Geoscience Reference
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geographic locations, or different depths in a drillhole.
The location assigned to the measurement is usually the
sensor location but may be some point between the trans-
mitter and the sensor. The resultant measurements, i.e. the
dataset, comprise a spatial series in the spatial domain.
Each of the measurements may comprise a single reading,
or may be a series of readings made over an interval of time
to form a time series in the time domain, or over a range of
frequencies to form a frequency series in the frequency
domain. In some geophysical methods (e.g. electrical meas-
urements), time- and frequency-series data provide the
information about the nature of the rocks at the measure-
ment location; and in other methods (e.g. seismic and
some kinds of electromagnetic measurements) they are
used to infer variations in the geology with distance from
the measurement location. This might be lateral distance
from a drillhole, but is most commonly depth below a
surface reading. The latter are then known as soundings.
Series of all types of geophysical data can be conveni-
ently treated as waves, and we use wave terminology
throughout the text. It is strongly recommended that those
readers unfamiliar with waves and their properties consult
online
Appendix 2
for details.
Exploration
model
Geophysical
model
Define survey objectives
(mapping vs targeting)
DATA ACQUISITION
Expected
responses
Survey design
Field surveys
Logistical &
cost constraints
Additional
geophysical
surveys
Topography
DATA PROCESSING
Reduction
Enhancement
Instrument
responses
Noise
characteristics
Multiple products
DATA DISPLAY
1D/2D/3D Products
Modify
based on
evolving
interpretation
Multiple products
DATA INTERPRETATION
Qualitative
Quantitative
(modelling)
Other
geoscientific
data
2.2.1
Absolute and relative measurements
Most kinds of geophysical surveys make absolute
measurements of the parameter of interest. This is not
always necessary; for some kinds of survey, notably gravity
and magnetic surveys, relative measurements provide suf-
ficient information. In general, relative measurements have
the advantage of being cheaper and easier to make than
absolute measurements.
A survey comprising relative measurements requires one
or more reference locations, called base stations, and the
measurements are said to be
Outcomes
(pseudo-geological map)
(drilling targets)
Figure 2.1
The principal stages of a geophysical programme in mineral
exploration: from identifying the objectives of the geophysical survey(s)
through to providing an interpretation of the subsurface geology.
2.2
Types of geophysical measurement
to the base stations. The
absolute value of the parameter at the base stations may be
known, in which case making comparative measurements
at other locations allows the absolute values to be deter-
mined elsewhere. For example, when we say that the
strength of Earth
'
tied
'
The parameters measured in the various types of geophys-
ical surveys described in
Section 1.2
are continuous, i.e.
they vary in time or space and without gaps or end. The
variations are an analogue representation of the physical
property variations that occur in the subsurface. Measuring
or sampling an analogue signal at discrete times or at
discrete locations is known as digitisation. The continuous
variation is then represented by a series of data samples
forming a digital series, a form most convenient for storage
and processing by a computer.
A geophysical survey consists of a series of measure-
ments made at different
is magnetic
field at a base station is
50,000 nanoteslas (nT), we are referring to the absolute
value of the
field. If the
field strength at a second station is
51,000 nT, then its relative value with respect to the base
station is +1000 nT (and the base station has a relative
value of
'
-
1000 nT with respect to the second station). If the
magnetic field at a third station has a relative strength of
+2000 nT with respect to the base station, then it has an
locations; usually different
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