GRAVITY

Innovation in the 1980s and 1990s has been dominated by new instrumentation. Only recently has processing and inter-pretation technology appeared which will extract the maximum value from the new instruments. In the following sections, the instrumentation advances are outlined. Processing and inter-pretation advances are sketched in the last section. Land gravity An excellent review of land gravity instruments was recently published by Chapin et al. (1999). Improvements have been made in usability of land gravity meters, but no fundamental design changes have appeared in commercially available in-struments for many years. What has created a dramatic change is the advent of the Global Positioning System (GPS). Al-though sufficiently accurate vertical control for precision mea-surements cannot be obtained from GPS without excessive ob-servation times, horizontal positioning is more than adequate with differential corrections, and the effort in surveying loca-tions is greatly reduced. In addition, reflectorless laser ranging has made it possible to obtain accurate near-zone terrain val-ues (Lyman et al., 1997). Thus, location and terrain data have improved substantially in the last decade, although generally not enough to challenge the accuracy of the gravity instruments themselves. Shipborne gravity As with land gravity, the major recent advances have been driven by the availability of GPS. Using Doppler GPS veloci-ties, vertical accelerations considerably superior to those avail-able from other sources may be obtained by differentiation. This significantly improves the accuracy of the vertical heave correction, which is the dominant term in the error budget. Under good conditions, accuracies of 0.2 mGal are said to be routinely available. Airborne gravity Experimental efforts to measure gravity on an airborne plat-form date back to the 1950s, but it was not until the 1980s that