GeoExplo Ltda.
Airborne Geophysical Survey
General -- Airborne Magnetic Surveys

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

General Introduction to Airborne Magnetic Surveys

Abstract

This article deals with general issues of airborne geophysical surveys with special emphasis on aerial magnetic surveys. Under the airborne magnetometer surveys section we discus: Basic Principles, Instrumentation, Magnetometers (fluxgate, proton precession and optically pumped magnetometers), Survey Operations, Contract Specifications, Survey Design and Noise.

 
Other useful links
Simon Fraser University Coarce on Magnetuc Surveys Good basic coarce on magnetic fields, magnetic survey and interpretation
The Berkeley Course in Applied Geophysics (Magnetics) The Berkeley Course in Applied Geophysics (Magnetics)
 

Table of Contents

1. INTRODUCTION
    1.1 Survey Costs
2. AIRBORNE MAGNETOMETER SURVEYS
    2.1 Basic Principles
    2.2 Instrumentation
        a) The Fluxgate Magnetometer
        b) The Proton Precession Magnetometer
        c) Optically Pumped Magnetometers. (Alkali or He vapor)
    2.3 Survey Operations
        a) Table 2.3-1 Contract Specifications
        a) Survey Design
        b) Noise
    2.4 Magnetic Gradiometer Surveys

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1. Introduction

During the past fifty years a very large number of both ground and airborne geophysical techniques have been developed to assist in mineral and hydrocarbon exploration. Airborne methods are usually the most cost effective tools available for both large regional reconnaissance surveys used as aids in geological mapping and for locating target areas for more detailed follow-up using helicopter borne instruments. Ground techniques are usually most effective when used to test targets discovered by the airborne surveys. In this workshop we will limit our discussion to airborne methods and only mention ground techniques in passing. We will concentrate on four of the most common types of aerial surveys:

We will also briefly introduce the very low frequency electromagnetic method or VLF. We will open with a brief discussion of the types of surveys relevant to the method and the types of exploration problems the method is normally used to solve before we discuss the basic principles of each method. Next we'll turn to survey design considerations and field practice, including instrumentation, field data processing and data quality control. Then we'll discuss final data compilation, and map and other data products. Finally, we'll briefly introduce some of the interpretation tools available for in-depth analysis of specific areas, particularly if geological, or other data is available for correlation with the geophysical data.

While we will be discussing these various methods separately, it is important to realize that geophysical techniques are most effective when two or more different types of data are collected during a single survey. For example, HEM and magnetic data are usually collected at the same time. It is also useful to include magnetic total field and vertical magnetic gradient, and perhaps VLF, instrumentation when conducting a radiometric survey.

Because aeromagnetic surveys are probably the most common type of airborne geophysical surveys, we will use this survey type as the vehicle for a discussion of field practice and many other elements of airborne geophysical surveying. However, many of the practices discussed for airborne magnetic surveys also apply to all other types of airborne surveying.

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  1.1 Survey Costs

It is difficult to predict the exact cost of conducting airborne surveys. Parameters that will influence the cost include: However, for rough comparison, an airborne survey will usually cost somewhere between US$10.00/km. to US$120.00/km. compared with the cost of conducting a deep reflection seismic survey which can be as much as US$1000.00/km. Other ground surveys will fall somewhere between these extremes. Of course, the geophysical information yielded by the airborne and ground surveys is very different, this is reasons choosing one over the other.

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2. Airborne Magnetometer Surveys

Aeromagnetic surveying is probably the most common airborne survey type conducted for both mineral and hydrocarbon exploration. Because total magnetic intensity and vertical magnetic gradient surveys are intimately related (total intensity data is always collected simultaneously with vertical gradient data) we will discuss both methods in this section, The type of aeromagnetic survey specifications, instrumentation, and interpretation procedures, will depend on the objective of the survey. Generally, we divide aeromagnetic surveys into two classes: regional and detailed surveys.

Regional surveys usually have a relatively wide traverse line spacing, 500 meters or more, and cover an area of at least 5,000 square kilometers. This class of survey is usually done for one of the following purposes:

  1. Geological Mapping to aid in mapping lithology and structure in both hard rock environments and for mapping basement lithology and structure in sedimentary basins or for regional tectonic studies.
  2. Depth To Basement mapping for applications to petroleum, coal and other non metallic exploration in sedimentary basins or mineralization associated with the basement surface such as strata bound Pb-Zn deposits or U-bearing basal pebble conglomerates.

