GeoExplo Ltda.
Geophysical Airborne Survey
ElectroMagnetic Methods (EM)

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Tel(56-2)326-5116
E-mail: surveys@new-sense.com Dr. W.E.S.(Ted) Urquhart

Electro-Magnetic Airborne Surveys

Abstract

This paper on airborne Electro-magnetic (AEM) techniques deals with a number of topics relating to airborne EM survey systems and methods. These AEM topics include: Basic Principles, Transient Airborne Electro-magnetics, Frequency Domain Airborne Electro-magnetics, Airborne VLF Electro-magnetics, Factors Affecting Detectability, Combined AEM/Magnetometer Surveys, Survey Data Presentation and Interpretation.

 
Other Useful Links
The Berkeley Course in Applied Geophysics (EM) The Berkeley Course in Applied Geophysics (EM)
 

Table of Contents

3. AIRBORNE ELECTROMAGNETIC SURVEYS
      3.1 Basic Principles
           3.1a Transient Airborne Electromagnetics
            3.1b Frequency Domain Airborne Electromagnetics
           3.1c Airborne VLF Electromagnetics
      3.2 Factors Affecting Detectability
      3.3 Combined AEM/Magnetometer Surveys
      3.4 Survey Data Presentation
      3.5 Interpretation
            3.5a Other Interpretation Methods
Appendix 1. Typical Electrical Properties
                    Selected Bibliography

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3. Airborne Electromagnetic Surveys

The general objective of AEM (Airborne ElectroMagnetic) surveys is to conduct a rapid and relatively low-cost search for metallic conductors, e.g. massive sulphides, located in bed-rock and often under a cover of overburden and/or fresh water. This method can be applied in most geological environments except where the country rock is highly conductive or where overburden is both thick and conductive. It is equally well suited and applied to general geologic mapping, as well as to a variety of engineering problems (e.g., fresh water exploration.)

Semi-arid areas, particularly with internal drainage, are usually poor AEM environments. Tidal coasts and estuaries should be avoided. Weathered maific flows can provide strongly conductive backgrounds, particularly flows of Tertiary or Quaternary age.

Conductivities of geological materials range over seven orders of magnitude, with the strongest EM responses coming from massive sulphides, followed in decreasing order of intensity by graphite, unconsolidated sediments (clay, tills, and gravel/sand), and igneous and metamorphic rocks. Consolidated sedimentary rocks can range in conductivity from the level of graphite (e.g. shales) down to less than the most resistive igneous materials (e.g. dolomites and limestones). Fresh water is highly resistive. However, when contaminated by decay material, such lake bottom sediments, swamps, etc., it may display conductivity roughly equivalent to clay and salt water to graphite and sulphides.

Typically, graphite, pyrite and or pyrrhotite are responsible for the observed bedrock AEM responses. The following examples suggest possible target types and we have indicate the grade of the AEM response that can be expected from these targets.

Conductive targets can be concealed by other geological conductors, "geological noise", such as:

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3.1 Basic Principles

Electromagnetic-induction prospecting methods, both airborne and (most) ground techniques, make use of man-made primary electromagnetic fields in, roughly, the following way: An alternating magnetic field is established by passing a current through a coil, (or along a long wire). The field is measured with a receiver consisting of a sensitive electronic amplifier and meter or potentiometer bridge. The frequency of the alternating current is chosen such that an insignificant eddy-current field is induced in the ground if it has an average electrical conductivity,

If the source and receiver are brought near a more conductive zone, stronger eddy currents may be caused to circulate within it and an appreciable secondary magnetic field will thereby be created. Close to the conductor, this secondary or anomalous field may be compared in magnitude to the primary or normal field (which prevails in the absence of conductors), in which case it can be detected by the receiver. The secondary field strength, Hs, is usually measured as a proportion of the primary field strength, Hp, at the receiver in percent or ppm (parts per million).

Anomaly = Hs / Hp.

Increasing the primary field strength increases the secondary field strength proportionally but the "anomaly" measured in ppm or percent remains the same.

Figure 3.1-1, from Grant and West, illustrates the general principle of electromagnetic prospecting.

EM responce sketch

Figure 3.1-1: A generalized picture of electromagnetic induction prospecting.

