Geology Reference
In-Depth Information
tidal currents, the survey ship needs to be anchored
to keep it on station while the gravimeter is on the
sea bed.
Gravity measurements can be made continuously at
sea using a gravimeter modified for use on ships. Such
instruments are known as shipborne, or shipboard,
meters.The accuracy of measurements with a shipborne
meter is considerably reduced compared to measure-
ments on land because of the severe vertical and hori-
zontal accelerations imposed on the shipborne meter by
sea waves and the ship's motion.These external accelera-
tions can cause variations in measured gravity of up to
10
6
gu and represent high-amplitude noise from which
a signal of much smaller gravity variations must be ex-
tracted.The effects of horizontal accelerations produced
by waves, yawing of the ship and changes in its speed and
heading can be largely eliminated by mounting the
meter on a gyrostabilized, horizontal platform, so that
the meter only responds to vertical accelerations. Devia-
tions of the platform from the horizontal produce
off-
levelling errors
which are normally less than 10 gu. Exter-
nal vertical accelerations resulting from wave motions
cannot be distinguished from gravity but their effect can
be diminished by heavily damping the suspension system
and by averaging the reading over an interval consider-
ably longer than the maximum period of the wave mo-
tions (about 8 s). As the ship oscillates vertically above
and below the plane of the mean sea surface, the wave
accelerations are equally negative and positive and are
effectively removed by averaging over a few minutes.
The operation is essentially low-pass filtering in which
accelerations with periods of less than 1-5 min are
rejected.
With shipborne meters employing a beam-supported
sensor, such as the LaCoste and Romberg instrument, a
further complication arises due to the influence of hori-
zontal accelerations. The beam of the meter oscillates
under the influence of the varying vertical accelerations
caused by the ship's motions.When the beam is tilted out
of the horizontal it will be further displaced by the turn-
ing force associated with any horizontal acceleration.
For certain phase relationships between the vertical and
horizontal components of motion of the ship, the hori-
zontal accelerations may cause beam displacements that
do not average out with time. Consider an example
where the position of a meter in space describes a circu-
lar motion under the influence of sea waves (Fig. 6.3).At
time
t
1
, as shown in Fig. 6.3, the ship is moving down,
displacing the beam upwards, and the horizontal com-
ponent of motion is to the right, inducing an anticlock-
t
2
t
3
t
1
t
4
Fig. 6.3
Cross-coupling in a shipborne gravimeter.
wise torque that decreases the upward displacement of
the beam. At a slightly later time
t
3
the ship is moving up,
displacing the beam down, and the horizontal motion
is to the left, again inducing an anticlockwise torque
which, now, increases the downward displacement of
the beam. In such a case, the overall effect of the hori-
zontal accelerations is to produce a systematic error in
the beam position. This effect is known as
cross-coupling
,
and its magnitude is dependent on the damping charac-
teristics of the meter and the amplitude and phase rela-
tionships of the horizontal and vertical motions. It leads
to an error known as the
cross-coupling error
in the mea-
sured gravity value. In general, the cross-coupling error
is small or negligible in good weather conditions but can
become very large in high seas. Cross-coupling errors
are corrected directly from the outputs of two horizontal
accelerometers mounted on the stabilized platform.
The inability to compensate fully for extraneous
accelerations reduces the accuracy of these shipborne
measurements to 10 gu at best, the actual amount
depending on prevailing sea conditions. Instrumental
drift monitoring is also less precise as base ties are, of
necessity, usually many days apart.
Cross-coupling is one of the major sources of error in
measurements of gravity at sea made with instruments
utilizing a beam-supported mass, and arises because of
the directional nature of the system. No cross-coupling
would occur if the sensor were symmetric about a ver-
tical axis, and since the late 1960s new marine meters
utilizing this feature have been developed.
The
vibrating string accelerometer
(Bowin
et al.
1972) is
based on the principle that the resonant frequency of a
short, vertical string from which a mass is suspended is