Detailed surveys have a line spacing of less that 500 meters and are done for a variety of reasons, usually in conjunction with other airborne methods. Reasons for conducting a detailed survey include:

  1. Direct prospecting for magnetic ores like magnetic iron ores, crome, asbestos-bearing ultramafic rocks, or kimberlites.
  2. Indirect prospecting, in combination with other methods or alone to:

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2.1 Basic Principles

Magnetometer surveys map local disturbances in the earth's magnetic field that are caused by magnetic minerals in the upper regions of the earth's crust. If no magnetic minerals were present in the crust the earth's magnetic field would be very smooth because it originates within the earth's core and therefore comes from a very great depth. This very broad, smooth magnetic field, the "geomagnetic field", is present in all survey data even though it usually cannot be perceived within the limited areas involved in contour maps of magnetically active regions like shield or many mountainous environments. It is a slow but steady change in the background levels of the measurements, generally rising as one moves northward or southward from the magnetic equator.

Fortunately, it can be removed from the data by a very simple mathematical procedure called, a spherical harmonic expansion, based on magnetic observations made at magnetic observatories around the world. The model that is used for doing this is called the "International Geomagnetic Reference Field" or IGRF, and its value is easily calculated at any given geographical position.

From the core of the earth to a depth below the surface where the temperature reaches a value of about 500 degrees C., the "Currie point", there are no additional contributions to the magnetic field. Above the "Currie point", which occurs at depths of between 5 and 15 km., some minerals acquire magnetic properties and, therefore, cause local disturbances, called magnetic anomalies, in the geomagnetic field. These minerals are a small number of oxides of Fe and Ti, and one of the family of pyrrhotites.

The most strongly magnetic and the most common magnetic mineral is magnetite. Others include maghemite, the titanomagnetites, and the titanomaghemites. Pyrrhotite is comparatively scarce compared with these ; and so, for practical purposes, magnetic contour maps can be viewed as giving information only on the distribution of magnetic iron oxides (chiefly magnetite) in the rocks that lie above the Currie point.

The intensity of magnetization, I, of a rock or mineral is measured in nano Teslas, nT, and is due chiefly to two factors as shown in equation 1:

I = K*f + R ................................. equation 1.
where:
K = the magnetic susceptibility of the material, which is largely determined by the magnetite content of the rock.
f = the local strength of the earth's field
and R = the remnant magnetic field of the rock, which is the magnetic component that was "frozen" into the rock by virtue of the ambient magnetic field at the time that it cooled to the Currie point or later while ferromagnetic grains were growing or undergoing chemical change. Remnant magnetization can also occur during hydrodeposition of very small, previously magnetized particles.

The first term in equation 1 is the "Induced" magnetic field, or the magnetization a rock obtains, by virtue of its susceptibility, through the applied field. It disappears when the rock is removed from the magnetic field.

The intensity of magnetization, however, is not fixed with respect to time and space. In aeromagnetic surveys we are usually interested in the spatial variations of the intensity of magnetization and, thus, the temporal variations must be identified and removed during data compilation. Three main types of temporal variations have been found to cause spurious errors in aeromagnetic data as follows:

1. Diurnal (24 hour variation):
  This variation usually has an amplitude of about 50 nT to 100 nT and is caused by large scale ionosphereic motions. It is removed by monitoring the field using a base station magnetometer or using a network leveling program.
2. Magnetic Storms:
  These are abrupt variations of several hundred nT and last for several hours. Because they follow cosmic ray activity they are probably related to solar activity. In many cases data collection must stop during a sever storm, hence the importance of using a base station.
3. Micropulsations
  These are very short period, 0.01 sec. to 10 sec. random variations having variable amplitude from about 0.001 nT to 10 nT. There are probably a variety of causes for micropulsations, including atmospheric electromagnetic activity. Micropulsations can be important in high sensitivity surveys.

An appreciation of the possible severity of the effect of temporal variations, sometimes referred to collectively as "diurnal" variations, can be gained from figure 2.1-1 which shows a comparison between shipboard magnetic variations and base station variation at a high magnetic latitude. Note that some of the "diurnal" fluctuations are as high as 500 nT. Because the ship travels much slower than an aircraft, it collects data over a much longer time period and so longer wavelength temporal variations are more evident in marine data than in airborne data.

diurnal variations
Figure 2.1-1: A comparison between base station magnetic variations and ship-borne variations

Typically a survey contractor will record the diurnal variations in a base station that is time synchronized with the mobile data acquisition system (usually using the GPS clock). The temporal variations recorded in the base station are them removed from the data collected in the mobile in post survey processing. If the diurnal variations are extreme (there are rules for this which will be defined in the "re-flight" specifications of the survey contract) then the data collected on the survey platform will need to be re-recorded.