Prospecting for anomalous zones is carried out by systematically traversing the ground either with the receiver alone or with the source and receiver in combination, depending on the system in use. In the case of airborne systems, the receiver coils are usually in a towed bird and the transmitter may be a large coil encircling a fixed wing aircraft, e.g. INPUT systems, or one or more small coils in the same bird that houses the transmitting coils, e.g. most HEM (Helicopter EM) systems.

There are two different basic systems commonly used to generate and receive the electromagnetic field: transient or "time domain" systems like INPUT,GEOTEM and MEGATEM and a/c. "frequency domain" systems like most HEM systems.

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Transient Airborne Electromagnetics

Historically, the most commonly encountered system of this type was the INPUT system. The newer systems GEOTEM and MEGATEM (Fugro Airborne Surveys) function in a similar way to INPUT, Thus for symplicity we will examine only the INPUT system. For those who would like to know more about the newer systems please link to GEOTEM, MEGATEM, or TEMPEST of Fugro Airborne Surveys.

In the INPUT system the transmitting coil, usually encircling a fixed wing aircraft, is energized by what is, essentially, a step current. In the absence of conductors, a sharp transient pulse proportional to the time derivative of the magnetic field is induced in the receiver. When a conductor is present, however, a sudden change in magnetic field intensity will induce in it a flow of current in the conductor which will tend to slow the decay of the field. Figure 3.1-2 illustrates this situation. The switching is repeated several times a second as the aircraft follows its flight line, so that the signal is virtually continuous.

The receiver "listens" only while the transmitter is "quiet" so that problems arising out of relative motion between transmitter and receiver, because the receiver is towed in a bird behind the aircraft, are virtually eliminated. Moreover, if the entire decay of the secondary field could be observed, the response would be equivalent to AC measurements made over the whole of the frequency spectrum. It is important to note in this connection, however, that not the decay function itself but only its time derivative can be recorded if a coil is used as the detector. This means that the anomalous fields which decay very slowly are suppressed in amplitude more than the others, and since these are the very ones generally associated with good conductors, there would seem to be an inherent weakness in this system. Because it is difficult to precisely synchronize the instant when the transmitter becomes "quiet" with the instant that the receiver begins to "listen", it is nearly impossible to record the entire function. This is equivalent to being unable to record many of the lower frequencies in the a-c spectrum. Th should be noted, however, that in the past several years, significant progress has been made in measuring the early time response.

Shetch of INPUT responce

Figure 3.1-2: A sketch of the INPUT transient airborne EM system operation. The primary field is a step function and the receiver records the decay of the field after the transmitter stops transmitting. (Grant and West 1965)

Typically, the time derivative of the decay function is measured using from six to twelve different time delays from the instant that transmitter stops transmitting before recording the signal received.

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3.1b Frequency Domain Airborne Electromagnetics

Historically, all helicopter-borne electromagnetic (HEM) systems, where of this type. There are a number of newer systems that employ the transient technique similar to the INPUT system but these will not be discussed here as they are as yet not widely used.

In the typical frequency domain helicopter EM system (HEM) both the transmitting coil set and the receiver coil set are housed in a rigid boom or "bird" that is towed beneath the helicopter. Commonly, this boom is from three to five meters long and contains from two to six coil pairs. Usually, half of the coils in each of the transmitter set and the receiver set are "co-axial", i.e. an axis normal to the plane of the coils passes through the centre of both coils. The second half of the coil sets are normally "co-planar", being equivalent to both the transmitting and receiving coil lying flat on the ground. Other coplanar orientations have been used occasionally. A diagram of this system is shown in figure 3.1-3. For clarity, the boom is shown over sized in this diagram. Note the stabilizing airfoil attached to one end of the bird.

Drawing_of_a_typical_EM_bird_system.GIF

Figure 3.1-3: Sketch of a typical HEM system configuration.

This system operates in precisely the manner described in section 3.1. The receiver measures the in-phase and out-of-phase, or quadrature, of the secondary field, expressed in ppm of the primary field. As we will discuss in the interpretation section, the two different coil orientations provide data that is useful in discriminating between dike like conductors that have considerable vertical extent and may be ore bodies, and horizontal sheet like conductors that are simply conductive overburden. The two coil orientations also provide additional information about the geometry of the target body. As is illustrated in the diagram, the system includes a second bird carrying a magnetometer. The magnetic data is often useful in discriminating between metallic and non metallic conductors and to assist in interpreting the geological setting of the conductor. Sometimes a VLF receiver is also included.

