Repeat microgravity surveys carried out using relative- and absolute-gravity meters are useful for identifying changes in subsurface mass, such as the volume of water stored in an aquifer. These surveys require careful field procedures to achieve the part-per-billion accuracy required to measure the small changes in gravity relevant for hydrologic studies. This chapter describes techniques and methods for carrying out gravity surveys, requirements for assuring high-quality survey results, and data processing and archival procedures. The focus is on acquiring and documenting repeat gravity surveys for monitoring changes in groundwater storage. Similar gravity surveys may be completed to evaluate other causes of mass change, such as those caused by magma movement below volcanoes. The methods are also useful for one-time surveys that map spatial gravity variations associated with geologic structures such as faults or sedimentary basins.
Repeat microgravity surveys can be carried out using relative-gravity meters, absolute-gravity meters, or both. Specific locations, known as gravity stations, are visited during each survey. Most commonly, absolute- and relative-gravity are combined using the least-squares method of network adjustment, much like benchmark elevations and relative-height differences in a leveling network. This chapter primarily describes the use of the A-10 absolute-gravity meter manufactured by Micro-g LaCoste, Inc., and relative-gravity meters made by LaCoste & Romberg (no longer in production) and ZLS Corporation, Inc. Field and office procedures are similar for other instruments such as the FG-5 absolute-gravity meter and Scintrex relative-gravity meters, but some adaptation may be required. Quality control for absolute-gravity data focuses primarily on proper field procedures and maintaining the time and distance calibration of the instrument. Quality control for relative-gravity surveys requires careful field procedures, an understanding of how the meter is behaving while in the field, and appropriate postprocessing.
The techniques and methods described in this chapter were developed over 30 years at the USGS Arizona Water Science Center and the Southwest Gravity Program and are the basis for many studies on groundwater-storage change and geologic structure. A description of the Program and complete bibliography is available at https://www.usgs.gov/centers/az-water/science/azwsc-capabilities-hydrologic-gravity-monitoring.
|Title||Procedures for field data collection, processing, quality assurance and quality control, and archiving of relative- and absolute-gravity surveys|
|Authors||Jeffrey R. Kennedy, Donald R. Pool, Robert L. Carruth|
|Publication Subtype||USGS Numbered Series|
|Series Title||Techniques and Methods|
|Record Source||USGS Publications Warehouse|
|USGS Organization||Arizona Water Science Center|
Gravity Data Spreadsheets
Jeff Kennedy, PhD
Gravity Data SpreadsheetsThese gravity data spreadsheets are Microsoft Excel documents containing functions that convert observed meter readings to gravity using the meter calibration table (for LaCoste &amp;amp;amp; Romberg relative-gravity meters), applies a tide correction, calculates and plots drift using the Roman (1946) method, and calculates average gravity differences between each station pair.
GSadjustGSadjust is the first comprehensive, publicly-available graphical interface for performing drift-correction and network adjustment for combined relative- and absolute-gravity surveys (Kennedy and others, 2021). The objective of network adjustment is to determine a single, best-fit gravity value at each station based on all available observations and their respective uncertainty. Typically the obse
Jeff Kennedy, PhD