Geophysical data acquisition, processing and interpretation - Geophysics for the Mineral Exploration Geoscientist - page 22

(Scott et al., 1990 ) . It is often dif cult to completely remove
the associated responses during data processing, again
because the required information about the local geology
is not available. The possibility of responses from the near-
surface needs to be accounted for when interpreting data
for bedrock responses.
At its most fundamental level, regolith and cover affect
geophysical responses by causing changes in the distance
between geophysical transmitter and receiver, and the bed-
rock geology (see Section 2.3 ) . The main problem pro-
duced by the near-surface environment is caused by its
tendency to contain complicated lateral and vertical
changes in physical properties. These create geophysical
responses, often of high amplitude, that interfere with
those originating from the bedrock. Moreover, they distort
the responses from bedrock sources measured at the sur-
face and, for active survey methods, the energy from the
transmitter before it penetrates the bedrock. Electrical and
electromagnetic methods are particularly susceptible to the
effects of the near-surface environment.
The degree and depth of weathering can be an import-
ant variable even when there is not a thick regolith
present. Some physical properties are altered by the
weathering process. Oxidation of magnetic iron-rich min-
erals can destroy their magnetism but, on the other hand,
physical property contrasts can be enhanced by
weathering. For example, weathering of the shallower
regions of kimberlites can preferentially enhance their
electrical conductivity. Mobilisation of materials in the
near-surface is another problem that can sometimes result
in geophysical responses that do not coincide with the
source of the materials. Radioactive elements are espe-
cially prone to mobilisation. The depth of erosion can
also be an important in

interest dif cult. Data processing to remove regional
responses is discussed in detail in Section 2.9.2 .

2.4.2 Methodological noise

Methodological noise may be introduced during data
acquisition, processing or display. Noise created during
data acquisition may be an unavoidable consequence of
the energy source (transmitter) creating a signal; it is
known as source-generated noise. Seismic data are a good
example because a variety of waves are created by the
seismic source, but only some of the waves are useful.
Noise originating within the recording system is known
as instrument noise but, although it cannot be totally
eliminated, modern digital instruments produce very little.
Some instruments are subject to drift, where the reading
associated with the same response varies slowly and sys-
tematically during the course of the survey. This temporal
change in the instrument

is behaviour is usually related to
changes in temperature. Readings from instruments having
a mechanical sensor, such as a gravity meter, sometimes
exhibit sudden jumps known as tares, which are related to
the mechanism undergoing small, but sudden, mechanical
changes. Both problems are monitored by making repeat
measurements at some pre-determined location, i.e. a base
station, during the course of the survey. Noise is also
caused by the inherent nature of the parameter being
measured, as in the measurement of radioactivity in the
radiometric method. Here the randomness in time of the
radioactive decay process produces statistical noise which
can only be reduced by observing and averaging more
radioactive

'

' events ' over a longer measurement period

(see Section 4.3.1 ).
Methodological noise can also be caused by errors in
orientation and position of the sensor. Accurate orienta-
tion of the sensor is fundamental to making many types of
geophysical measurements. For example, gravity measure-
ments require the sensor to be accurately aligned in the
vertical, and, for all methods, directionally dependent gra-
dient measurements (gradiometry, see Section 2.2.3 )
require the gradiometer to be orientated consistently in
the same direction for the entire survey. Positioning
requirements depend on the type of geophysical survey.
For example, gravity measurements require the sensor
elevation to be measured to within a few centimetres, and
to less than 1 cm for high precision work; whereas meas-
urements to within about a metre are acceptable for mag-
netic and radiometric measurements. Processing of survey

sgeophysical
response. For example, kimberlites exhibit vertical
changes in physical properties associated with crater
facies, blue and yellow ground, etc. (Macnae, 1995 )and
will appear different depending on the depth of erosion.
Also, porphyry style mineralisation and epithermal
deposits have zoned alteration haloes and appear different
according to which zones coincide with the ground sur-
face (Irving and Smith, 1990 ) .
At the other end of a spectrum are responses originating
from depths beyond those of immediate interest, or from
regional-scale geological features. These forms of geological
noise are known as the regional response. It is characterised
by smooth, long-wavelength variations which often make
accurate definition of the shorter-wavelength variations of

uence on a target '