Picture_of_HEM_system_during_launch

Figure 3.1-4 shows a photograph of one of a typical HEM systems being launched for survey operations. This system includes co-axial and co-planar coil pairs to measure the electromagnetic field at four frequencies simultaneously. "Clicking" the mouse over the picture will enlarge the picture (27kb).

Figure 3.1-4: The typical HEM bird configuration being launched for survey operations. Note that this system also includes a magnetometer bird between the helicopter and the EM bird.

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3.1c Airborne VLF Electromagnetics

With VLF systems the primary field is supplied by powerful radio transmitters used for military communications and navigation. The receiver usually consists of a coil and supporting electronics towed in a bird. Figure 3.1-5 shows the positions of current VLF EM transmitters and approximate ranges of reception.

VLF station locations

Figure 3.1-5: The locations and ranges of VLF EM transmitting stations.

Because the available frequencies are high (15-22 Khz) the systems are particularly susceptible to geologic noise. Also, because the transmitters are controlled by the military, they may not always be operating for the entire period that a survey is in progress. They are also limited in terms of available primary field directions which will not always be well coupled with the favorable geologic strike.

Note: A number of the stations shown in the above picture are no longer operating.

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3.2 Factors Affecting Detectability

There are at least six factors that determine whether or not a particular conductor will be detectable with any EM system.

1. Signal-to-noise ratio:

In practice, because of "system noise" (Ns) and "geological noise" (Ng), the ability of a system to recognize and measure an anomaly is limited by the "signal-to-noise" ratio:

Signal-to-noise = Hs / (Ns + Ng)

Because Hs and Ng are proportional to the primary field strength Hp, and Ns, in frequency-domain systems, usually contains elements proportional to Hp, there is little to be gained by increasing the primary field power. In time domain systems Ns is not greatly affected by Hp, so extra power does result in increased signal-to-noise. Attempts to increase the signal-to-noise are sometimes made by increasing the distance between the transmitter and receiver. This results in roughly the same Hs and Ng but often a lower system noise Ns. However the longer bird required to achieve this is more prone to flex, and thus may actually display increased system noise Ns. In addition, the larger bird is heavier and more difficult to handle and thus may reduce survey productivity, increasing cost. In conductive areas Ng may be higher, thereby offsetting any advantage of lower Ns.

2. Penetration

The penetration of an AEM system is its effective depth of exploration. Commonly, this is taken to include the elevation of the system above ground, as this is also affected by local environment and flying conditions.

In general, systems with large transmitter-receiver coil separation, usually referred to as Tx-Rx, have greater penetration than those with small separations. Penetration is closely related to signal-to-noise, as the system that produces a larger anomaly from a given conductor can, of course, look further into the ground. Penetration is usually defined as the maximum depth at which a large vertical sheet will produce a recognizable anomaly of at least twice the amplitude of the system noise.

3. Discrimination

The discrimination of an AEM system is the ability of the system to differentiate between conductors of different physical properties or geometric shapes. Discrimination, particularly between flat lying surficial conductors and steeply dipping conductors, is vitally important. Good discrimination can be achieved in HEM systems by using several frequencies and both co-axial and co-planar coil pairs.

4. Resolution

Resolution refers to the ability of an AEM system to recognize and separate the interfering effects of nearby conductors. A system that does this well also produces sharp anomalies over isolated or discrete conductors. Resolution generally increases with decreasing flight elevation and coil separation. Typically the HEM systems have better resolution than the fixed wing time domain systems.

5. Conductivity-Width Aperture

All AEM systems are, to some extent, aperture-limited. Below a certain "response factor", which includes the conductivity and dimensions of the conductor, the anomaly produced by the system will be below the recognition level. At the upper end of the response factor, some systems are limited and others are not. The ones that are not limited sometimes cease to be multi-channel systems and lose their discrimination. Time domain systems like INPUT are aperture limited.

6. Lateral Coverage

In addition to penetration, the lateral coverage of an AEM system is important because it dictates, to some extent, the maximum distance between survey lines, which in turn affects the cost of exploration. Alternatively, at a given survey line spacing, a system with good lateral coverage will have a better chance of detecting a conductor that lies between two survey lines. Like penetration, lateral coverage generally increases with increasing coil separation.

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3.3 Combined AEM/Magnetometer Surveys

In general there are three steps involved in planning a survey of this type. We will outline these steps and then give a few examples of how to plan the survey.

