Part II

Processingr presentation ^and

interpretation ^techniques

CHAPTÐR 3. EFFECTIVE DISPLAY

OF DATA ³²

Chapter ³

Effective display of aeromagnetic and aeroradiometric data

The information available

in

aeromagnetic and aerotadiometric data is generally under-utilized, and

this

is

in part

due

to

^poor display of the

data.

The effective display of a data set

is

crucial

to its

efficient

interpretation

(Rajagopalan and Boyd,

in press). To

maximize

the

amount of

information

extracted

from the data

set and overcome

the limitations

imposed

by

using only one

kind

of display

format,

several dispiays should be used

to

provide different perspectives.

Conventional methods

of

displaying aeroradiometric data, namely as contour maps or pro-

files,

have tended

to

downgrade

the information

content

and restrict the integration of

this

data

set

into the interpretation

moclel.

Digital

images have transformed

the

interpretation of aeroradiometric

data

and emphasize the importance of matching data sets and displays. This chapter ^deals mainly

with

the display of aeromagnetic data as there are fewer ways of displaying aeroradiometric data effectively.

It is

assumed here

that digital data

are available

and that all

necessary corrections and reductions

(diurnal

correction, levelling, removal

of the International

Geomagnetic Reference Field. . .

)

have been applied

to the

data set.

Methods for

the

display of fine detail

in

aeromagnetic data and for the production of digital images which have been developed by

the

author are presented as separate sections.

Examples presented

in this

chapter are taken from

the

study area. Except

for

the image in Figure 3.5 (which covers almost all the study area), every other map shows different presentations of

the

aeromagnetic data over the lower

left part

of the study area.

3.1 Systematic approach to displaying geophysical data

Selection of the

format

and design of the displays (function displayed, whether as contour maps or images or profiles, scale of presentation, use of colour) is dependent on the facilities available and on these geophysical

criteria:

aims of interpretation, magnetic characteristics and geology of the area, and the survey specifications. This chapter analyzes the qualities of good displays, and

the

geophysical

criteria

which govern

the

choice

of

displays.

A

systematic approach leading to the selection of the most effective displays is presented here and demonstrated through examples

from

the study area.

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for line 260. These ^are produced at a large scale for ^use

in

quantitative analysis.

CHAPTER 3. EFFECTIVE DISPLAY

OF DATA ³³

3.1.1 ^{Display formats}

Airborne geophysical data are generally collected along proflles which are oriented

in

the same

direction.

The direction of the

flight

lines is usually ^chosen to be perpendicular to the dominant strike of the regional geology. The frequency

of

data collected along the profiles is

the

sample spacing and

the

distance between adjacent lines

is

the

flight line

spacing.

The

nominal flight spacing

is

12 times

the

sample spacing

for the CRA

data and 6 times

the

sample spacing for the Pacific

Expl.

data (see Appendix

A).

Tie lines used for levelling the survey a¡e also used to display and

interpret

the suïvey. The varying density of information

in

directions parallel and perpendicular

to

the

flight

line direction is an

important

factor when planning displays.

Standard display formats can be classed

into

two groups: profile displays and gridded data displays. The

f.rst

group includes single profiles of the measured geophysical field, combinations

of

different functions

of the

measured fields, as

well

as stacked profiles

of a

single function of

the

measured

data.

Contour maps and

digital

images are examples

of

gridded data displays.

Each display serves different

but

complementary purposes (these are

outlined briefly

below and

at length in the

following sections) and together

they highlight

different features

of

the aeromagnetic and,

to

a lesser extent,

the

aeroradiometric signal, which would otherwise have remained undetected.

Multi-profiles,

for

a single

flight

line,

in

which several functions of the aeromagnetic field are displayed are invaluable

for

quantitative

interpretation.

Figure 3.1 shows, for line 260, the

total

magnetic freld,

its

upward continued field,

its

computed one-dimensional vertical gradient

(VG)'

and the

flight path

and

altitude

during the

flight.

^{Both the}

total

magnetic field and

its

vertical gradient can be used

to interpret

anomalies

quantitatively

(Chapter

4). The

one-dimensional vertical gradient was computed

from total

magnetic fleld profile data using a cluarlrature

filter

(differencing

interval:

4, sampling

interval:

1) developed

by

Paine (1986).

On a map of stacked profi.les, several

flight

lines are displayed on

the

same map sheet

with the

base

line

of

the

displayed

function

being the

flight path.

Stacked profiles (see Figure 3.9) provide an alternative map display

to

contour maps and digital images and can be used to locate anomalies accurately, provide

quality

control on the survey, and map linear anomalies.

Gridded data displays provide an overview of

the

data,

permit

easy correlation

rvith

other data ^sets and aid regional and qualitative

interpretation.

These displays ^have several advantages over displayed

profiles: they

are visually pleasing and provide a good

pictorial

representation

of

many recorded

data points.

However,

fine detail in the

magnetic

field may be

obscured,

distorted or wrongly

represented

in

such displays

(Mclntyre,

1981), since

gridding

packages cannot completely compensate

for

the varying density of data points

in

different directions.

Contour maps (Figure 3.2) ^are familiar to most interpreters. In

tightly

folded legions, contour maps can help indicate magnetic trends which may otherwise be hard to

identify.

On the other hand, deviations

in

flight line paths are hidden,the choice of contour levels is

difrcult,

individual trends may be obscured, significant but low-amplitude anomalies may be overshadowed, and the map may be misleading because of

its

bias towards certain

trends.

Small scale contour ^maps may have poor resolution due

to

closely spaced contour lines.

Instead

of

producing contour maps

from the

gridded data,

pixel or digital

irnages may be

produced. These may be

in

colour

or in

shades of grey (Plates

3

and

4). They

can easily be enhanced

to

delineate directional trends and are

particularly

effective

at very

small scales at which other forms of presentation may become

too

complex

to

analyze.