airborne magnetic data compilation and interpretation

8/17/2019 Airborne Magnetic Data Compilation and Interpretation

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GeoExploLtda. Geophysical Airborne Survey

Compilation and InterpretationSantiago Chile

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

!andate Client

"epresentative QC

#fficer

"esponsibilities

$urvey $peci%cations

and Typical Tests

"e&uired for a!agnetic $urvey

Table of Contents '.( )ata Compilation '.(.( )atabase '.(.* +light Path Plotting '.(., -eveling '.(. !apping the Total !agnetic /ntensity '.* /nterpretation '.*.( /nterpreting !agnetic )ata '.*.* Quantitative /nterpretation

0ppendix (: Typical !agnetic $usceptibilities of 1arth !aterials

$elected Bibliography 22 0irborne !agnetometer

$urveys

5.1 Data Compilation

Before the availability of high speed portable personal computers all data

compilation was done long after survey 3ying was complete and we had to wait

for many wee4s to see the %rst map products. 5ith the advent of the integratedairborne geophysical systems and PC based 6in %eld6 geophysical data

compilation system pioneered by 7igh2$ense 8eophysics now part of +ugro

0irborne $urveys data can be compiled in the %eld on a daily basis. #n2siteprocessing not only provides an excellent means of &uality control but provides

map results for immediate evaluation planning and decision ma4ing.

0irborne data processing is achieved in the %eld or on the office computer

systems by using a highly optimi9ed 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

techni&ues associated with processing demand a special database architecture.

ery fast data access times are essential to both database management and

processing. To ac&uire the necessary speed one re&uires a highly optimi9ed

random access database with a host of features for loading managing and

manipulating data including: ;

!ultiple channels of information to accommodate both single and a variety

of multi2parameter surveys.

report summaries on data inventory

archiving functions

search functions

data corrections and channel manipulation functions.

functions to merge base station and survey platform data

higher level functions for leveling griding contouring and imaging data

pro%le manipulation and presentation functions

etc.

5.1.2 Flight Path Plotting

0n important part of data compilation and an essential part of data &uality

control is to be able to plot and label the 3ight path annotate the lines with

%ducials and produce pro%le maps similar to the one shown in %gure (.

+igure (:!agnetic pro%les plottedalong recorded positionsin pro%le form.


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#ther important capabilities re&uired for 3ight path and data control and display

include multichannel pro%ling and survey amount calculations.

5.1.3 e!eling

-eveling of airborne magnetic data is re&uired primarily to remove the eustments. 0 s4illed processor then uses advanced tools to %ne tune the

corrections. /mportant capabilities of the leveling system include:

Creating traverse line 2 control line intersection lists.

Building a traverse?control line intersection database.

0utomatically calculating intersection corrections with a manual override.

Tool 4it for interpolating smoothing etc.

/nterpolating between intersection corrections and applying the corrections

to the database.

5.1." Mapping the Total Magnetic Intensit#

5hile pro%le maps are useful for some interpretation methods a two

dimensional map usually contoured and coloured is re&uired to fully interpret

the data in the ma>ority of magnetic surveys. Before a two dimensional map can

be plotted the aeromagnetic data must be interpolated onto an e&uispaced grid

@or matrixA. Thus an efficient and accurate gridding algorithm must be included

in the data compilation system. !ost contractor=s data compilation system

includes several gridding algorithms but for airborne data the bi2directional

spline @usually employing some form of damping such as the a4ima splineA

approach is usually most accurate and has fewest side e


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+igure * shows an example contoured map of the total magnetic intensity.

+igure *: 0 typical colour contoured map of part of a total magnetic intensity grid.

$ometimes a coloured map @without contoursA of a grid is useful for &uic4 visuali9ation of the data on a computer screen. Coloured maps li4e the one

illustrated in %gure ,. are used to &uic4ly loo4 for data artifacts or 6in2%eld6

evaluation and interpretation for rapid follow2up. ote that a number of

lineations which may be evidence of diorite di4es are clearly evident in the data.

/n addition some lineations show evidence of lateral o


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gridding one samples along the line every 'D metres and then splines across the

lines to interpolate points every 'D metres in the orthogonal direction. This

results in ta4ing approximately between every tenth @*DD4m?hrA to every %fth

@,ED 4m?hrA point along the line and then inventing out of %ve points across

the lines. Thus the gridding process throws out between FDG and HDG of the

pro%le data and then creates HDG of the data between the lines to construct the

grid for further contouring imaging or grid %ltering processes.

0lthough 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 wor4ing with a

interpreted subset of the real data set which is contained in the one dimensional

pro%le information. The pro%le data is harder to wor4 with but as usual there is

no substitute for ward wor4 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 pro%le data./n section '.* we will discuss a few of the available aids and methods of

interpreting the geological meaning of the geophysical data.

