Geoscience Reference
In-Depth Information
Anomalies of hundreds of gravity units are common
over large igneous intrusions and sedimentary basins, but
most mineral deposits produce responses of only a few
gravity units. An anomalous response of 0.1 gu is 1 part
in 10
8
of the Earth
Gravity gradient measurements are mostly made from
the air because they are signi
cantly less affected by the
large accelerations associated with the movement of
the aircraft than measurements of normal gravity (see
We describe the instruments and procedures used for
measuring gravity and gravity gradients on the ground and
in the air. Reduction of the gravity data requires the time of
each reading and accurate GPS-based positioning data to
be recorded with each gravity measurement. Detailed
knowledge of the local terrain is also required, and air-
borne surveys often include instruments to survey topog-
raphy beneath the aircraft. A description of downhole
gravity surveying is beyond our scope; the interested reader
s gravity
field, so gravity measurements
and survey procedures need to be capable of resolving
these extremely tiny responses. Modern instruments can
measure gravity to the required accuracy. The problem is
that the responses of interest are small compared with
variations in the gravity field due to factors such as height,
topography, the rotation of the Earth, and the attractions
of other bodies in the solar system. This requires a survey
strategy designed to ensure that reduction of the data can
remove or reduce these effects. Unfortunately, compen-
sation for some of these effects cannot be achieved to a
level comparable to the accuracy of the survey instruments
and sometimes not to the level of accuracy needed to
recognise the signals of interest. The principal problem is
that suf
ciently accurate topographic and density infor-
mation from the survey area is usually not available and,
inevitably, simplifying assumptions must be made which
reduce the accuracy of the results.
Geophysical surveys may measure gravity or spatial
variations in gravity, i.e. gravity gradients (see
Section
2.2.3
)
. As noted in
Section 3.3.2
,
measurements of one type
may be used as a basis for converting readings into data of
the other type so in some ways the distinction is artificial.
The instrument for measuring gravitational acceleration is
known as a gravity meter (Chapin,
1998
). Gravity surveys
for mineral exploration measure differences in gravity
between the survey stations and a survey base station, i.e.
relative measurements, and not the absolute value of grav-
ity (see
Section 2.2.1
). It is not necessary to measure
absolute gravity because the objective is to identify relative
changes in gravity related to near-surface density vari-
ations. Most gravity measurements are made on the
ground. Downhole and underground measurements are
possible, but are uncommon in mineral exploration. Meas-
urements can also be made from the air, known as aero-
gravity for
fixed-wing aircraft and heligravity when made
from a helicopter, but their accuracy is severely limited
by the need to remove the much larger accelerations
associated with the movement of the aircraft. One solution
which has shown promise is to mount the sensor on
an airship, which is inherently a very stable platform
(Hatch and Pitts,
2010
).
A gravity gradiometer measures gravity gradients, the
reading being the gradient in one or more directions.
'
3.3.1
Measuring relative gravity
A gravity meter measures the gradient of the gravitational
potential in the vertical direction, i.e. the vertical attraction
of gravity (
Figs. 3.3d
and
3.15a
)
. Instruments used in
exploration operate on the principle of measuring differ-
ences in the tension of a spring from which a small mass is
suspended. This is one of the masses in
Eq. (3.3)
, the other
being the combined mass of the Earth. In principle, the
mass is attached to one end of a beam which is pivoted at
its other end, and suspended horizontally by the spring. In
reality, it is a more elaborate and complex arrangement of
springs and pivots arranged to obtain a large dynamic
range and the desired measurement stability.
An important consideration is the gradual change in
spring tension with time due to changes in the temperature
and elastic properties of the spring and the beam, which
cause corresponding changes in the meter reading. This is
known as instrument drift. It is a particular problem with
older style instruments, but modern gravity meters operat-
ing under electronic control have very little drift. The
mechanical nature of gravity sensors means they are also
subject to tares. These are sudden changes in the drift rate,
i.e. there is a sudden
in readings, usually indicating
that the sensor has suffered either mechanical or thermal
'
'
jump
'
and may require laboratory maintenance. Instru-
ment effects are monitored during a survey by periodically
making repeat readings at some reference location
throughout the day. An example of a modern gravity meter
used for exploration work is shown in
Figure 3.10
.
Since gravity is measured in the vertical direction, it is
necessary to
shock
'
'
level
'
the instrument prior to making a
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