GeoExplo Ltda.
Geophysical Airborne Survey
Compilation and Interpretation

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

Airborne Magnetic Data Compilation and Interpretation

Abstract

Quality Control is one of the most important parts of the survey operations. This QC paper deals with the what should be expected of a Quality Control officer and discusses the following topics: Basic Principals, QC Mandate, Client Representative, QC Officer Responsibilities, Survey Specifications and Typical Tests Required for a Magnetic Survey

 

Table of Contents

    5.1 Data Compilation
         5.1.1 Database
         5.1.2 Flight Path Plotting
         5.1.3 Leveling
         5.1.4 Mapping the Total Magnetic Intensity
    5.2 Interpretation
         5.2.1 Interpreting Magnetic Data
         5.2.2 Quantitative Interpretation
    Appendix 1: Typical Magnetic Susceptibilities of Earth Materials
    Selected Bibliography -- Airborne Magnetometer Surveys

5.1 Data Compilation

Before the availability of high speed, portable personal computers, all data compilation was done long after survey flying was complete and we had to wait for many weeks to see the first map products. With the advent of the integrated airborne geophysical systems and PC based "in field" geophysical data compilation system, pioneered by High-Sense Geophysics now part of Fugro Airborne Surveys, data can be compiled in the field on a daily basis. On-site processing not only provides an excellent means of quality control but provides map results for immediate evaluation, planning and decision making.

Airborne data processing is achieved in the field or on the office computer systems, by using a highly optimized binary data base systems to manage the large volume of data associated with this type of surveying.

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5.1.1 Database

Because large volumes of data are collected in airborne surveying, the techniques associated with processing demand a special database architecture. Very fast data access times are essential to both database management and processing. To acquire the necessary speed one requires a highly optimized, random access, database with a host of features for loading, managing, and manipulating data including:

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5.1.2 Flight Path Plotting

An important part of data compilation, and an essential part of data quality control, is to be able to plot and label the flight path, annotate the lines with fiducials, and produce profile maps similar to the one shown in figure 1.

Magnetic profile map

Figure 1: Magnetic profiles plotted along recorded positions in profile form.

Other important capabilities required for flight path and data control and display include multichannel profiling and survey amount calculations.

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5.1.3 Leveling

Leveling of airborne magnetic data is required primarily to remove the effects of temporal variations in the earth's magnetic field. While leveling is an art, the modern leveling system automates many of the repetitious tasks associated with leveling to quickly produce a good first approximation of the required adjustments. A skilled processor then uses advanced tools to fine tune the corrections. Important capabilities of the leveling system include:

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5.1.4 Mapping the Total Magnetic Intensity

While profile maps are useful for some interpretation methods, a two dimensional map, usually contoured and coloured, is required to fully interpret the data in the majority of magnetic surveys. Before a two dimensional map can be plotted, the aeromagnetic data must be interpolated onto an equispaced grid, (or matrix). Thus an efficient and accurate gridding algorithm must be included in the data compilation system. Most contractor's data compilation system includes several gridding algorithms but for airborne data the bi-directional spline (usually employing some form of damping such as the akima spline) approach is usually most accurate and has fewest side effects. Capabilities of the gridding module include the following features:

After the data has been gridded at an appropriate grid interval, a host of digital processing techniques can be applied to the data as aids in interpretation. However, at the compilation stage, the basic product that is almost always required is a map of the total field and other primary data. Most often, a contour map is required, thus a contouring algorithm must be included in the data compilation system that includes:

Figure 2 shows an example contoured map of the total magnetic intensity.

Typical colour contour map

Figure 2: A typical colour contoured map of part of a total magnetic intensity grid.

Sometimes a coloured map (without contours) of a grid is useful for quick visualization of the data on a computer screen. Coloured maps like the one illustrated in figure 3. are used to quickly look for data artifacts or "in-field" evaluation and interpretation for rapid follow-up. Note that a number of lineations which may be evidence of diorite dikes are clearly evident in the data. In addition, some lineations show evidence of lateral offsets which may be related to faulting. The colour scheme, in the figure, indicates magnetic lows as blue, and highs as red.