Step 1. Define the Target and Geological Environment

Target

Defining parameters

Large massive sulphide lens Type, attitude, strike and composition.
Small massive sulphide lens Type, shape, attitude and composition
Veins or other discontinuous mineralization Type, extent, strike and mineral assemblage.
Shear zone or fracture hosted non-conductors Type, strike, alteration, water content

Geological environment criteria.

Step 2. Determine Factors Affecting Survey Performance

The topography and physiography of the area:

Step 3. Select the AEM System

The following are examples of different targets in three areas in Canada (Seminar presented by Dr. N. Paterson). The target, and topographic/physiographic conditions also differ between these areas.

Target 1:

A large stratabound volcanogenic Cu-Zn sulphide body somewhere in a 1000 km2 area in north west Quebec. Thick (30 to 60 meter), partly conductive overburden covers a country rock that is a mixture of felsic and intermediate metavolcanics, greywacke, quartzite, banded iron formation, intrusive granite and minor gabbro. The area is flat and swampy and the only access is from Mattagami, 150 km. away. The area is to be flown in summer.

AEM system requirements:

Appropriate systems:

Target 2:

A large stratabound massive Pb-Zn body in 150 km2 area in the Yukon. Very steep topography. Little overburden except in valleys. High tree cover. Country rock is phyllite, argillite, shist, intermediate volcanics and granite. The area is 130 km. from Ross River, Yukon and 25 km from a private airstrip at Anvil. The area is to be surveyed in summer.

AEM system requirements:

Appropriate systems:

Target 3:

Small Cu-Zn sulphide lenses somewhere in a 500 km2 kilometre area of north west Newfoundland. The bed-rock is intermediate-mafic metavolcanics with some ultramafic intrusives and minor metasediments. The terrain is moderately hilly covered by 10-20 meter high trees. There is little overburden and what there is, is virtually non-conductive. There is good access to villages in the area by road and the nearest airstrip, Cornerbrook, is 120 km. away. The area is to be flown in winter.

AEM system requirements:

Appropriate systems:

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3.4 Survey Data Presentation

In addition to a digital data file, the results of an AEM survey, the data, is presented in a variety of formats. Some contractors only present the EM anomaly locations plotted on the flight path maps, together with a coding indicating anomaly strengths and certain parameters derived by computer-modeling the anomaly sources as vertical sheets. Before the advent of personal computers with their interactive display capabilities, stacked profiles of the EM, altimeter, magnetic, and sometimes, spheric noise data used to be a common form of data presentation. However, because handling the large amount of paper involved was always an onerous task and most explorationists can now display profiles, using their computer, directly from the digital data base, it is no longer common to produce hard copy profile displays.

Typically contractors present EM data in two principle formats:

Total Field Magnetics and VDV Map Enlarge TFM Map (110 kb) Enlarge VDV Map (108 kb) EM Profiles and Resistivity Map Enlarge EM Profile Map (100 kb) Enlarge RES Map (136 kb)

Figure 3.4-1 illustrates a typical suite of final maps of both the magnetic data and the EM data, including the interpretation map, that survey contractor would deliver after the completion of a combination Magnetic-HEM survey operations and the required compilation and interpretation phases of data analysis. Moving the mouse over the pictures will allow you to see different presentations. "Clicking" on the visible version will produce an enlargement.

Interp and EM Profiles  Map Enlarge EM  Profile Map (100 kb) Enlarge Interp Profile Map (136 kb) Enlarge EM  Profile Map (100 kb)

Figure 3.4-1: A typical Interpretation map that result combined HEM and magnetic survey.

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3.5 Interpretation

Most survey contractors limit their interpretation to a systematic analysis of the more promising anomalies using a vertical sheet as the conductor model. This is normally done, using a computer program, after the local base level for estimating anomaly amplitudes has been carefully determined. Anomaly selection is done by, judiciously, using the shape of calculated models of various conductors, vertical sheets, flat lying surficial sheets, etc. similar to the ones shown in figure 3.5-1.

HEM anomaly shaped

Figure 3.5-1: A sketch illustrating the theoretical HEM anomalies caused by simple conductor shapes. When multiple conductors are present, the shapes illustrated will be modified by neighbouring anomalies.

Nomograms exist, such as the one illustrated in figure 3.5-2 by which similar analysis can be made from profile data. Both procedures produce estimates of conductance, called the conductivity-thickness product (which is the product of the conductivity of the tabular source and its thickness), and the depth to the source from the sensor. The sensor height, as recorded by the radar altimeter, is then subtracted from the depth to give an apparent depth below ground.

