Fixed Frequency Electromagnetic (EM) Profiling

Outline | Detail | Results |

OUTLINE

This is an active method that uses an electromagnetic (EM) signal to detect variations in subsurface conductivity. The EM signal is generated by a transmitter coil from where it radiates into the ground generating eddy currents within any subsurface conductors. These produce a secondary EM signal that is detected on surface by a receiver coil. The spacing between the coils determines the maximum depth of investigation in any given setting. Instruments commonly used in environmental and engineering applications are capable of site investigation to depths of between 1-30m. However, target resolution varies inversely with depth.

DETAIL

Electromagnetic profiling utilises a time-varying electromagnetic field (the primary field) to induce eddy currents within subsurface conductors. These currents result in a secondary magnetic field that is measured together with the original transmitted signal, using a receiver coil on the EM instrument. The secondary field is then separated into two orthogonal components, the real and imaginary (quadrature) components, representing respectively the vector components of the field in-phase and 90 degrees out of phase with the primary. The quadrature component provides a measure of the apparent ground conductivity whilst the real (in-phase) component is responsive to buried metallic objects.

The depth of penetration attained is dependent on a number of factors including the ground conductivity, the loop spacing and the orientation of the primary field (dipole orientation). A number of EM instruments are available which together provide a depth of investigation range of between 0.5m and 30m. The use of three or more loop spacings in both dipole orientations enables quantitative modelling of the depth to individual conductive layers. This is commonly known as EM depth sounding.

Shown on the right is an EM31 Ground Conductivity Meter manufactured by Geonics Inc. of Canada. This instrument gives a maximum depth of investigation of approximately 6m in vertical dipole mode (dependent on ground conductivity). Data from the EM31 instrument is logged automatically using an on-board data logger and can be periodically downloaded to a field computer during the day for quality checking and initial data processing.

RESULTS

The results of EM profiling surveys are normally presented as colour-coded grids (as illustrated) or less often as contour plots. The computer fits an artificial surface to the data set by dividing the area into a series of 'cells'. The value of each cell is based on the values of the closest surrounding sampling points. Each cell is then assigned a colour based on its value.

Shading of the data (as shown here) using an imaginary light source often helps enhance subtle anomalies within the data set.

Buried conductors generally appear as complex anomalies due to their dependence on the relative orientation of the transmitter-receiver intercoil axis and the target. This governs the amplitude and direction of coupling of the conductor with the primary field. Linear conductors, such as pipes or faults, will normally appear as either a single linear high or a low flanked by two lower amplitude highs, depending on whether the intercoil axis is orientated parallel or perpendicular to the strike of the conductor respectively. Readings are normally negative over large sheet conductors (such as reinforced concrete) due to saturation of the instrument electronics.

The image above represents the results of an EM31 electromagnetic profiling survey over the base of a lagoon. The NW-SE trending linear highs extending across the centre of the site represent a fault line. This intersects a number of circular anomalies (marked 3) which were interpreted as sink holes.

Time Domain Electromagnetic Sounding (TDEM)

Outline | Detail | Results |

OUTLINE

This active method involves inducing eddy currents within subsurface conductors using pulsed electromagnetic (EM) energy transmitted from a square loop of wire located on the ground. The decaying secondary EM signal induced by these eddy currents is measured over a series of time windows immediately after the transmitted signal is shut-off using the transmitter loop, or more commonly, a smaller second receiver coil located at the centre or to the side of the transmitter loop. TDEM soundings are capable of providing information on the conductivity of different layers within the subsurface to depths of between 3-1000m.

DETAIL

This active method measures the bulk electrical resistivity of the ground by inducing eddy currents in subsurface conductors using pulsed electromagnetic energy transmitted from a square loop of wire laid on the ground. The decay of these induced currents results in a decaying secondary magnetic field which is measured over up to 30 time increments ('gates') immediately after termination of the transmitter pulse. Measurement of the secondary field can be made using either the transmitter loop or more commonly with a separate receiver coil located at the centre or to the side of the transmitter loop.

