CHAPTER 3. EFFECTIVE DISPLAY OF DATA 40

AMPLIFIED PROFILES

CHAPTER 3. CHAPTER 3. EFFECTIVE DISPLAY OF DATA 40

time. The

display

is

reasonably

faithful to the original

data

set. The

visual display

is

clear,

making it

possible

for the interpreter to

delineate trends, and

in the

process,

to

distinguish between signal and

noise. The

map

of the

profiles can be overlain

on other

maps and used, together

with

profiles

and

contour maps

of the total

magnetic

intensity, to

provide

a

mole complete

interpretation

of aeromagnetic ^{dat a.}

3.3 Digital images

Imaging techniques have immeasurably improved the display and interpretation of gridded data sets. Techniques

for digital

image processing applied

to LANDSAT

data have been and can ^be extended

to all

geophysical data sets, including aeromagnetic and aeroradiometric

data.

Digital images can be produced

from

gridded

data

sets using an image processor and display device.

Essentially,

the

gridded

data

set

is

defined

to

consist

of

^pixels

which

are

then

mapped

to

an image where

the

colour

or

grey level

of the

image

pixel is

a

function of the

value

of the

grid pixel.

Digital

image processing has ^a number of advantages: a wide range of data types and formats can be easily incorporated

into the

same image

format for

analysis, data collected

at

different scales or

with

different resolutions can be processed

to

a common map projection and scale, and most

important of all, two or

more

data

sets can be combined

or

merged

into

one display so

that

correlations can be readily identified (Guinness et

alr

1983). The different images may be

integrated on ^a

TV

screen (or colour

printer),

by creating a colour composite image, by flickering between

the

different images registered over each other,

or

by using a

split

dcreen. The colour composite may be

a

simple

t'addition"

where each

of two

or three data sets

is

assigned one of the

primary

colours, and the composite colour for

anypixel

is the addition of the primary colour intensities,

or

a more complex image may be created, where one data set is used

to

control the intensity and the other data set the hue of the colour for each pixel (see Guinness et al., op. cit.).

The major interpretation advantages lie in the visual delineation of large scale features which might

not

have been recognized

in

other, more conventional, displays such as contour maps and

profiles.

Images are

the

most satisfactory

form of

displaying aeroradiometric

data

(see Smith, 1985) as

the

data set is inherently noisy and tl"Le signal

to

noise

ratio is

generally

low.

Digital processing enabled Kowalik and Glenn (1987)

to

make a direct comparison between TANDSAT and aeromagnetic

data

and improve

structural interpretation. Similarly, Karner et al.

(1987) have shown

that

"geotectonic images", formed

by integrating British National Gravity

and Topographic Data Bank

with

SEASAT derived gravity anomalies and

with

gridded bathymetric data, can benefit tectonic studies. Subtle

structural

associations

with

gold mineralization have been traced

from

images of magnetic data of the

Yilgarn

Province (Isles ^eú ø/., 1988).

One of the basic activities which constitute

digital

image processing is image enhancement.

This

includes increasing

or

decreasing

the

contrast, edge sharpening

or

smoothing,

or,

more simply, altering the image

in

^some respect which facilitates the

interpretation

of

its

information content

(Hord,

1982). The image contrast may be enhanced

by

stretching the histogram either

linearly or

non-linearly (histogram eclualization). Further,

the

contrast may also be improved by using fewer grey levels

than

the maximum provided and thus mapping all

grid

values

within a digital

range

to

a specified grey level (density

slicing).

Linear features are formed

by

^edges.

Edges may be enhanced

by

using non-directional and directional

filters. With

non-directional filters, linear features

in all

directions get enhanced, maximum enhancement being obtained for edges parallel to either diagonal of the

filter.

Directional filters are used to enhance specific linear trends

in

an image. Usually, the analyst

will

run filters

in

different directions (e.g. N-S, NE-SW,

Figu¡e

3.12:

Grey-scale shaded-reiief images

of total

magnetic

intensity. The

elevation and azimuth of thc

ill¡minating

"source" is given for the three ima'ges from top

to bottom:

^{15o and} 225o;30" and 135o; 45o and _'[5o.