Because a two dimensional contoured map or couloured image is produced from a grid of data values derived from the measured data by interpolation, it is the most elementary example of an interpretation of the magnetic or other data.

It is important to note, that the two dimensional type of presentation is a result of considerable degradation and interpolation of the data. Consider that in modern survey systems, magnetic data for example, is measured at 10 times a second. At fixed wing survey speeds of between 200 and 360 km/hour this results in a sample interval of between 5.5 and 10 metres. If the traverse lines are spaced 250 metres apart one usually uses a 50 cell size for an interpolated grid for two dimensional display. To achieve this using bi-directional spline gridding one samples along the line every 50 metres and then splines across the lines to interpolate points every 50 metres in the orthogonal direction. This results in taking approximately between every tenth (200km/hr) to every fifth (360 km/hr) point along the line and then inventing 4 out of five points across the lines. Thus the gridding process throws out between 90% and 80% of the profile data and then creates 80% of the data between the lines to construct the grid for further contouring, imaging or grid filtering processes.

Although, the two dimensional display of data is the most common method of viewing and interpreting data, because of the ease of use and the ability to superimpose other types of parametric data one is only working with a interpreted subset of the real data set which is contained in the one dimensional profile information. The profile data is harder to work with but as usual there is no substitute for ward work if one is interested in getting the most out of a data set.

There are a number of commercial processing software systems available that include sophisticated routines to produce a host of other interpretation products and aids, either from grids, or from more importantly the measured profile data. In section 5.2, we will discuss a few of the available aids, and methods, of interpreting the geological meaning of the geophysical data.

Colour contour map

Figure 3: A grid of total magnetic intensity data displayed on a computer in colour. screen as a coloured map.

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5.2 Interpretation

There are, at least, two different types of interpretation, of all geophysical data, involved in the exploration process.

The first type of interpretation is the relatively straight-forward process of using mathematical techniques to enhance various characteristics of the observed data and relate them to possible physical causes relevant to the distribution of the particular property of the source of the phenomena; for example, enhancing a magnetic trend to try to determine if there is a preferential direction to the orientation of the distribution of magnetite, and indicating this trend on a map, and perhaps, commenting on a possible source of the trend, e.g. a fault or dike. This type of interpretation is the only type associated with most survey contracts simply because most of the information required for the second type of interpretation is not available to the geophysical contractor.

The second, and most difficult and speculative but perhaps the most important and certainly the most useful to the exploration geologist, type of interpretation requires correlation between different types of geophysical data as well as with geological and geochemical information. Because of the requirement of intimate knowledge and access to the full spectrum of data collect for a particular project, this type of interpretation is usually done by the exploration company itself or by a that companies preferred consultant. In reality there is not enough of this type of interpretation carried out and as a result the airborne geophysical survey client is typically not maximizing the value of the geophysical survey. This failure to fully integrate the survey data into the exploration process has resulted in numerous missed deposits and provides a wealth of opportunity for those that are willing to rework old data sets in a comprehensive way.

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5.2.1 Interpreting Magnetic Data

In section 5.1 we mentioned that a contour map of the total magnetic intensity is the most elementary form of an interpretation. It is an interpretation in the sense that it is a 2 dimensionally continuous model of the magnetic field derived from discrete data measured along specific lines. From this limited data, and a number of assumptions about the field, such as that the field is continuous and does not contain any sudden spikes or steps, and the particular interpolation method used to grid and contour the data will not introduce any artificial features, we arrive at a map of the magnetic field. In fact, particularly if the flight lines are inappropriately oriented or spaced, different gridding and contouring algorithms will produce different maps. Thus, a contour map is an interpretation of the measured magnetic field itself.

There are a variety of other ways of interpreting the measured values other than contouring them. For example, we may prepare a coloured map where the colour of any given point on the map is determined by a colour scale based on the measured or interpolated value of the magnetic field at that point. A few other possibilities include:

For example, figure 4. shows a monochrome pseudo shadow map of data in the same area as is shown by the colour map of figure 3. The illumination of this map is produced by a source located directly to the north at an angle of 30 degrees above the horizon. This placement of the source will tend to emphasize east-west structures at the expense of north south oriented features. We see a variety of trend directions within this map, three of which we have indicated by the coloured lines drawn across it. The green north-east trending lineations appear to be interrupted by the nearly east-west trending red lineations and the southern blue lineation. When this type of observation is correlated with geological information and other geophysical data it is often possible to identify the geological nature of the sources that produce the magnetic trends indicated.

