INTRODUCTION
Traditionally,
magnetic data have been used in early phases of exploration programs to
map depth to magnetic basement and define the basin architecture. Recent
improvements in acquisition and processing technology, together with
more detailed understanding of structural styles in exploration areas,
allows us to now say:
“ Magnetic data are not just
for the basement anymore.”
This article describes
methods of interpreting magnetic anomalies. Fundamental concepts, or
“rules-of-thumb,” including wavelength, amplitude, methodology, interpretation (geologic) concept, and
depth-to- magnetic -source analyses, are summarized, along with modeling,
trend and lineament analyses, and filtering. Although
there are certainly alternative approaches and/or techniques that may be
used, the purpose here is to provide a framework for geoscientists who
may be unfamiliar or do not regularly work with magnetic data.
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Figure
Captions
Figure 1: Three-dimensional
perspective of magnetic data and Earth model.
Figure 2: Two-dimensional
cross-section.
Figure 3.
Two-dimensional magnetic model along a seismic line.
Figure 4. Filtering magnetic data is a
qualitative aspect of interpretation. Graphic (a) shows total-intensity
magnetic anomalies, with major trends identified. Graphic (b) shows
filtered magnetic anomalies, with additional – more subtle – trends
identified.
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Rules
of Thumb
Wavelength
In general, the wavelength of an anomaly is
proportional to the depth of the magnetic source body that produces it (Figure
1). More correctly, depth is related to the horizontal distance of
the slope of the anomaly.
As with other geophysical data, long
“wavelengths” are related to deep sources (or events), and short
“wavelengths” are related to shallower sources. Outcrops of the San
Juan volcanics in southwestern Colorado have narrow, high frequency
anomalies, while the deep basement in the Williston Basin is represented
by relatively broad highs and lows. High-frequency anomalies are also
observed over the Devil’s River Uplift in West Texas. Adjacent to the
uplift, anomalies are broader, indicating a dramatic deepening of the
basin.
On a magnetic map, an anomaly high is not
necessarily produced by a structural high. Rather, an area of closely
spaced, sharp, short wavelength anomalies implies shallow basement and an
area of smooth, broad, long wavelength anomalies implies deep basement.
With a practiced eye an interpreter can quickly pick deeper from shallower
areas.
Amplitude
The amplitude value is proportional to the
magnetic susceptibility contrast in the rocks beneath the magnetometer.
“ Susceptibility ” is a measure of the ease with which a rock can be
magnetized. Geologically it can be thought of as a measure of the
magnetite content, although a few other minerals may contribute under
special circumstances.
Amplitude does decrease with increasing
distance from the source, but not to an extent that effects the following
concepts.
Amplitudes can generally be divided into
categories of hundreds of nanoteslas (nT), tens of nT, and ones of nT. The
nanotesla (nT) has been adopted by our industry as the “official” unit
of measure for magnetics. It replaces the gamma (y); in other words, 1 nT
is equal to 1 gamma (y).
Lithologic variations in magnetic basement,
or the presence of igneous rocks within the sedimentary section, generally
produce anomalies with the highest amplitudes. For example, the
magnetization of intra-basement features may be stronger than surrounding
basement rocks. In this case, large amplitude anomalies would be observed
where basement structures are not present.
The East Coast Magnetic Anomaly, with an
amplitude of several hundred nT, is related to the contact between oceanic
and continental crust and to possible intrusive rocks along it. In the
Black Warrior Basin of northwestern Mississippi, an area of low magnetic
intensity is bordered by high-amplitude anomalies and is, in fact,
structurally high. The basement in this area is, in fact, structurally
high – as proven by several exploration wells.
To summarize, high-amplitude anomalies
typically reflect lithologic contrasts, whereas anomalies produced by
structures are usually more subtle. Anomalies with amplitudes on the order
of:
100s nT – are
related to lithologic variations in basement or igneous rocks with the
sedimentary section.
10s nT – are
related to basement structures (supra-basement).
1s nT – are related
to sedimentary magnetization contrasts.
Methodology
A typical approach for interpreting magnetic
data involves geologic research, including an assessment of existing
geologic and geophysical control, depth-to- magnetic source estimation, 2-D
and 3-D forward modeling, data inversion, analyses of anomaly trends
(using observed data and its derivatives), and data filtering.
It is not necessary to follow a specific
order when applying these elements, but final products usually involve
producing geologic map(s) that incorporate information from one or more
elements.
Geologic
Concept
The most important element required for
interpreting magnetic data is a geologic concept or structural model. We
are never blind; that is, even if the only data available in an area are
magnetic data, we know the area is in a rift setting, or a foreland basin,
or along a passive margin, etc.
