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General Comments
 Gravity
measurements are a simple and inexpensive source of information about the
subsurface structure of an exploration target. These data require only some
simple data reduction to be converted into interpretable anomaly values.
By “anomaly,” we
mean departures from values that can be calculated for a simple model of the
Earth, including compensation for the variations in gravity due to measurements
being made at different elevations. The overall gravity field varies from about
978 gals (1 gal = 1 cm/s2) at the equator to about 983 gals at the
poles. These variations easily can be accounted for by using the international
gravity formula in which the predicted value of gravity varies with latitude.
We often find
anomalies of a few milligals to be geologically significant, and the effect of
changes in elevation alone is about 0.3 milligal/meter. Thus, a crucial
consideration in gravity surveys is the measurement of the elevation where the
measurement is made. In the case of land surveys, determining the elevation to
an accuracy of a few centimeters involves a larger effort than making the actual
gravity readings. Because of the large effects topography has on gravity
measurements, correcting for these effects in mountainous regions poses a
special challenge. However, the broad availability of digital elevation models
worldwide, thanks to efforts such as the Shuttle Radar Topography Mission, and
the ability of computers to handle massive data sets have brought this problem
under control.
The specific type
of anomaly that is usually employed in gravity studies is the Bouguer anomaly,
which is called the Complete Bouguer anomaly if the reduction process includes
terrain corrections. Much has been made of the fundamental ambiguity that
gravity anomalies clearly reveal the presence of mass, but allow for an infinite
combination of density-volume products to model a specific anomaly.
Given geologic
constraints, drill hole data and supporting geophysical data, this ambiguity can
be drastically reduced. Thus, these data are particularly useful in:
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Determining the regional
structural setting of an exploration project.
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Helping optimize seismic data
acquisition.
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Providing structural
information in areas outside seismic data coverage or at depths that exceed
the limits of seismic control.
Classic
applications of gravity data that define prospects directly include:
If one has a good
geologic understanding of an area, the qualitative interpretation of gravity
data is in fact quite straightforward. The key is thinking about density
contrasts -- sediments (2.5 gm/cm3) versus basement (2.7 gm/cm3)
-- that would produce a particular anomaly. For example, a gravity high could be
due to a structural uplift that has brought denser (older and more compacted or
cemented) rocks near the surface, and a gravity low could be due to a
sedimentary basin that contains rocks less dense than the surrounding geology.
Two-dimensional
models are a more quantitative form of interpretation and should be derived in
much the same way as the construction of a geologic cross-section in that the
process should involve integration of all available information.
Three-dimensional modeling also is often undertaken.
Gravity studies
often begin with public domain regional scale data sets consisting of point
measurements taken on land, track data recorded from ships or aircraft, or even
data derived from satellites. Land and marine measurements recently have been
compiled into a large database for the contiguous United States, and a North
American database effort is well under way. These databases are the result of a
cooperative effort by the U.S. Geological Survey, the National
Geospatial-Intelligence Agency, NOAA/NGS (National Oceanic and Atmospheric
Administration / National Geodetic Survey) and university groups.
Figure Captions
Example:
Rio Grande Rift
The current
version of this database can be accessed at http://paces.geo.utep.edu,
and an example of the data is available for the southern Rio Grande rift region
(Figure 1). The Rio Grande rift is a major
continental rift zone that is associated with a series of deep basins that both
follow and cut across older features. The data shown in Figure 1 were gridded
and contoured to produce the Bouguer anomaly map shown as
Figure 2. This map is dominated by a strong regional
increase of anomaly values (~100 milligals) from northwest to southeast that
obscures the more local anomalies due to the basins.
This regional
anomaly is primarily due to the crustal thinning across the Rio Grande rift and
to a batholith that is found beneath the Datil-Mogollon volcanic field to the
northwest. These effects illustrate a well-known problem in gravity studies;
namely, the separation of the regional field from the more local anomalies in
which we are interested.
In
Figure 3, the regional field has been removed by
applying a simple band-pass filter to the data, so that the gravity lows due to
the basins become evident. The filtered map clearly shows the north-south
trending basins associated with the Rio Grande rift and provides a quick outline
of their geometry and relative depth.
The map’s southern
portion shows strong northwest trends that reflect Laramide uplifts.
Summary
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The analysis of gravity data
is a very cost effective exploration tool.
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Regional data often are
available from public domain sources and can provide a useful starting
point.
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Many data brokers provide
more detailed data at a reasonable price.
Any gridding and contouring software can be used to
turn the gravity measurements at points into maps.
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