Electro-Magnetic Airborne Surveys

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3. Airborne Electromagnetic Surveys

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.

• Massive volcano-sedimentary stratabound sulphide ores of Cu, Pb, Zn, (and precious metals), usually with pyrite and/or pyrrhotite. Fair to good AEM targets accounting for the majority of AEM surveys.
• Carbonate-hosted Pb-Zn, often with marcasite, pyrite, or pyrrhotite, and sometimes associated with graphitic horizons. Fair to poor AEM targets.
• Massive pyrrhotite-pentlandite bodies containing Ni and sometimes Cu and precious metals associated with noritic or other mafic/ultramafic intrusive rocks. Fair to good AEM targets.
• Vein deposits of Ag, often with Sb, Cu, Co, Ni, and pyrite in volcanic and sedimentary rocks. Generally poor AEM targets.
• Quartz veins containing Au with pyrite, sometimes also with Sb, Ag, Bi, etc., in volcanic or sedimentary (and possibly intrusive) rocks. Poor AEM targets.
• Skarn deposits of Cu, Zn, Pb, and precious metals, usually with pyrite and magnetite, around igneous intrusions. Fair to poor AEM targets.

• Lateral variations in conductive overburden.
• Graphitic bands in metamorphosed country rock.
• Altered (to clay facies) mafic-ultramaific rocks.
• Faults and shear-zones carrying appreciable groundwater and/or clay gouge.
• Magnetite bands in serpentinized ultramafics.

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3.1 Basic Principles

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.

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

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

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Transient Airborne Electromagnetics

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.

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)

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3.1b Frequency Domain Airborne Electromagnetics

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.

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

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

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

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

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3.2 Factors Affecting Detectability

1. Signal-to-noise ratio:

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

2. Penetration

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

4. Resolution

5. Conductivity-Width Aperture

6. Lateral Coverage

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3.3 Combined AEM/Magnetometer Surveys

Step 1. Define the Target and Geological Environment

Geological environment criteria.

• Depth and conductivity of the overburden. Consider the underlying bed-rock geology, residual or transported soil, and the Quaternary history of the area.
• The conductivity of the bed-rock and the presence of bed-rock conductors.
• The strike and dip of the formations.
• The possible presence of magnetic bodies.
• The depth range to the conductors of interest.

Step 2. Determine Factors Affecting Survey Performance

The topography and physiography of the area:

• Is the area flat or hilly? Only a helicopter can maintain required ground clearance safely in hilly terrain.
• The extent and height of tree cover will effect flight elevation.
• Presence of cultural features like pipelines or other conductors and or interference from power lines may be important.
• Determine access to the area and the required logistics. These factors will affect survey production and therefore cost.
• Mobilization to and from the area: how far and how long?
• Ferry distance from base camp to the survey area(s).
• The shape and size of survey block(s). Line length, spacing and the total kilometerage will affect survey production and therefore cost.
• Presence of obstructions such as power lines or towers that my cause a safety risk for low flying aircraft and/or bird and cable assembly

Step 3. Select the AEM System

Target 1:

AEM system requirements:

• Good penetration.
• Tolerance to conductive overburden.
• Good discrimination because geologic conductors such as graphite, sulphide and iron formation are likely.
• Good lateral coverage and aperture are desirable.
• Low flying cost if possible.

Appropriate systems:

• Helicopter EM: - will require a fly camp and gasoline dump. May be relatively expensive, especially if the line spacing must be reduced because of limited lateral coverage of the system. This system will produce the best discrimination between graphitic and sulphide conductors and has good surficial to bedrock discrimination.
• INPUT: - has the necessary characteristics but could have a problem with atmospheric noise in the summer months. A 400 meter line spacing would be appropriate so cost would be relatively low but does not have as good discrimination as the HEM system.

Target 2:

AEM system requirements:

• Good discrimination and resolution because the expected graphitic conductors are important markers.
• Good sensitivity to poor conductors. This requires that high frequencies be available.
• Good performance in steep terrain.
• Adequate penetration of at least 75 meters.
• Flight lines are short so the aircraft must have good turn-around capability.
• Because the program is small, mobilization costs must be low.

Appropriate systems:

• Multi coil Helicopter EM with at least one frequency over 3000 Hz has all of the necessary characteristics. This type of system can be installed in a local helicopter are preferable in order to reduce mobilization costs. 150 m line spacing is appropriate.

Target 3:

AEM system requirements:

• Good resolution because the mineralization is, typically, in small pods, often in graphitic host rock and sometimes as steeply dipping pipes.
• Good response to poor conductors. Typical massive sulphide conductance in the area is 1-3 mhos. The host rock and the overburden are relatively non-conductive.
• Good lateral coverage is required because the conductors are of irregular strike and dip.
• A depth of penetration of about 75 meters is adequate.

Appropriate systems:

• A multicoil helicopter EM system with at least one frequency over 3000 hz and, perhaps, with VLF. In this case the VLF may add aperture, lateral coverage and penetration, and help to discriminate between long (formational) and short (lens type) conductors at very low additional cost. The system can be based in a village in the area.

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3.4 Survey Data Presentation

Typically contractors present EM data in two principle formats:

• As profile maps showing the in-phase and quadrature components of complimentary co-axial and co-planar frequency pairs plotted as coloured profiles on the flight path. This map also shows the locations of significant EM anomalies displayed using an icon code to indicate the calculated conductivity-thickness product of the source assuming that it is a vertical sheet. The process of "picking" and modeling these anomalies will be described in more detail in the interpretation section.
• As a coloured map of the apparent resistivity with embedded contours calculated from the coplanar or coaxial EM data. This map shows the apparent ground resistivity assuming the ground to be of uniform conductivity both laterally and vertically. These maps are helpful in outlining conductive overburden and showing discrete bed-rock conductors. Actual values of resistivity bear little relation to the true resistivities of the overburden or bed-rock features.

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.

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

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3.5 Interpretation

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.

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.

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

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

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.

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.

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

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:

• The assumption of vertical dip: If the conductor is non vertical, its apparent depth will be underestimated but the conductance estimate will not be greatly affected.
• The assumption of semi-infinite size: The depth estimate tends to be too great and the conductance too low.
• The assumption of non-conductive host rock and overburden: If the conductor is in contact with a conductive host rock or overburden, the quadrature anomaly will be enhanced and the depth and the conductance will both be underestimated. If the conductor underlies, but does not contact the conductive overburden the depth and conductance will both be overestimated.
• Failure to correctly remove local EM background: If the residual effect of local EM background has shifted the assumed EM base level used in the calculation, the depth estimates will be too low and the conductance underestimated.
• Presence of magnetite: suppression of the in-phase anomaly by magnetic susceptibility in the case of conductors in magnetic environments will lead to underestimating conductance and wild line to line variations in the estimate.

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3.5a Other Interpretation Methods

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.

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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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