In the case of horizontally layered materials the induced current loop will diffuse outwards and downwards with time whilst gradually decaying in amplitude. The speed of this diffusion and the amplitude of the secondary magnetic fields are related

to the conductivity of individual subsurface layers. As a consequence thin resistive layers are generally invisible to TDEM soundings. Use is made of a very early time (capable of 1.2msec turn-off time) NanoTEM system with Zonge GDP-32 receiver owned by GSI (UK) Ltd. This system provides improved resolution of the near surface as compared with 'traditional' TDEM systems. The depth of investigation of a TDEM survey is dependent on the moment of the transmitted signal together with the conductivity of the subsurface layers. A larger moment (achieved through an increase in the loop size and/or transmitter current) and an increase in ground resistivity will result in increased signal penetration.

Modelling of TDEM sounding data can be carried out using commercially available software programs as well as proprietary software. Background noise levels are commonly measured at each sounding location so that the quality of the data can be effectively monitored. Data falling below the measured noise level is discarded from the modelling process. A hand-held conductivity meter is used to log existing core from calibration boreholes and provide control for the modelling process.

RESULTS

Time-domain EM sounding results are generally presented as combined plots of the calculated apparent resistivity versus time and the modelled resistivity versus depth. The dotted lines in the second graph indicate equivalent resistivity models that also satisfy the observed data. Equivalence results from the inability to uniquely resolve the thickness and resistivity of a layer from its conductance value (thickness/resistivity). As long as the resistivity and thickness are changed within limits to give the same ratio there will be no appreciable variation in the apparent resistivity curve.

Where a series of TDEM soundings are undertaken along a profile or series of profiles, 2D sections can be constructed by extrapolating the various layers indicated in the 1D models. If the data set is extensive then isopach maps of the depths to individual layers may also be produced.

Time Domain Electromagnetic Profiling

Outline Detail | Results |

OUTLINE

This active method involves inducing eddy currents within subsurface conductors using pulsed electromagnetic (EM) energy transmitted from a square loop of wire located on the ground. The decaying secondary EM signal induced by these eddy currents is measured over a series of time windows immediately after the transmitted signal is shut-off using the transmitter loop, or more commonly, a smaller second receiver coil located at the centre or to the side of the transmitter loop. TDEM soundings are capable of providing information on the conductivity of different layers within the subsurface to depths of between 3-1000m.

DETAIL

Time domain electromagnetic profiling utilises a pulsed electromagnetic wave to induce secondary EM fields within subsurface conductors. The duration of these secondary fields, following shut off of the transmitter pulse, is dependent on the conductivity of the target, such that fields induced in metallic targets dissipate much slower than those resulting from ground conductivity. The secondary response is measured over a set time interval during the off period between pulses using one or more receiver coils. By measuring this response after dissipation of the ground component, the response from metallic targets is enhanced. TDEM profiling is consequently highly sensitivity to both ferrous and non-ferrous metals.

The EM61 comprises a portable coincident loop time-domain transmitter and receiver together with an additional receiver loop. The latter enables depth-to-target estimations and discrimination between near surface and deeper target response.

The EM61 uses a pulse frequency of 150Hz and a high powered transmitter providing a maximum depth of investigation of up to 5m (depending on target size). Output from the instrument is the integral of the time-gated secondary response, expressed in millivolts (mV).

Commercial software exists for estimating depth to source using the raw data from the two receiver loops. When combined with the high sampling rate of the EM61, this enables rapid screening of a site for potential hazards such as unexploded ordnance (UXO) and buried drums.

RESULTS

The results of TDEM profiling surveys are normally presented as colour-coded grids (as illustrated at left) similar to other 2D profiling methods such as magnetics and horizontal loop EM. As a result of the large range in values encountered in a TDEM profiling survey (typically 2-2000mV) the colour-coded image is normally scaled logarithmically in order to provide detail at both ends of the data range.

Very Low Frequency (VLF) Surveys

Outline | Detail | Results |

OUTLINE

Very-Low-Frequency (VLF) surveying is a continuous-wave (frequency domain) electromagnetic technique that uses low-frequency radio transmissions as the source. When these intersect a buried conductor they induce eddy currents that generate a secondary magnetic field concentric around the source of the currents. VLF surveys involve measuring the orientation of this field. As the instrument passes perpendicularly over a vertical target the vector orientation changes from a maximum on one side to a minimum on the other side. The method is primarily used in mineral exploration work but has also been successfully applied in engineering and groundwater surveys to detect conductive fault zones and other sub-vertical conductors.

