Geology Reference
In-Depth Information
previously possible. The factor preventing more wide-
spread deployment of the meter is its large cost.
The measurement of gravity from aircraft is complex
because of the large possible errors in applying correc-
tions. Eötvös corrections (Section 6.8.5) may be as great
as 16 000 gu at a speed of 200 knots, a 1% error in veloc-
ity or heading producing maximum errors of 180 gu and
250 gu, respectively. Vertical accelerations associated
with the aircraft's motion with periods longer than the
instrumental averaging time cannot readily be cor-
rected. In spite of these difficulties, tests undertaken in
small aircraft (Halpenny & Darbha 1995) equipped with
radar altimeters and GPS navigation have achieved re-
sults which differ from those obtained with underwater
meters by an average of -2 gu and standard deviation
27 gu. Bell
et al.
(1999) describe a more modern set-up
for airborne gravity surveying, which is now in use
commercially. A system is also available for use with a
helicopter (Seigel & McConnell 1998) in which the
gravimeter is lowered to the ground by a cable, levelled
and read remotely, so that measurements can be made
where landing the aircraft is impossible.
The calibration constants of gravimeters may vary
with time and should be checked periodically.The most
common procedure is to take readings at two or more
locations where absolute or relative values of gravity are
known. In calibrating Worden-type meters, these read-
ings would be taken for several settings of the coarse ad-
justing screw so that the calibration constant is checked
over as much of the full range of the instrument as
possible. Such a procedure cannot be adopted for the
LaCoste and Romberg gravimeter, where each different
dial range has its own calibration constant. In this case
checking can be accomplished by taking readings at
different inclinations of the gravimeter on a tilt table, a
task usually entrusted to the instrument's manufacturer.
Permanent
magnet
Coil
Induced
magnetic
field
DC
current
Servo
loop
Permanent
magnetic
field
Permanent
magnet
Fig. 6.4
Principle of the accelerometer unit of the Bell marine
gravimeter. (After Bell & Watts 1986.)
proportional to the square root of gravity. Changes in
this frequency provide a measure of changes in gravity.
Gravimeters based on this mechanism have never found
much favour because of relatively low reported accura-
cies and erratic drift.
The most successful axially symmetric instrument to
date is the
Bell gravimeter
(Bell & Watts 1986).The sensing
element of the meter is the accelerometer shown in Fig.
6.4 which is mounted on a stable platform. The ac-
celerometer, which is about 34 mm high and 23 mm in
diameter, consists of a mass, wrapped in a coil, which is
constrained to move only vertically between two per-
manent magnets. A DC current passed through the coil
causes the mass to act as a magnet. In the null position,
the weight of the mass is balanced by the forces exerted
by the permanent magnets. When the mass moves ver-
tically in response to a change in gravity or wave acceler-
ations, the motion is detected by a servo loop which
regulates the current in the coil, changing its magnetic
moment so that it is driven back to the null position.The
varying current is then a measure of changes in the verti-
cal accelerations experienced by the sensor. As with
beam-type meters, a weighted average filter is applied
to the output in order to separate gravity changes from
wave-generated accelerations.
Drift rates of the Bell gravimeter are low and uniform,
and it has been demonstrated that the instrument is
accurate to just a few gravity units, and is capable of
discriminating anomalies with wavelengths of 1-2 km.
This accuracy and resolution is considerably greater than
that of earlier instruments, and it is anticipated that much
smaller gravity anomalies will be detected than was
6.5 Gravity anomalies
Gravimeters effectively respond only to the vertical
component of the gravitational attraction of an anom-
alous mass. Consider the gravitational effect of an anom-
alous mass
d
g
, with horizontal and vertical components
d
g
x
and
d
g
z
, respectively, on the local gravity field
g
and its
representation on a vector diagram (Fig. 6.5).
Solving the rectangle of forces gives