Some contractors have developed interactive computer programs that allows the interpreter to "pick" the anomalies directly from a display on the computer screen and immediately see the results of the conductance/depth calculation. This permits the interpreter to alter both the map scale and the profile data scale quickly to insure that all features, regardless of amplitude, are fully assessed.

Nomogram for Typical EM system

Figure 3.5-2: A nomogram used to estimate the conductivity-thickness product and depth to the source.

Typical Bedrock Conductor

Figure 3.5-3: This profile map shows a typical bed-rock conductor anomaly.

In figure 3.5-3, note that the anomaly has a characteristic signature. The positive coaxial response (the red line for the inphase component and the blue for the quadrature) is mirrored by a low in the coplanar response (maroon for inphase and teal for quadrature).

Figures 3.5-4 and 3.5-5 illustrate the signatures of a surficial conductor and of a conductor which contains significant magnetite content. Note that the surficial conductor is broad and lacks the high coaxial / low coplanar response of the vertical sheet anomaly in figure 3.5-3. The magnetite response is negative in the in-phase component.

Typical Surface Conductor

Figure 3.5-4: Typical signature of an HEM anomalies due to near surface "surficial" material. Note that the quadrature response of the coaxial, (blue), and coplanar, (teal), profiles are nearly identical while there is no inphase response for either coil pairs.

EM_Magnetite_Responce

Figure 3.5-5: Typical HEM response of a conductor that contains a significant amount of magnetite.

In figure 3.5-5: Note that both the coaxial and coplanar in-phase response is strongly negative while there is little or no quadrature response from either coil pair.

While the process described above does produce very useful information about the relative importance of various anomalies in the EM data, it has severe limitations including:

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3.5a Other Interpretation Methods

For reasons similar to those suggested in the section on magnetic interpretation, detailed interpretation of specific anomalies on a map is almost always done by the exploration managers personnel. Many of these methods rely on the application of sophisticated modeling algorithms. Figure 3.5-6 shows an example of an HEM model of two conductive plates in one such modeling program.

EM_Model

Figure 3.5-6: A calculated theoretical coaxial inphase electromagnetic response of two dipping conductive dikes.

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Appendix 1: Typical Electrical Properties of Earth Materials.

Rock, Mineral, etc. Conductivity (mohs/meter) Resistivity (ohm-meters)
Bornite 330 3 x 10-3
Chalcocite 104 10-4
Chalcopyrite 250 4 x 10-3
Galena 500 2 x 10-3
Graphite 103 10-3
Marcasite 20 5 x 10-2
Magnetite 17 x 10-4 - 2 x 104 5 x 10-5 - 6 x 10-3
Pyrite 3 0.3
Phrrhotite 104 10-4
Sphalerite 10-2 102
Igneous and Metamorphic Rocks 10-7 - 10-2 100 - 107
Sediments 10-5 - 5 x 10-2 20 - 105
Soils 10-3 - 0.5 2 - 103
Fresh Water 5 x 10-3 - 0.1 10 - 200
Saline Overburden 0.1 - 5 0.2 - 1
Salt Water 5 - 20 0.05 - 2
Sulphide Ores 10-2 - 10 0.1 - 100
Granite Beds and Slates 10-2 - 1 1 - 100
Altered Ultramafics 10-3 - 0.8 1.25 - 103
Water-filled faults/shears 10-3 - 1 1 - 103

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Selected Bibliography -- Airborne ElectroMagnetic Surveys

Grant, F.S. and West, G.F., 1965, Interpretation Theory in Applied Geophysics, McGraw-Hill Book Company.

Fraser, Douglas C., 1976, Resistivity Mapping with an Airborne Multicoil Electromagnetic System: Geophysics, vol. 41, no. 1 (Fedbruary 1976).

Fraser, Douglas C., 1979, The Multicoil II Airborne Electromagnetic System: Geophysics, vol. 44, no. 8 (August 1979).

Paterson, Norman R., 1982, Use of Airborne E.M. (AEM) in Exploration for Bedrock Conductors, in Mining Geophysics Workshop., Paterson Grant and Watson Limited.

Paterson, Norman R., 1982, Prospecting by Combined AEM/Magnetometer Surveys, in Mining Geophysics Workshop., Paterson Grant and Watson Limited.

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