Geophysical data acquisition, processing and interpretation - Geophysics for the Mineral Exploration Geoscientist - page 20

Table 2.1 ( cont. )

Electrical and
electromagnetics Source of noise Gravity Magnetics Radiometrics Seismic

Topography Terrain-related

responses
Variable relative
positions of
sensors and
anomalous bodies

Terrain-related
responses
Variable relative
positions of sensors
and anomalous bodies

Terrain-related responses
Variable relative positions
of sensors and anomalous
bodies

Terrain-related
responses
Variable relative
positions of transmitters,
sensors and anomalous
bodies

Variable relative
positions of
sensors and
anomalous
bodies

Man-made Responses due to

large buildings
and excavations,
such as open pits

Responses from
ferrous objects, roads,
pipelines, etc.
constructed of
magnetic materials
and electrical
powerlines

Responses from materials
created by radioactive
fallout from nuclear
explosions and reactors

Responses from metallic
objects, electrical
powerlines and
electromagnetic
transmissions

Sound of motor
vehicles and
heavy
machinery

to this type of surveying in very steep terrain than the less-
manoeuvrable fixed-wing aircraft. Whether drape is main-
tained or not depends upon the width (wavelength) and
slope of the topographic variation, the speed of the aircraft
and its climb capability. Small topographic features tend to
be ignored ( Figs. 2.6 and 2.7a ) producing spurious noise in
the measured geophysical response. Another problem for
airborne surveys is that an aircraft can descend faster than
it can climb. When the terrain begins to rise rapidly in
front of the aircraft, the survey pilot must anticipate the
need to gain height so as to pass safely over the hill, and so
begins ascending before the feature is reached. The result is
an increase in terrain clearance adjacent to the hill. The
easier descent on the other side of the hill often means it is
possible to re-establish the specified terrain clearance rela-
tively quickly. The same occurs when surveying across a
valley. The result is illustrated by the actual

a)

Survey aircraft unable
to maintain
constant ground
clearance

Prevention
of access

Localised
terrain-related
anomalies

Responses from rocks
not below the
sensor

Unexpected
physical-
property
contrasts

b)

Lateral & vertical
changes in
physical
properties

Variable separation
of sensor &
bedrock

Degenerated
input/response

Altering
physical-
property
contrast

Cover

Transportation
of source
materials

Bedrock

flight paths
from an aeromagnetic survey across part of the Hamersley
iron-ore province in northern Western Australia ( Fig. 2.6 ).
The terrain clearance not only varies across the hill, but
also depends on the direction the aircraft is travelling with
respect to the topography. For surveys where adjacent
traverses are flown in opposite directions (see Section
2.6.3.3 ) , the terrain clearance is fairly consistent for alter-
nate traverses and different for adjacent traverses resulting
in corrugations, i.e. traverse-parallel artefacts caused by
readings being anomalously high and then low on alternat-
ing traverses.
When measurements are made on the ground the geol-
ogy may be adjacent

Figure 2.5 Environmental noise associated with (a) topography and
(b) the near-surface environment. Cover could be unconsolidated
sediments, regolith, glacial till or ice.

achieved depends on a number of factors, notably how well
the local geology (and associated physical property vari-
ations) and topography are known.
Historically, airborne surveys were usually conducted at
a constant barometric altitude above the undulating ter-
rain. Modern airborne surveys are draped over the terrain
as the aircraft attempts to maintain constant height above
the undulating ground surface, i.e. they are flown at
constant terrain clearance. Helicopters are better adapted

to, or even above, the sensor

( Fig. 2.7b ). This is also a common occurrence for