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2.2 Instrumentation

The magnetic field is measured by sampling the total field, or a component of the field, at either discrete times (usually at intervals of from .1 sec. to 1 sec), or by recording the field continuously along the flight lines. While there are at least five different types of magnetometers that are used to collect various types of geophysical data: the Fluxgate, Proton Precession, Optically Pumped, Overhauser, and Squid, (superconducting quantum interference device) only the first three are in common use for aeromagnetic data collection. Thus we will only describe the operation of the three most common types.

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a) The Fluxgate Magnetometer

The fluxgate magnetometer is based on the properties of the magnetization curves of a highly magnetically permeable material. Two bars of this material are oriented such that, in the absence of a magnetic field, the magnetization curves of the two bars are equal but opposite. In the presence of a magnetic field, the magnetization of the two bars is different and the difference is measured as a voltage in an output coil.

This type of magnetometer has an accuracy of from 0.1 to 2 nT and produces a continuous analogue output profile that, in modern instruments, is digitized for processing. It records either the total magnetic intensity or one of its three vector components and has a wide dynamic range.

A three axis fluxgate magnetometer typicaly is included in all airborne survey systems to support data correction for magnetic interference from the aircraft or other sources. It three axis fluxgate magnetometer records the orientation of the aircraft in the earth's main magnetic field and this information is used to correct the main magnetic measurments for the orientation effects of the aircraft in a process called "magnetic compensation" correction.

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b) The Proton Precession Magnetometer

The proton precession magnetometer is based on a property of the atomic nucleus. If the electrons spinning about the nucleus are deflected from the direction of the earth's field the distorted spin axis will precess back to the direction of the earth's field. The precession frequency, called the Larmor frequency, is related to the magnitude of the magnetic field by:
f = A*F ................................. equation 2.
where:
f = the Larmor frequency,
A = the atomic constant for the element processing,
F = magnetic field strength, in geophysics, the earth's total field.

The sensor consists simply of a bottle of material containing hydrogen nuclei, such as water or a hydrocarbon, with a coil of wire, the induction coil wound around the bottle. A cyclical microvolt signal is generated in the coil whose frequency, the Larmor frequency, can be measured. For accurate total field measurements (0.1 nT to 1 nT) the frequency must be measured to 1 part in 100,000 to 1,000,000. Because of the necessity to continually deflect the electronic spin vectors, the measurements are not continuous. This type of magnetometer has an accuracy of from 0.1 to 1 nT, and produces an intermittent (digital) sample in intervals of from 0.5 sec. to 1 sec. It records the total magnetic intensity and has a limited dynamic range.

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c) Optically Pumped Magnetometers. (Alkali or He vapor)

The optically pumped magnetometer also depends on the precession property of the atomic nucleus. In this case the sensors bottle contains helium or an alkali vapor. The atoms of the vapor are illuminated by polarized light of a lamp that contains the same vapor as the bottle, and the intensity of the light transmitted through the vapor is recorded by a photo cell. This illumination excites the valence electron of each vapor atom to a higher energy level and the vapor becomes transparent to the light. An external magnetic field also changes the energy level of the atoms. If an alternating magnetic field at the Larmor frequency is applied to the vapor at right angles to the earth's field the atoms energy levels are returned to the lower level where the vapor becomes opaque to the light. A feedback loop between the photo cell and the coil can be established such that the system oscillates at the Larmor frequency and so changes frequency as the intensity of the magnetic field changes.

A schematic diagram of a typical optically pumped magnetometer is shown in figure 2.2-1, (Hood, Peter, 1969)

opticaly pumped magmetometer

Figure 2.2-1: Self-oscillating alkali-vapor magnetometer

The instrument will not operate when the ambient magnetic field is either parallel to, or normal to, the optical axis of the gas cell. This requires that the orientation of the sensor be changed, particularly at low latitudes and when crossing the equator.

Because of this instruments high accuracy, reasonably wide dynamic range, and ability to sample the field very rapidly, most contractors use this type of magnetometer for all aeromagnetic surveys. The MiniMag magnetometer card included in the Integrated Airborne Geophysical System is designed to operate with up to four optically pumped sensors. The Larmor frequency of the magnetometer is resolved to 1 part in 96,000,000 without filtering, 10 times per second. A special feature of this system is that the aircraft's heading is monitored by the system and is used by the MiniMag magnetometer module to automatically toggle the cesium sensor's polarity as necessary at low magnetic latitude.