Shadow map of Total Field Mabnetics

Figure 4: A monochrome shadow map of the total intensity data in figure 3. Note that this interpretation enhances some of the trends within the field at the expense of others. The coloured lines indicate three trend directions.

For comparison, figure 5. shows a second vertical derivative, calculated from the total field, of nearly the same map area. This map emphasizes the shorter wavelength magnetic anomalies thus giving us different information about the magnetic field than does the previous figure. Note that the trends indicated in the shadow map are also evident in this map but they are portrayed differently.

By a careful comparison of the maps shown in figures 3, 4 and 5 we see that these various interpretations of the magnetic field can reveal different information about the field. These differing interpretations can be invaluable aids while attempting a type of interpretation that is more important to exploration for minerals or petrolium, i.e., interpreting the geological meaning of the geophysical data. In this case, the offsets of and /or truncation of the north west trending lineations by the nearly east-west lineations obvious in the second vertical derivative map and indicated by the black lines, suggest that this E-W trend may be due to faulting.

Second Vertical Derivative Colour Map

Figure 5: A calculated, coloured, second vertical derivative of the total magnetic intensity of the same area as shown in figure 4. The black lines indicate possible fault zones

Figure 6. shows a comparison between magnetic data collected useing a fixed wing aircraft, collected using a helicopter data measured measured on the ground. Note that most of the detail seen in the ground magnetic survey is also present in the helicopter aeromagnetics.

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Helecopter data Ground Data Helicopter Data Fifed Wing Data

Figure 6: A comparison between fixed wing, helicopter and ground magnetic contoured maps. Moving the mouse, over the left third of the picture will reveal the Fixed wing data, over the center of the picture will show the Helicopter data and over the right side the Ground data.

The Helicopter data gives a much more detailed view of the magnetic character of the geology than the fixed wing survey principally because the fixed wing survey was flown at 120 metres of the ground on 200 metre spaced lines whereas the helicopter survey was flown with th magnetic sensor 30 meters of the ground on 50 metre spaced lines. The helicopter survey also provides greater data continuity than the ground survey since the ground survey sensor is to close to the surface and is influenced by surface material. Thus the helicopter survey provides the best data for interpretation and was used to produce the the interpretation map bellow:

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Magnetic Interpretation Magnetic Interpretation Helicopter Data

Figure 7: Interpretation of the helicopter magnetic data. Moving the mouse over the left half of the picture will display the helicopter magnetic data contour map.

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5.2.2 Quantitative Interpretation

There are numerous methods, algorithms and software programs in use for the quantitative interpretation of magnetic data, we will only look a few possible methods that can be used to extract additional geological information from the magnetic data. Depth to source information is contained in the shape of the anomaly. Because of the obvious importance of the thickness of the sedimentary section to a hydrocarbon explorationist, the depth to source, usually referred to as the depth to the magnetic basement, is of critical importance. In addition, depth information may be important when potential mineral deposits are covered by a thick layer of either consolidated or unconsolidated overburden.

The wavelength of magnetic anomalies is a fundamental result of the depth of burial. Attenuation caused by thickness of non-magnetic overburden is due, almost entirely, to the increase in distance between the sensor and the magnetic source. This effect, for the case of dikes, is illustrated in figure 8.

Flight Hight Effect

Figure 8: Magnetic anomalies due to shallow and deeply buried bodies.

Indeed, following Vacquier et. al. (1951), by calculating theoretical models of simple bodies and then deriving graphical estimators from the models, we can use the anomaly shape to obtain a first approximation to the source depth under the assumption that the model is a reasonable approximation to the source geometry. An example of this approach, for a vertically dipping dike at a high magnetic latitude is shown in figure 9.