We also know the survey’s location; hence,
we know the attitude of the magnetic field or its inclination and
declination and strength. The poles of the Earth’s magnetic field are
not aligned exactly with its geographic poles, and therefore inclination,
declination and field strength indicate the direction and magnitude of the
field relative to geographic position.
When interpreting geophysical data, it is
most important to apply known geologic control to constrain the
interpretation.
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Depth-to- Magnetic
Source
Depths determined from magnetic data can be
confidently estimated to about +7 percent. When an entire data set
is interpreted by consistent methods, the interpretation map will show
structural highs and lows that are relative to each other. Although depths
are not known exactly, the horizontal positions of anomalies are directly
related to locations of interpreted sources, so there is no ambiguity with
regard to geographic position (Figure 2).
There are many depth-to- magnetic -source
estimation techniques, manual and automated. The important thing to
remember when applying these techniques is to be consistent. The end
product will then be a map of posted values that are all relative to each
other. It is helpful to generate hypothetical 2-D models, incorporating
the appropriate magnetic -field attitude and strength in order to see
relationships between structures and the position of anomalies over them (Figure
2).
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Modeling
A
two-dimensional magnetic model (Figure 3) can be created along a seismic
line in order to check, for example:
If
an interpreted depth to magnetic basement is reasonable.
If
a sedimentary structure is supported by a basement structure.
If
a feature on a seismic section is salt or igneous, etc.
This
type of modeling is called forward modeling. For inverse modeling, the
observed data and a starting model are used. Then either model geometries
or magnetic susceptibilities are modified until the calculated field
produced by the model “fits” the observed field. Three-dimensional
modeling is similar, utilizing gridded data and surfaces.
Two
variables are involved in modeling: magnetic susceptibility and geometry
of source bodies. Using control such as seismic, gravity and well data,
geometries may have little variability – thus modeling involves
adjusting magnetic susceptibility . If there is no control other than
magnetic data, then it is best to keep susceptibilities constant and
modify geometries.
Magnetic
data also can be used to constrain interpretations of other data sets. For
instance, geological cross-sections are interpretations, and magnetic
interpretations can improve such work in areas of ambiguous geology.
It
is easy to create a complex model, with an excellent match between
computed and observed magnetic anomaly profiles, that far exceeds
available control. Therefore, it is:
Not
appropriate to modify geometry and susceptibility in magnetic models
randomly with no control.
Not
appropriate to model using filtered data, because we do not know if the
component of the magnetic field removed by the filter is also removed in
our model.
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Trend
and Lineament Analyses
Depth-to- magnetic
source estimation and modeling are quantitative techniques. An important
qualitative technique is analyses of trends and linears. Trends can be
analyzed using profiles or gridded data and generally consist of lines
drawn on a map that may correspond to edges of structures, faults, or
partitions of the data character (Figure 4).
Subtle
linear breaks in magnetic data, especially where correlated with features
identified from other data sets, may indicate positions of complex
structures in the prospective section. For example, part of the data may
be characterized by short wavelength, high-amplitude anomalies, and
another part of the data may be characterized by longer wavelength
anomalies.
Geologic
examples are accommodation zones in rifts, wrench anticlines in convergent
settings, and even zones of fracturing. Trends also may be defined as the
termination of linear anomalies.
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Filters
Filtering
magnetic data is also a qualitative aspect of interpretation (Figure
4).
The objective of filtering data is to separate anomalies by wavelength,
and this operation can be performed several ways through manual and
automated techniques. The most effective way to filter is with an
understanding of the geologic control and an idea of the desired filtered
results.
A
typical process involves producing suites of filtered maps and assessing
their character with geologic control. Filtering data is a powerful tool
and often leads to important conclusions, but its use should be driven by
the nature of the geologic problem to be solved.
Recent
advances in navigation (Differential GPS positioning), computer systems,
and processing now allow extremely subtle anomalies to be resolved. For
example, anomalies produced by small magnetization contrasts within
sedimentary rocks can be confidently mapped. Filtering and trend analyses
are techniques especially suited for interpreting these subtle anomalies.
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Summary
Interpretation
of magnetic data should include elements of both qualitative and
quantitative analyses, which in turn should be guided by geologic
concepts. This does not mean that the interpretation should be forced to a
rigid concept, but that the end result must be geologically plausible
given the control.
The
interpretation should contribute to the overall geologic picture, and our
understanding should be modified and improved by the data. On the other
hand, quite often we generate more questions that may be as useful as the
geologic questions already answered by our interpretation of magnetic
data.
Fundamental
understandings of magnetic data and interpretation techniques, as outlined
here, are valuable tools that geoscientists can use to gain insight and
improve their geologic knowledge of an area. As with geology, often the
subtle features of the data – and their meaning – are most important.
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