<p><strong>Measurement techniques</strong></p><p>High-precision aerial gravity surveys can be carried out by relative spring meters, with ties to stable reference stations or absolute measurements for time-lapse studies. Instrument drift is controlled by frequent repeat measurement and repeatability of 1-3 &#181;Gal has been common. &#160;Free-fall gravimeters are heavier and costlier but provide absolute values and are immune to drift. Superconducting gravimeters are stationary and provide sub-&#181;Gal resolution over days and weeks, while drift uncertainty can build up to several &#956;Gal over years. Cold atom gravimeters are under development and may provide yet another survey alternative in the future.</p><p>Multiple sensors and multiple repeats are effective ways of improving survey precision, as much of the noise reduce at random noise (sqrt(N)). This holds also for the sensor drift residuals. An efficient, transparent and reproducible processing software is an integral part of such techniques.</p><p><strong>Surface stations</strong></p><p>Stability of measurement platforms over years is required for &#181;Gal time-lapse precision and can be achieved by installing geodetic monuments. For optimal monitoring of targets like a producing oil, gas or geothermal field, a water reservoir or a volcano, a grid of stations with spacing equal to or smaller than the overburden thickness is required. Surface subsidence or uplift requires sub-cm precision which can be obtained by optical leveling, InSAR or GPS.</p><p>Accuracy</p><p>Station repeatability is a robust accuracy measure for relative surveys with multiple occupations of each station. Together with multiple sensors they provide abundant statistics. The redundancy also allows for in-situ calibration of parameters for scale factor, tilt and temperature by minimizing residuals. Time-lapse precision can be judged at stations with minimal or known subsurface changes, and will be affected by gravity survey precision, accuracy of measured depth changes and other time-lapse effects such as benchmark stability and time-lapse signals outside interest. Groundwater variations could be one such noise term, unless the purpose is hydrology monitoring.</p><p>Efficiency and cost</p><p>Most microgravity projects have been carried out in a research or development setting, with one sensor, few stations repeat and implicit capital and personnel cost. In a more industrial setting, efficiency is likely to improve, together with reduced survey cost. More instruments and measurements will likely reduce the personnel and mobilization portion of the cost. Precision/cost tradeoffs and value of data will determine the economics of a project, whether in a scientific or commercial setting.</p><p>Conclusion</p><p>Currently proven survey repeatabilities of 1-2 &#181;Gal may be regarded state-of-the-art and become commonplace for microgravity surveys using relative gravimeters. This can widen the range of applications and reduce monitoring intervals. Further instrument developments may improve this limitation.</p>
A time-lapse gravity survey of the Coso geothermal field, China Lake Naval Air Weapons Station, California
