CHAPTER 3. EFFECTIVE DISPLAY OF DATA 36

VERTICAL GRADIENT CONTOURS

CHAPTER 3. CHAPTER 3. EFFECTIVE DISPLAY OF DATA 36

Where vertical gradient data have not been collected,

it

is possible

to

approximate both the one-dimensional (Paine, 1986) and

the

two-dimensional gradient using

the profile

and gridded

total

magnetic fleld data sets respectively. The computed one-dimensional gradient ovet horizons

which

are continuous, and whose

strike

does

not

parallel

the flight path direction,

compares favourably

with the

measured gradient over

the

same.

The

two-dimensional

vertical

gradient was computed

from

a 15

x

15 convolution operator supplied

by T.H. Whiting.

Unlike the relatively smoothly varying

total

magnetic field which can be effectively displayed

in

many different formats, the dominance of high frequencies

in

the gradient data does not lend

itself to clarity in

contour map dìsplays.

This

can be observed

by

comparing Figures 3.2 and 3.3. The map needs to be

in

colour to enable easy recognition of highs and lows (this is desirable

in

maps of

the total

^field

but

indispensable

in

those of gradient

data).

Contouring algorithms designed

to

handle smoothly

varying data

cause

a

large number

of

closures

to be

drawn on gradient maps, and

the

choice

of

contour levels

is

restricted

by the

large order of magnitudes

of

gradient va.lues. Images

of the

gradient avoid

the

problems associated

with

contouring the

function. In

any case, the two-dimensional computation introduces spurious anomalies. Stacked profiles of the vertical gradient are ^a much more satisfactory form of display (compare Figures 3.9 and 3.3) and

will

be dealt

with in

Section 3.2.

3.1.5 Interpretation stages

As the interpretation proceeds from the

initial

evaluation of the survey and terrain, through the various stages,

to

the

final writing

of the report, the map requirements of the interpreter varies.

The

flow chart

shown

in

Figure 3.4 suggests a possible selection

of

displays

at

different stages

in

the

interpretation

process.

Survey and terrain evaluation

Once located data tapes are made available, the data should be represented as a small scale map of the

total field.

Gridding the data (either coarsely or on the

finai grid)

and then

plotting

as a

contour map

is

a good

start.

From

this

map, the interpreter

will

be able to judge whether the survey was

flown

successfully and

the

data correctly levelled and reduced.

The

map

will

^also

show

what the

magnetic

terrain is

lil<e and

the

number and characteristics

of

different zones.

This information is used

to

^plan

further

displays.

If

the map indicates an unacceptable amount of noise

(in

the

form

of stripes, herringbone patterns. . .

),

additional processing

to

smooth and

filter the

data may be recluired. Poorly levelled and noisy data sets can be

better

displayed as

pixel images rather

than

as contour maps.

P

reliminary interpretation

The next

stage involves

the

production

of

maps

at a

suitable working scale. Black and white contour maps of the

total

field are standard and relatively inexpensive compared to hard copies

of

digital

images. The

grid

spacing should be intermediate between

the

nominal

flight

line and sample spacing

- ^the

^closeness

^{to the}

^{sarnple spacing}

^will

^{depend on}

the quality of

the data.

The

scale

of the map

should

be

such

that

adjacent

flight

lines are less

than 1cm apart. A

scale

larger than this

makes

trend

analysis

difûcult.

Colouring

the

contour maps serves the dual purpose of promoting

familiality with

the data and providing _{a display}

in

which magnetic zones may be more easily identifled.

A

photographically reduced coloured version of the contour

Figure

3.5:

Colour composite image

of total

magnetic

intensity (blue) and total

garnma ra-

diation

count

(red).

The image was displayed on

the AUDIA

system (see Section 3.3.1) and photographically reduced and compiled. The colour scale is an increasing scale with

light

colours representing high values. Data

for

the lower

right part of the study

area was

not

available at the time the image was produced. The single most

important

advantage of using colour rather than grey-scales is

that

two or more data sets may be merged.

CHAPTER 3. EFFECTIVE DISPLAY

OF DATA ³⁷ maps of

the

study area has been reproduced

in

Figure

5.4.

The scale of the original maps was 1 : 50 000.

Detailed interpretation

At this

stage, working scale maps of the

total

magnetic

field,

and possibly

its

vertical gradient and upward continued field, and of the geology are desirable. The frequency content of the data set and

the aim of the interpretation

are used

to

decide

which

displays are most

suitable. If

geological maps aïe available

at

scales larger

than

the working scale, then the interpreter may wish

to

produce magnetic maps

at that

same scale

for

easy comparison. Overlays of black and

white

geological maps are convenient

to

use. Stacked profiles of the vertical magnetic gradient

(or

some

other

derivative) can be used

in trend

analysis and mapping

of

magnetic horizons.

Regional

structural

features may be delineated from enhanced

digital

images.

Q

uantitative interpretation

Side by side

with

maps for qualitative interpretation, the interpreter

will

need ready access to the recorded

total fleld

data,

at

a

vertical

scale suitable

for quantitative interpretation.

Modelling

of

anomalies

is

usually carried

out

on graphs

of

profiles

of the total

magnetic

field,

and these may be displayed on papet

or

on a graphics

terminal

connected

to

a computer. The graph for each

line

should display some combination of the following

functions:

the

total

magnetic field,

the

ground cleanance,

flight path,

and perhaps functions

of the total field

such as

its

upward continued values and derivatives. Stacked profiles of the

total

field are not usually recommended:

they take up

too

much paper, are cumbersome

to

handle and are unsuitable

for

trend analysis.

Integration of inforrnation

Overlaying maps on

light

tables

is the

conventional method

of

integrating geological and ^geo- physical

information.

The difficulties associated

with

this method (mainly the problems of exact positioning and of comparing data collected

at

different scales and formats) can be overcome by using imaging techniclues.

Any data

set which

is

available

in digital form, or

can be digitized (e.g. geological maps), can be integrated

with

aeromagnetic data through imaging.

Radiometric total count data

integrated

with total

magnetic

field data

are presented in Figure 3.5. Such an image permits the visual correlation between the radiometric and magnetic properties of various rocks types.

In this

flgure, a l-ight

pink

(nearly

white)

colour characterizes rocks (migmatites and granites)

wliich

give rise

to

magnetic anomalies and radiometric highs.

Such rocks are seen

in

the central

portion

of the figure.

Light

blue patterns indicate rocks which have a high magnetic anomaly associated

with

them

but

low radiometric count.

3.2 Stacked Profiles

Stacked profiles provide an alternative map display

to

contour maps and

digital

images. The most

important

difference is

that

no gridding of the data is required and therefore

fldelity

of the

data

can be

maintained.

Stacked profiles can be used

to

locate anomalies accurately, provide

quality

control on the survey, and present another perspective.

6109000 6109000

6090000 6090000

29s000

EASTING (metres)

³¹

Figure 3.6: Stacked prolìles of vertical magnetic gradient using a sm*J vertical scale.

6109000 6109000

6090000 6090000

295000

EASTING (metres)

31

at,

o o E

(5

z

t-

fE

o z

U,

o o

E

o z

T Þ

fE

o z

Figure 3.7: Stacked profiles of vertical magnetic gladient using a large vertical scale.