+igure ,: 0 grid of total magnetic intensity data displayed on a computer in

colour. screen as a coloured map.


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5.2 InterpretationThere are at least two di


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5e may produce a hori9ontal gradient map by calculating and colouring or

contouring the di


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+or comparison %gure '. shows a second vertical derivative calculated from the

total %eld of nearly the same map area. This map emphasi9es the shorter

wavelength magnetic anomalies thus giving us di


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+igure E: 0 comparison between %xed wing helicopter and ground magnetic

contoured maps. !oving the mouse over the left third of the picture will revealthe +ixed wing data over the center of the picture will show the 7elicopter data

and over the right side the 8round data.

The 7elicopter data gives a much more detailed view of the magnetic character

of the geology than the %xed wing survey principally because the %xed wing

survey was 3own at (*D metres of the ground on *DD metre spaced lines

whereas the helicopter survey was 3own with th magnetic sensor ,D meters of

the ground on 'D metre spaced lines. The helicopter survey also providesgreater data continuity than the ground survey since the ground survey sensor is

to close to the surface and is in3uenced by surface material. Thus the helicopter

survey provides the best data for interpretation and was used to produce the theinterpretation map bellow:


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+igure J: /nterpretation of the helicopter magnetic data. !oving the mouse over

the left half of the picture will display the helicopter magnetic data contour map.

5.2.2 $%antitati!e Interpretation

There are numerous methods algorithms and software programs in use for the

&uantitative interpretation of magnetic data we will only loo4 a few possible

methods that can be used to extract additional geological information from themagnetic data. )epth to source information is contained in the shape of the

anomaly. Because of the obvious importance of the thic4ness of the sedimentary

section to a hydrocarbon explorationist the depth to source usually referred toas the depth to the magnetic basement is of critical importance. /n addition

depth information may be important when potential mineral deposits are

covered by a thic4 layer of either consolidated or unconsolidated overburden.

The wavelength of magnetic anomalies is a fundamental result of the depth of

burial. 0ttenuation caused by thic4ness of non2magnetic overburden is due

almost entirely to the increase in distance between the sensor and the magnetic

source. This e


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+igure H: !agnetic anomalies due to shallow and deeply buried bodies.

/ndeed following ac&uier et. al. @(F'(A by calculating theoretical models of

simple bodies and then deriving graphical estimators from the models we can

use the anomaly shape to obtain a %rst approximation to the source depth underthe assumption that the model is a reasonable approximation to the sourcegeometry. 0n example of this approach for a vertically dipping di4e at a high

magnetic latitude is shown in %gure F.


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+igure F: The derivation of 6slope6 estimators for source depth from a theoretical

magnetic model. ote that at this magnetic latitude the source depth ) is about

twice the hori9ontal component of the length of the most steeply dipping 3an4sof a north2south pro%le across the anomaly.

0 variety of mathematical modeling techni&ues can ma4e 6automatic6 depth

estimates. 0 few of these are 5erner deconvolution 1uler deconvolution and

6inverse6 magnetic modeling.

5e usually refer to 6direct6 or 6forward6 modeling as the process of calculating

the magnetic response from the parameters of the source and 6inverse6

modeling as calculating a parameter e.g. depth of the source from the magnetic

response assuming that the source is a particular simple shape. !any

commercially available programs have been developed to permit an interpreterto 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.


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+igure (D: The theoretical magnetic response calculated along two pro%le lines

of a number of dipping di4es and a sphere.

!any exploration companies have developed other interpretation techni&ues

that are uni&ue 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.

+igure (D. 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. $imilarly 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

pro%les or contour maps of the observed %eld.


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6ood, P.-., Ir!ine, -.., and 6ansen, -., 1442 The 0pplication of the 0eromagnetic 8radiometer $urvey Techni&ue to 8old 1xploration in the al d=#r

!ining Camp Quebec. Canadian !ining Lournal vol. (D, no. FI pp.*(2,F

Ir!ine, -., Cepella, 7., and Pa#ne, T., 1/43 Menting 8radiometer $ystempresented at the ', annual $.1.8. /nternational !eeting and 1xposition -os

agas evada .$.0.

Misener, -ames D., 1/42 0irborne !agnetometer $urveys in !ining8eophysics 5or4shop. Paterson 8rant and 5atson -imited.

eford, M.(., 1/" !agnetic 0nomalies #ver Thin $heets: 8eophysics . *Fpp. ',*2',E

(#pe, .., 81/019 0pplication of !agnetic $urveys in Petroleum 1xploration1xploration )epartment 0moco Production Company Tulsa #4lahoma .$.0.

:a%ier, :., (teenland, ;elson Clarence, oland,., and

airborne magnetic data compilation and interpretation

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