Geoscience Reference
In-Depth Information
Summary
.....................................................................................................
￿ Geophysical measurements respond to changes in the physical properties of the subsurface. Some common causes
include a body of mineralisation, a contact of some kind, or the effects of weathering and alteration. These cause
deviations from the background geophysical response and are called anomalies.
￿ Geophysical responses measured at the detector become weaker the further the source is from the transmitter (if there is
one) and the detector. Increasing source-detector separation causes the geophysical response to extend over a wider
area, i.e. there is an accompanying increase in wavelength of the anomaly.
￿ Responses not associated with the objective of the survey constitute noise. Environmental noise may be geological,
e.g. terrain-related, or man-made, e.g. energy originating from powerlines. Methodological noise comprises
unwanted consequences of the geophysical survey itself and of the processing of the geophysical data. A feature
in the data caused by noise is referred to as an artefact.
￿ The two most common objectives of mineral geophysics are to map the local geology and/or to measure
responses originating from the mineralised environment. Mapping surveys provide essential geological context,
and in areas of poor outcrop may comprise the only useful form of 'geological' map available. Surveys for
exploration targeting may be intended to detect or to define the geological/geophysical features of potential
significance.
￿ A geophysical survey consists of a set of discrete measurements of what is a continuously varying parameter. In order
for the samples to accurately represent the true variation in the parameter, the measurements must be made at an
appropriate interval. Failure to do so results in aliasing, i.e. the creation of spurious long-wavelength variations in
the data.
￿ Geophysical data processing comprises two distinct stages: data reduction and data enhancement. These are followed
by data display. Reduction is the application of a variety of corrections to the raw survey data to remove obvious errors
and correct for sources of noise. Data enhancement involves modifying the data in specified ways with numerical
algorithms. We recognise three basic kinds: the merging of repeat readings (stacking); the comparison of two
measurements (ratios); and more mathematically sophisticated manipulations of the data (filtering). Image processing
methods are used to display, visually enhance and integrate multiple datasets to create readily interpretable pixel images
of the survey data.
￿ Interpretation of geophysical data begins with a qualitative analysis of the data. This usually involves target
identification and/or the creation of a pseudo-geological map. This is often followed by matching the measured response
of selected features with the computed response of a model, i.e. a (simplified) representation of the subsurface geology.
This is a form of quantitative analysis.
￿ The first step in making a pseudo-geological map is the creation of a structural framework from the linear
and curvilinear features identified in the data. These may be interpreted from the alignment or truncation of other
features, or areas of pronounced gradients; and they usually coincide with contacts and/or faults/shear zones.
Next, consistent textural and tonal characteristics are used as the basis for mapping pseudo-lithological
subdivisions.
￿ Modelling provides important information about source geometry and its location. However, it is important
to recognise the limitations of the result, which is ambiguous (non-unique). Ambiguity can be reduced
by incorporating petrophysical and geological data. Varying the model parameters and observing the effects
on the computed response can quantify the range in model parameters producing an acceptable match
to the data.
 
Search WWH ::




Custom Search