During vertical magnetic gradient surveys, two alkali vapor optically pumped magnetometers are operated together at a fixed distance apart and will therefore measure the vertical gradient of the total field, the difference between the two measurements, as well as the total field itself. It is, of course, possible to measure the horizontal gradients as well, but these are not usually geologically useful. Figure 2.2-2 shows a Navajo aircraft with a vertical magnetic gradiometer system installed as a tail stinger. This configuration is ideal for conducting both regional and detailed surveys where the magnetic data is the primary product of the survey and it permits the carrying on other instruments for multi-parameter surveys.

airborne vertical gradient platform
Figure 2.2-2: A airborne survey Navajo aircraft with the twin stinger vertical magnetic gradiometer installed. Each of the two tale stingers houses an optically pumped magnetometer.

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2.3 Survey Operations

Many of the following comments are applicable to all geophysical surveys; not just aeromagnetic surveys. In some cases, the details of the application of the ideas presented here will need to be altered to suit the specific needs of electromagnetic or radiometric data. In the sections where these data types are discussed, we will indicate where the requirements of that specific type of data may differ from those discussed here.

The final products of an airborne geophysical survey, in this case aeromagnetic, are usually one or more maps such as the contoured total magnetic field, coloured and/or shadow maps of the total field or a parameter derived from the total field, and a digital data file recorded on a convenient medium that contains the time, location and value of each measurement and any other information relevant to that measurement. Because we actually measure the field only at discrete points in time and along flight lines, all maps, other than profile maps, are an interpolation of the measured data. The selection of survey parameters by the exploration manager, such as line spacing, altitude, and the orientation of the traverse lines as well as compilation and presentation procedures and an evaluation of the anticipated noise - both temporal and geological - should all be made with the desired accuracy of the final products as your guide. These parameters should be detailed in the Contract Specifications. For multi-method surveys, the survey parameters should be chosen to fit the needs of the method most sensitive to them, or in cases of conflict, of the method expected to yield the most useful data. Typical major points that we recommend that should be covered by an airborne survey contract are listed in table 2.3 - 1.

When evaluating competitive bids for airborne surveying the following two general catagories should be considered and each proposal graded under these points.

    1. Quality of the proposal (Technical), Company Experience,

    2. Project personnel (Qualifications), Delivery dates and Price.

Three of the most important factors to be specified for any airborne geophysical survey are:
  1. The flight height
  2. the traverse line separation
  3. The traverse line orientation (direction)
For aeromagnetic surveys, the number of, or separation between, control lines, used when leveling the data, should also be specified. We will deal with these parameters in some detail.

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    Table 2.3-1 Contract Specifications

  1. General Specifications
  2. Survey Equipment Specifications
  3. Survey Flying Specifications
  4. Data Compilation and Interpretation

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2.3a Survey Design

The selection of traverse line spacing and flight elevation depends critically on the selection of the line direction. The selection of line direction depends on two main consideration, the magnetic inclination in the survey area (sometimes called the magnetic latitude of the area), and the geological strike of the phenomena of most importance to the exploration objective.

Figure 2.3-1 shows north-south oriented magnetic profiles across a dipole source at various magnetic latitudes. In the southern hemisphere south is towards the right hand side; in the northern hemisphere north is towards the right. Note that the anomaly produced by the dipole is symmetrical at the pole and at the equator, but is non-symmetrical everywhere else. This implies that the true shape of a magnetic feature is best defined along a north south traverse in most areas of the world. Thus the preferred traverse flight line direction would be north-south if the anomalies in the area were distributed randomly. Because regional surveys are conducted over very large areas usually containing various geological strike directions, a north-south traverse line orientation is usually preferred for aeromagnetic surveys.

Magnetic anomaly shapes

Figure 2.3-1: Profiles of total intensity anomaly from a dipole source at north geomagnetic latitudes where i = 0, 15, 30,45 ,60, 75 and 90 degrees.

However, if the survey area is known to contain a pronounced geological strike direction and the magnetic latitude is either very high or very low it may be advantageous to orient the traverse line direction perpendicular to the geological strike direction. The advantages of this orientation arise because many of the most useful magnetic features arise from linear features like dikes and or faults, and by orienting the traverse lines at right angles to these features, we can be confident that only a few anomalies will not cross the selected flight lines.

The respective values of line spacing and height, for all types of geophysical surveys, should be selected in order to reduce the amount of aliasing to less than 5% in the recorded data. Aliasing of data occurs when we try to extract anomalies or signals possessing a wavelength k less than twice the sample, or line, spacing Dx. This idea is illustrated, for simple sine waves, in figure 2.3-2.

Example of aliasing

Figure 2.3-2: Illustrating the effect of aliasing on two sine waves having wavelengths of 1/4Dx and 3/4Dx respectively. Note that, when sampled as indicated by the circles, we cannot tell which wave is actually present.

