances as well, and may encourage
users to take renewed advantage of the simplicity of both operation and
interpretation inherent in G-SEM.
G-SEM
Applications
The number and range of G-SEM applications increased
rapidly after 1980 with public interest in environmental monitoring and
groundwater assessment. The
categorization and examples of applications presented herein are drawn from an
annotated bibliography of more than 250 entries maintained by Taylor (1999).
The basis of the applications is contrast in
geological EM properties, especially conductivity, for which Keller (1988) and
Palacky (1988) provide excellent summaries.
A useful distinction may be made between two groups
of applications. The first group deals
with geological exploration, and the second group deals with the detection (and
monitoring) of anthropogenic features.
Although the characteristics of some features may be identical across
group definitions, the preparation and techniques for surveying are influenced
by group considerations, such as the physical surrounding and social
significance of the feature.
The most common feature for geological exploration
is the freshwater aquifer; other features are mineral resources, cavities,
faults and soils.
Freshwater aquifers that are more conductive than
their hosts tend to be on-or-near the surface of buried igneous- or
carbonate-bedrock. The largest
conductivity-contrasts occur where igneous bedrock is deeply weathered, and the
aquifer matrix consists of weathered material confined in a trough or fracture
zone. Due to lower contrast with
bedrock, aquifers of coarse glacial-or-other sediments present a greater
challenge for EM, but one that many times has been successfully met.
Freshwater aquifers are less conductive than their
hosts when they are confined by conductive materials, such as shales, clays and
saline groundwater. In these
situations, surveyors seek depressions in the conductive material that may be
filled with pools of freshwater.
Clay-caps on kimberlite pipes and bitumen-saturated
sands have been mineral-resource targets of G-SEM surveys, but exploration for
aggregates is a more frequent application.
Aggregates, which are relatively free of clay, present a resistive zone
if they are enclosed laterally and at depth by finer, more conductive material.
In general, soils are amenable to surveying with
G-SEM, due to their layered structure and the link between clay-content and
conductivity. Geotechnical
investigation is the principal application for mapping soils and frozen
ground. Other applications deal with
soil salinity, with regard to agricultural productivity or the corrosion of
buried utilities.
Faults and cavities become more easily mapped as the
conductivity increases of the material they contain. Air-filled structures are undetectable if they are hosted by
resistive material, and difficult to interpret if they are hosted by conductive
material.
The second group of applications deals with man-made
features. Some of these, such as buried
metal and disturbed ground, are unchanging and one-time surveys are designed
for their detection. Others, resulting
from the chemical contamination of soil and groundwater, may be repeated as
part of a monitoring process.
Although the confined shape and conductivity of
buried metal-objects cause G-SEM systems to respond beyond the
low-frequency-approximation, metal detection is an important application as it
is intimately related to conductivity sounding for archaeology, environmental assessment
and forensic investigation.
Disturbance can alter the conductivity of the
ground, by changing its physical or its electrical structure. Through careful use, G-SEM systems have
detected varied features, such as ancient burial-excavations and clandestine
tunnels. G-SEM systems have also
located sites of ancient fires, through the altered magnetic susceptibility of
the heated ground.
The most frequent G-SEM application is the
delineation of conductive contamination in soils and groundwater. Fertilization, irrigation and other
practices change the conductivity of agricultural soil. The decomposition of domestic waste produces
salts and acids that give landfill-leachate its conductivity.
Many industrial and commercial activities generate
conductive wastes, such as the processing of agricultural products, chemicals,
coal, forest products, metals, minerals and ores, as well as petroleum
production, power generation and road maintenance.
Applications that require special expertise in
selected situations are the monitoring of hazardous wastes and contamination
from organics.
Summary
G-SEM is unique among EM techniques for the
simplicity with which its sampling volume is defined and its results are
interpreted.
The first G-SEM instrument developed for use at
surface incorporated a perpendicular arrangement of transmitter and
receiver. Almost all instruments in
current use incorporate a horizontal co-planar arrangement.
