Gravity data can be collected at sea using shipboard gravity
meters or underwater gravity meters. Since underwater gravity
meters are operated on the ocean bottom, they can provide gravity
data detail that is lost in the averaging process used by
shipboard meters. Underwater meters are usually used in shallow
water, but can be modified for use at almost any depth. Deep
water operations are often slow and therefore expensive.
Underwater gravity meters can also be useful in swamps, on
muskegs, frozen lakes, ice islands or any other place where there
is so much motion that a land gravity meter would not operate
satisfactorily. For these applications an underwater gravity
meter with a light fiberglass housing is used. If necessary the
underwater gravity meter can be transported by helicopter which
can hover over the gravity station while taking a reading.
The accuracy of underwater gravity data decreases with depth
due to errors in measuring water depth and uncertainty in the
position of the gravity meter. The accuracy also depends on
how the gravity meter is handled. The inherent precision of the
gravity meter is about 0.01 mgal. The rough environment of the
sea may cause errors in underwater surveys greater than those
encountered in a land survey.
In actual sea operation under normal conditions, base station
checks indicate an accuracy of about 0.1 mgal, although careful
surveys have been made with appreciably higher accuracy. An
accuracy of 0.1 mgal is generally adequate because uncertainties
in water depth and latitude often give errors of this magnitude.
Water depth is usually measured with pressure gauges whose
accuracy is not much better than 1/2%. A 0.6 meter (2 foot) error
in depth corresponds to about 0.1 mgal. Also, a 160 meters (500
foot) error in the northsouth direction corresponds to about
0.1 mgal at a latitude of 30 degrees. An overall accuracy of
about 0.2 mgal is considered good in a survey in water 160 meters
(500 feet) deep.
Underwater gravity meters are essentially remotecontrolled
highlydamped land gravity meters. Unlike land meters,
underwater gravity meters must be designed to withstand very
rough treatment. They may receive a hard bump when they hit the
ocean floor, be accidentally dragged on the ocean bottom, or hit
the side of the ship when being hoisted out of the water.
Seismic motion of the ocean bottom is a primary problem in
making underwater gravity measurements. This seismic motion is
caused by wave action and is most troublesome on muddy bottoms at
depths less than 15 meters (50 feet). The vertical component of
this seismic action is often greater than can be tolerated in the
gravity meter beam without introducing mechanical hysteresis
errors in the gravity meter spring.
The first LaCoste and Romberg underwater gravity meters were
built about 1948 and designated the "W" series. In this
design, the seismic motion problem was solved by putting the
gravity meter on a servocontrolled elevator which approximately
counteracted the vertical seismic motion. The acceleration of the
servo was controlled so that the gravity meter beam was held (for
a while) at any point in its scale regardless of the seismic
motion.
The next generation of underwater meters were designated the
"H" series. The seismic motion problem was solved by
highly damping the gravity meter beam, much the same method as
used in modern shipboard gravity meters. A highly damped gravity
meter is very slow to read by the standard land meter nulling
technique, therefore a new method of reading was developed.
The meter was adjusted to infinite mechanical sensitivity,
and the reading was made by measuring the velocity of the beam.
If the velocity was not zero, a correction was made for the
observed beam velocity. By using this method, a highly damped
meter can be read as fast as an ordinary gravity meter. The
"H" model gravity meter was simpler to operate than the
elevator type "W" series, although it still required a
cable with many conductors due to the analog remote control
system containing numerous relays.
The "U" series of underwater gravity meters were designed in 1986 and first delivered in 1987. This design employs digital telemetry thereby reducing the number of conductors required in the control cable. Meter control is performed by software operating on an IBMcompatible personal computer and interfaced to the meter through a standard serial port. A method of reading the meter was employed using a calibrated electrostatic force to null the meter beam.
