A side‐by‐side test of four land gravity meters

Abstract

Four different land gravity meters were run side by side on a test range in eastern Texas. This test range was designed to duplicate typical field survey conditions encountered in gravity measurements on 3-D land seismic surveys. A profile of 270 stations was acquired with a station spacing of 33.5 m (110 ft). The four meters were Scintrex CG-3, Sodin, LaCoste & Romberg (L&R) G-meter, and Edcon/L&R Super-G meter. The purpose of this study was to determine which of the four meters had the best balance of accuracy, convenience, and speed of measurement for these specialized surveys. All four meters had their limitations, and no single meter was greatly superior to the others. The CG-3 was awkward in leveling, had a large (368 μGal per day) drift rate, and had some quirks in its user interface. The Sodin meter had problems controlling its temperature and, at one point, had a very high 935 μGal per day drift rate. It had the poorest accuracy of the four. The G-meter lacked a convenient digital user interface; thus, it was susceptible to operator blunder in misreading or recording the data. The Super-G meter also had some problems in its user interface, had only a 6-mGal range without a range change, and was the heaviest of the field instruments. An unbiased statistical estimator was developed to analyze the data. The technique determined the best three meters at each station. Both the G-meter and the Super-G meter produced the most accurate data. They had an average absolute deviation from the mean of 20 and 23 μGal, respectively. In comparison, the two quartz meters—the CG-3 and the Sodin—had an average absolute deviation from the mean of 31 and 46 μGal, respectively. We would like to see the manufacturers improve (or add, in the case of the G-meter and the Sodin) their digital interfaces. Additionally, we would like to see upgrades in the thermal control systems in all the meters. The single largest amount of station time was devoted to leveling (rather than reading) the meters. Significant advances can be envisioned to create fast, automated leveling systems in all the meters.