DETAIL

Very Low Frequency (VLF) surveying is a continuous-wave (frequency domain) electromagnetic technique that utilises high power, low frequency radio transmissions as the source. Eleven major transmitters located across the globe generate these transmissions, providing a range of frequencies from 3kHz to 24kHz. Their primary use is in communication with submerged submarines and for long-range radio positioning.

Sensed at a distance greater than a few tens of kilometres the EM transmissions act as plane waves propagating outwards horizontally. When these waves intersect a buried conductor they induce eddy currents that generate a secondary magnetic field concentric around the source of the currents. The strength of the eddy currents is greatest when the long axis of the conductor is oriented parallel to the direction of propagation (i.e. on a radial from the active transmitter). In this orientation the magnetic vector is acting tangentially. Modern VLF instruments enable measurements to be carried out at a number of different frequencies in sequence in order to ensure optimum secondary field signal strength. However, the orientation of the survey lines still has to be chosen to lie perpendicular to the expected orientation of the targets.

VLF surveys involve measuring the orientation (tilt-angle/dip-angle) of the vector summation of the primary (horizontal) and secondary magnetic field vectors. As the instrument passes perpendicularly over a vertical target the vector orientation changes from a maximum on one side to a minimum on the other side. The point at which the reading changes from positive to negative is termed the 'cross-over' point and lies directly above the conductor. If the conductor dips then the anomaly shape will be distorted in either the positive or negative sense (depending on dip direction).

The VLF method is primarily used in mineral exploration work but has also been successfully applied in engineering and groundwater surveys to detect conductive fault zones and other sub-vertical conductors.

RESULTS

The data acquired during modern VLF surveys normally comprises at least three separate parameters of the secondary magnetic field, including the amplitude of the field and its quadrature (imaginary) and in-phase (real) components relative to the horizontal primary field. Results are presented as profiles against distance as indicated below. Filtering of the real component or dip-angle measurement is often carried out in order to produce a maximum over the crossover point (the point lying directly over the anomaly), thus aiding interpretation.

Advanced filtering of the real component of the secondary field can be used to produce a current density pseudosection as illustrated in the figure below. This provides an indication of current concentrations and their spatial distribution that approximately reflect the depth and location of sub-surface conductors.

Ground Penetrating Radar (GPR) Profiling

Outline | Detail | Results |

OUTLINE

This is an active method that uses a towed antenna to pulse microwave electromagnetic energy into the subsurface. As the polarised pulse travels downwards it interacts with materials within the ground and part of the energy is reflected back to the antenna at surface. The GPR unit measures the amplitude of the reflected signal and the time delay between the transmitted and received pulses in order to map subsurface features. The frequency of the antenna can be changed depending on the required depth of investigation and the nature of the expected target. Whilst the method is highly versatile it is not suitable for use over highly conductive ground.

DETAIL

Ground penetrating radar profiling involves transmitting pulses of electromagnetic energy at microwave frequencies (typically 50-1000MHz) into the subsurface and measuring the amplitude and travel-time of the returned signals.

The signal is introduced into the ground as polarised pulses via an antenna that produces energy of a specific central frequency. Each pulse propagates downwards through the ground where it may interact with subsurface materials in a variety of ways (these include attenuation, reflection, refraction, diffraction and scattering). However, the two most important physical conditions which impact on the behaviour of radar waves are the materials' dielectric properties and its conductivity.

The dielectric constant of the medium determine the velocity of the EM wave; the lower the dielectric the faster the propagation of the wave. A sudden reduction in the dielectric constant, such as might occur at a geological boundary, will result in an increase in the velocity of the wave and a consequent reflection of some of the energy back to the surface (analogous to the reflection of seismic energy in reflection profiling). Slowing of the EM wave also results in a concomitant energy loss. This explains why radar penetration depth is limited in a water-saturated material as groundwater has a dielectric constant of approximately 81.