A G-SEM system that incorporates a horizontal
transmitter, a perpendicular receiver and a horizontal co-planar receiver
sounds two sampling-volumes simultaneously, yielding additional information
about the geo-electric section. This
dual system represents a practical implementation of advances in
electronic-signal processing.
The dual system is suitable for the wide range of
G-SEM applications, which include the monitoring of contamination in soil and
groundwater, the detection of disturbed ground and buried metal for
archaeology, environmental assessment and forensic investigation, and
exploration for groundwater, minerals, soils and geological structures.
References
Doll, H.G., 1949, Introduction to induction logging
and application to logging of wells drilled with oil base mud: Journal of
Petroleum Technology, 1, 148-62.
Howell, M., 1966, A soil conductivity meter:
Archaeometry, 9, 20-23.
Keller, G.V., 1988, Rock and mineral properties:
Electromagnetic Methods in Applied Geophysics-Theory, 1, 13-51.
McNeill, J.D., 1980, Electromagnetic terrain
conductivity measurement at low induction numbers: Geonics Ltd., Technical Note
TN-6.
Palacky, G.J., 1988, Resistivity characteristics of
geologic targets: Electromagnetic Methods in Applied Geophysics-Theory, 1, 53-129.
Taylor, R.S., 1999, An annotated bibliography of
applications of geo-conductivity meters: Dualem Inc.
Wait, J.R., 1962, A note on the electromagnetic
response of a stratified earth: Geophysics, 27, 382-85.
Wait, J.R., 1982, Geo-electromagnetism: Academic
Press Inc.
Addendum
The preceding document discusses the cumulative
sensitivities with depth of the perpendicular (PRP) and horizontal co-planar
(HCP) geometries. Cumulative
sensitivities are useful for the quantitative interpretation of conductivity
layering within the DOE of a G-SEM instrument.
To supplement the description of the volumes sampled
by G-SEM, this addendum discusses the incremental sensitivities with depth of
the HCP and PRP geometries, and illustrates them (see Figure 5) for a popular
G-SEM instrument, the DUALEM-2. The
Tx-Rx separation of the DUALEM-2 is 2 m for the HCP geometry, and 2.1 m for the
PRP geometry.
The formulae for the incremental sensitivities are
taken from Wait (1962) and McNeill (ibid.).
Figure 5 shows that the PRP geometry of the DUALEM-2
is most sensitive to the earth at the instrument (i.e. zero depth). Sensitivity decreases gradually to a depth
of about 0.2 m, and then more rapidly.
Sensitivity at the nominal PRP DOE of 1.2 m is about one-fourth of the
maximum at the instrument.
In contrast, the figure shows the HCP geometry is
insensitive to the earth at the instrument, but the sensitivity rises rapidly
to peak at a depth of about 0.7 m.
Sensitivity at the nominal HCP DOE of 3 m is, again, about one-fourth of
the maximum.
Figure 5: Incremental sensitivity
with depth.
Without consideration of incremental sensitivity,
users of G-SEM might assume that the instrument samples somewhat uniformly to
the DOE. A better guide might be the
depths between which the sensitivity exceeds, say, half of its maximum
value. Using this guide for the
DUALEM-2, we would conclude that the PRP geometry samples from the instrument
to a depth of about 0.8 m, and the HCP geometry samples between depths of about
0.2 m and 2 m.
Since G-SEM sensitivity is linearly proportional to
Tx-Rx separation, the curves of Figure 5 are valid for other instruments with
different separation, provided that the depth scale is adjusted
accordingly. For example, the depth
scale for the DUALEM-4 would range from 0 to 6 m.
Thus we can generalize the half-sensitivity guide
for all G-SEM instruments, e.g. the PRP geometry samples to a depth of about
four-tenths the Tx-Rx separation, and that the HCP geometry samples between
depths of about one-tenth and one Tx-Rx separation.