CHAPTER 3, EFFECTIVE DISPLAY

^{OF DATA}

4t

NW-SE and

E-W)

and produce separate

prints

of each trend-enhanced

image.

Examples and use of these filters is described

by

Sabins (1987).

Especially when viewing extremely large data sets (covering areas of the order of hundreds of thousands of square kilometres), the

digital

image cannot be matched

in its ability

to display

information in a

comprehensible

format. ^The

coarseness

of

contour maps

in

comparison

with

the subtle features detected

in

images can be ^seen

in

maps of Canada presented

by

Dods eú a/.

(1g8b). The maps include shaded-relief images. The impression received

is

akin

to

viewing an

illuminated

topographical surface.

This

effect

is

achieved because variation

in light

and ^shade is one of the depth ^cues required by the human ^eye to identify a three-dimensional ^shape

(llorn,

1981).

By this

means the magnetic data acquires

a "texture"

^and

this texture

reflects on the regional geology and is used

to

subdivide the region

into

magnetic terranes'

The

technique

for

using

light and

shade

in pictorial

representation

of

three-dimensional shapes has been used

by

artists

for

many centuries.

Horn (op. cit.)

developed an algorithm which could simulate the effects of the

illumination

of ^a

hilly

region. The apparent brightness of

a surface element depends on its local gradient. Complicating factors ^{such as} mutual illumination

of

surface elements and shading

by

neighbouring elements, which are

in

any ca,se irrelevant to magnetic data, were

not

considered. The brightness of the pixel is taken

to

be proportional to

the

cosine of the angle between the direction of the

illumination

source and the surface normal.

The brightness is

primarily

sensitive

to

the horizontal gradient

in

two orthogonal directions and is therefore similar

to

the horizontal gradient map.

As in

directionally

filtered

maps, shaded relief maps can be varied

to highiight

trends in different directions. The ^variables

in

a shaded relief map are

the

azimuth and elevation of the

illuminating

source. The effect of varying

the

azimuth is similar

to

directional

filteting. If

the source

is in the NW, then

N\M trends aïe suppïessed relative

to NE trends. If the

source is shifted

to

the SE, the directional enhancement stays the same.

But

the maximum rellectance ^is shifted from northwest of the anomalous trend

to

southeast of

it.

Lowering the elevation of the souïce can be used.

to

exaggerate

the "terrain

elevation" and

to highligtrt

subtle

and/or

^weak magnetic features.

Weak

but spatially

coherent

trends, and

curvilinear

and dendritic patterns

can

be

more clearly delineated from

digital

images,

particularly

shaded-relief maps, than from more ^conven-

tionai

displays (Chandler, 1985). Figure 3.12 ^shows a set of

digital

grey-scale images' The data set

is

common

to

most

other

figures

in this chapter. The

effect

of

varying

the

azimuth and elevation of the source is self evident.

3.3.1- Image ^displaY

Images ^are usually displayed on ^a

TV

^{screen connected to a computer,}

^with

^{hard copies produced}

only after the interpretation

^{has been}

flnished. The

disadvantages are

that

these copies may be expensive

to make.

Also, while large scale structures and trends may be delineated ^easily on images,

the

positioning of these structures is easier on contour maps

than

on images. Low- amplitude and regionally restricted anomalies may be obscured on images through the using of

only

a few grey levels

or

colours

(typically

^less

than

16).

As imaging is often carried out on special purpose interactive plocessols connected to display systems

*ti.h

^may be "ephemeral", €.B.cathode ray tube

(CRT)

screens'

or _"hard

copy" such as

ink-jet

^printers,

^the

^use

^of this

powerful interpretive

tool is

generally

limiterl to

^{those who}

have ^access

to

dedicated image processing and display equipment.

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