Depth estimations

Figure 9: The derivation of "slope" estimators for source depth from a theoretical magnetic model. Note that, at this magnetic latitude the source depth D is about twice the horizontal component of the length of the most steeply dipping flanks of a north-south profile across the anomaly.

A variety of mathematical modeling techniques can make "automatic" depth estimates. A few of these are Werner deconvolution, Euler deconvolution, and "inverse" magnetic modeling.

We usually refer to "direct" or "forward" modeling as the process of calculating the magnetic response from the parameters of the source and "inverse" modeling as calculating a parameter, e.g. depth, of the source from the magnetic response assuming that the source is a particular simple shape. Many commercially available programs have been developed to permit an interpreter to model a wide variety of geophysical data types, both airborne and ground, and calculate either the forward response of the model or, by inverse modeling, the value of a parameter from the geophysical response of the source.

Figure 10. illustrates a forward magnetic model scenario using simple model geometries. The some system are also capable, in the case of magnetic and gravity data, of modeling the response of very complex geometries using an assemblage of vertical polygons, each having many sides. Thus, it is possible to test the validity of a depth to basement interpretation by modeling the response that the interpreted surface would produce and comparing it to the observed response. Similarly, the gravity response of the complex shape of, for example, a salt dome or ore body, can be modeled and the results compared with either data profiles or contour maps of the observed field.

Model Visualization

Figure 10: The theoretical magnetic response, calculated along two profile lines of a number of dipping dikes and a sphere.

Many exploration companies have developed other interpretation techniques that are unique to their particular needs. They have the advantage of having access to data that is not generally available to survey contractors or to their competition.

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Appendix 1: Typical Magnetic Susceptibilities of Earth Materials.

Rock (Mineral) Type Susceptibility (c.g.s.)
Magnetite0.3 to 0.8
Pyrrhotite0.028
Ilmenite0.044
Specularite0.004
Iron Formation0.056
Basalt0.00295
Diabase0.00259
Rhyolite0.00112
Gabbro0.00099
Granite0.00047
Other Acid Intrusives0.00035
Ely Greenstone0.00009
Slates0.00005
Sedimentary Rocks0.00001 to 0.001

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Selected Bibliography -- Airborne Magnetometer Surveys

Cameron, J.B. and Eby, T.W., 1971, Introduction to Gravity Exploration, Exploration Department, Amoco Canada Petroleum Company Ltd.

Goh, Rocque, 1972, A Marine Magnetic Survey in the Mackenzie Bay / Beaufort Sea Area, Arctic Canada; M.Sc. Thesis, Department of Geophysics, The University of British Columbia, Vancouver B.C., Canada.

Grant, Fraser S., 1982, Regional Magnetic/Gravity Data in Selecting Areas for Exploration, in Mining Geophysics Workshop., Paterson Grant and Watson Limited.

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

Hood, Peter, 1969, Airborne Geophysical Methods, Section 2, Aeromagnetic Methods: in Advances in Geophysics Volume 13, pp. 4-41, New York, Academic Press.

Hood, P.J., Irvine, J.L., and Hansen, J., 1882, The Application of the Aeromagnetic Gradiometer Survey Technique to Gold Exploration in the Val d'Or Mining Camp, Quebec. Canadian Mining Journal, vol. 103, no. 9; pp.21-39

Irvine, J., Cepella, O., and Payne, T., 1983, Kenting Gradiometer System, presented at the 53 annual S.E.G. International Meeting and Exposition, Los Vagas, Nevada, U.S.A.

Misener, James D., 1982, Airborne Magnetometer Surveys, in Mining Geophysics Workshop., Paterson Grant and Watson Limited.

Reford, M.S., 1964, Magnetic Anomalies Over Thin Sheets: Geophysics, V. 29, pp. 532-536

Sype, W.R., (1971), Application of Magnetic Surveys in Petroleum Exploration, Exploration Department, Amoco Production Company, Tulsa Oklahoma, U.S.A.

Vaquier, V., Steenland, Nelson Clarence, Roland,G., and Zietz, Isidore, 1951, Interpretation of Aeromagnetic Maps: Geological Society of America Memoir 47.

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