In the limiting case k is called the Nyquist wavelength Kn, where:

Kn = 2 Dx

Any anomaly with a "true" wavelength less than Kn will not be identified, and will have the effect of distorting the good data that posses wavelengths longer than the Nyquist.

When dealing with an assemblage of magnetic sources it can be shown that the amount of aliasing is simply related to a ratio of the sensor height above the source to the line spacing. In hard rock environments, the sensor height will usually be the distance from the sensor to the surface; however in areas covered by sediments or other non magnetic material, this height will be the flight height plus the thickness of the overlying non-magnetic sediments. As a rule of thumb, the line spacing should equal the sensor height for complete definition of the anomalous magnetic field. However, economic considerations may require a larger line spacing. In this case, the amount of detail required in the survey will depend on the desired use of the data and will, in turn, determine the permissible level of aliasing. Suggested optimum line spacing for given sensor heights is specified in figure 2.3-3 based on a selection of desired products. The larger value in each range may be used if the line direction is perpendicular to the strike of the majority of magnetic structures.

Effect of flight height

Figure 2.3-3: Optimum line spacing vs. height for aeromagnetic surveys. Note that line spacings should be smaller if very sophisticated interpretation methods are going to be applied to the data.

Control lines are flown to permit leveling of the survey data. In small surveys, at least three control lines should be flown at right angles to the traverse line direction. In large surveys, control lines should be spaced at intervals of five to ten times the traverse line spacing as is illustrated in figure 2.3-4.

Typical flight plan

Figure 2.3-4: A typical flight path pattern flown during geophysical surveys.

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2.3b Noise

Table 2.3-2 summarizes the sources of, and typical ranges of, expected noise for various types of aeromagnetic surveys.

Table 2.3-2: SURVEY ACCURACY (In Nanoteslas)

Survey sensor Alkali Vapor Proton Fluxgate
Resolution .01 -.25 .1 - 1 .1 - 2
Instrumental error .01 - .5 .1 - 1. .5 - 1.
Diurnal etc. .5 - 2. .5 - 2 .5 - 2
Positioning Errors .25 - 5 .25 - 5 .25 - 5
Total .77 - 4.75 .95 - 9 1.35 - 10

As is evident in this table, the major noise sources are the temporal changes and positioning errors. the Contractor's technicians and geophysicists monitor the temporal changes using the Ground Monitoring System and both network and micro-leveling methods to eliminate virtually all of the data errors that arises from temporal changes in the field. Figure 2.3-5 shows the display screen from this module with the magnetometer data, fourth difference, and the altimeter profiles displayed in real time.

Positioning errors arise from the inability to navigate and record the sensor position with absolute precision. Before the advent of differential GPS positioning systems, these errors could be quite large. For example, in an area where the normal geomagnetic field gradient is of the order of 5 nT per horizontal kilometer and 31 nT per vertical kilometer a magnetic reading will be in error by 1 nT if it is misplaced by 150 meters horizontally or 30 meters vertically. To attain a one-tenth nT accuracy the sensor position must be known to within 15 meters horizontally and 3 meters vertically. The typical GPS Navigation Module can provide a differentially corrected position accurate to within about 3 meters. Thus it is now possible to attain survey data accuracy of the order of one tenth of a nano tesla in magnetic surveys having gradients similar to those described above. While the gradients described above, are rather gentle in a hard rock environment such as a shield area, they are common in sedimentary environments important to hydrocarbon exploration where it is important to accurately define very subtle anomalies.

Within the GPS Navigation Module, all data from a high accuracy, high speed GPS receiver card, such as the Novatel 12 channel card, is available to the Integrated Geophysical system directly on the systems computer bus. Prior to initiating flight operations, or at any time during flight, the status of the GPS satellites can be monitored. Satellite or radio link real time differentially corrected GPS (DGPS) systems are also available.

Typically the contractor's Airborne Geophysical System enables the geophysical technician to quickly and thoroughly monitor system performance on the ground in preparation for flight. Once cleared for operation the pilot takes over, selects a survey area on the touch screen, and begins data collection. An on-board operator or navigator is not required under most survey conditions and the weight saving can be converted to extra fuel and, hence added range and productivity. Figure 2.3- 6 shows the Navigation Module screen as it would be seen by the pilot during survey operations.

Figure2.3-6: Typical on-board control display screen. The screen at the left shows a test of the uncorrected GPS position of the aircraft at rest while the right screen illustrated an outline of the survey area with completed lines indicated.

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2.4 Magnetic Gradiometer Surveys

This section is not finished yet. Sorry

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