Marine Gravity

Marine gravimeter system KSS32M with the gravity sensor on the gyro-stabilized platform Source: BGR

Systematic marine gravity measurements on vessels started in the early 1960s with the development of useable marine gravimeters. Great improvement of the measuring accuracy are achieved by developments of the control electronics and above all enhanced navigation data gained by global navigational satellite systems, like GPS.
The calculation of gravity from satellite altimetry measurements resulted in global datasets, providing free-air gravity anomaly values for nearly all oceanic areas of the Earth. These data are well applicable for overview maps or large-scale survey, but cannot replace shipboard measurements. The problems of the gravity data derived from satellite measurements are their limited spatial resolution and systematic errors especially in shallow water areas. In addition for detailed modelling good water depth data are needed and whenever available also seismic data. Good gravity values in combination with these additional data can be obtained at sea only with research vessels.
Marine gravity measurements seem to be a nearly insolvable problem at first sight considering that gravity changes should be measured with an accuracy of 1 mGal. This unit commonly used in geophysics corresponds to about 10^-6 g, that means one millionth of the Earth’s gravity acceleration. As on vessels already during moderate seastate conditions the disturbing inertial accelerations reach up to 0.1 g, the gravity signal has to be separated from a noise signal which is about one hundred thousand times higher.
Nevertheless this can be achieved with instruments, which are based on the rather simple principle of a spring balance. BGR is running for this purpose the marine gravimeter system KSS32M of Bodensee Gravitymeter Geosystem GmbH (BGGS). The KSS32M consists basically of the gravity sensor GSS30, which is installed on a gyro-stabalized platform. All electronic units including the power supply are integrated in the platform. A notebook is used for the control of the system and the data acquisition.

Principle sketch of the gravity sensor GSS30 Source: BGR

The gravity sensor GSS30 consists of a moveable tube-shaped mass with a coil at its bottom and parts of a capacitance transducer at its top. All this is suspended on a metal spring and guided frictionless by 5 threads. If the system is kept on a stabilized platform in a vertical position, the mass can move only in the vertical direction and is measuring thus this gravity component only. This measuring principle is completely insensitive to horizontal accelerations, which means a big advantage compared to beam type instruments, where horizontal accelerations affect also disturbances in the vertical direction. This so-called cross coupling effect has to be corrected by complex and error-prone procedures when beam type gravity meters are used.

The main part of the total gravity acceleration is compensated by the mechanical spring, but gravity changes are compensated and detected by an electromagnetic system. The displacement of the spring-mass assembly with respect to the outer casing of the instrument is measured using a capacitance transducer. This signal is converted by a feedback system into an electric current which is proportional to the gravity change. This effects by magnetic forces on the coil, which is moving within a permanent magnet, that the spring is returned to its zero position. The current value across the coil is thus a measure for the gravity change. In order to suppress the effect of the seastate the sensor is damped strongly by the feedback system and successive digital filters, which are selectable.

The measured values, however, are not useable without further data processing. The main steps of the data processing are:

• A time shift of the measured data of 76 seconds to compensate for the time delay due to the inevitable damping.
• The Eoetvoes correction removes the vertical components of the disturbing inertial forces (centrifugal force and Coriolis force). These forces occur inevitably when a moving measuring system is used on the rotating Earth. The quality of this correction depends crucially from the quality of the navigational data, especially from the ship’s velocity.
• Subtraction of the normal gravity value, whereby the latitude dependent gravity of a distinct reference ellipsoid (here: GRS80) as reference datum for the gravity anomalies is defined.

• The drift correction, which is determined by tying in the harbor readings of the sea gravimeter to stations of known absolute gravity values in the starting and the arriving harbors. On the basis is of this gravity tie measurements in the harbors, the shipboard data are also referenced to the global gravity net IGSN71. The drift of our instrument is usually relatively small and amounts to about 1 to 2 mGal per month.

Having carried out all these corrections, we obtain as result the so-called free air gravity anomaly. The data analysis of recent survey cruises show, that the data accuracy is generally better than 1 mGal. This results besides from the high quality of the gravimeter also from today’s excellent navigational systems.