The conductivity of the substrata is the most important factor determining the rate of signal attenuation. Materials with high conductivities will cause rapid dissemination of the transmitted pulse through the transformation of the EM energy into heat, as ions within the medium become excited (similar to the effect of a microwave oven). Signal loss is consequently greatest in clayey soils. Signal loss can also result from scatter of the transmitted pulse during interaction with large inhomogeneities within the subsurface, such as cobbles or bricks.

The antennae used in a GPR survey are selected on the basis of the depth of interest and the size of the target. Penetration depth varies inversely with frequency and the higher the central frequency of the antenna, the smaller the size of object that can be resolved.

GPR is currently the subject of active research with respect to contamination plume mapping and automatic target identification.

RESULTS

Ground penetrating radar data is generally presented as grey scale images (radargrams) illustrating the amplitude of the reflected radar energy against two-way time (on the vertical axis) and with distance along the survey line (horizontal axis). In the example strong reflections and multiple reflections appear as bright white areas on the radargram. These are typical of buried metallic features or voiding. The above example illustrates three adjacent underground storage tanks. The vertical lines represent survey markers indicating distance along the survey line. Where an appropriate dielectric constant is available the travel time axis can be converted to indicate approximate depth.

Controlled source AudiomagnetoTellurics (CSAMT)

Outline | Detail | Results |

OUTLINE

Conventional magneto-telluric survey techniques, such as natural-source MT and audio frequency MT, utilise the magnetic and electric components of naturally occuring magneto-telluric fields in order to map variations in subsurface resistivity to depths of up to several hundred kilometres. CSAMT is a specific derivation of conventional natural-source and audio frequency magneto-telluric methods that utilises an artificial source (typically in the range 0.1Hz to 10kHz) in addition to the natural fields. This provides a more reliable and stronger signal and enables imaging of shallower targets than would otherwise be possible with low frequency natural signals alone.

DETAIL

Temporal variations in the Earth's magnetosphere and ionosphere, caused by factors such as the solar wind and diurnal variation of the Earth's magnetic field, result in natural low-frequency magneto-telluric fields across the globe which induce alternating telluric currents within the ground. Higher frequency signals resulting from worldwide electrical storms are superimposed on these low-frequency fields. Conventional magneto-telluric survey techniques, such as natural-source MT and audio frequency MT, utilise the magnetic and electric components of the MT fields and currents in order to map variations in subsurface resistivity to depths of up to several hundred kilometres. However, the erratic nature of the source in terms of strength and direction mean that the signal has to be stacked for long periods of time at each station.

CSAMT is a specific derivation of conventional natural-source and audio frequency magneto-telluric methods, that utilises an artificial source (typically in the range 0.1Hz to 10kHz) in order to speed up data acquisition and provide a more reliable and stronger signal. The source normally comprises either a loop or long grounded dipole of up to several kilometres length. The dipole may be combined with a second orthogonal transmitter in order to provide two source polarisations. Simultaneous measurements of five separate parameters are taken at each location; the two components of the electric field and the three components of the magnetic field. Electric field measurements are acquired using orthogonal dipoles whilst the magnetic field vectors are measured using multi-turn high permeability coils. Modern CSAMT instruments also enable measurement of natural and audio-frequency MT signals in order to provide an extended exploration depth range (the lower the frequency the greater the depth of investigation).

Measurement of the change in the electric and magnetic fields over a range of frequencies enables an apparent resistivity sounding curve to be constructed. Apparent resistivity is combined with a measure of the phase difference between the electric and magnetic components. Over isotropic homogeneous ground the magnetic component will lag behind the electric component by Pi/4. However, if the resistivity varies with depth the measured phase difference will be different. Joint inversion of the data using both phase and apparent resistivity provides a more robust interpretation. The data are normally displayed as apparent resistivity versus frequency and phase difference versus frequency plots.

RESULTS

Raw results from CSAMT surveys are commonly displayed as log-log graphs of apparent resistivity and phase against frequency (see right above). However, a number of other plotting conventions may be adopted depending on the particular parameters being measured.

Combination of 1D resistivity inversions or joint phase/resistivity inversions leads to the formation of 2D pseudosections of resistivity versus depth. In the image right low resistivity areas are displayed in blue. High resistivities are in red. The low resistivity area below Station 200 displayed a coincident IP chargeability high and was later identified as a sulphide ore body.