GSadjust 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 observations are relative-gravity differences between stations, observed with a relative-gravity meter, and absolute-gravity measurements at individual stations, observed with an absolute-gravity meter.
Data in GSadjust are organized in a hierarchical tree. At the top-most level is the Campaign, which contains all of the data in a project, typically from a particular field area. Each Campaign has one or more surveys. Each survey represents all of the data that are combined in a single network adjustment (usually collected over days to weeks). Each survey has one or more loops; the drift correction is specified on a per-loop basis. Each loop has two or more station occupations, with at least one repeat. Finally, each station occupation has one or more samples (usually at a 5 second interval for the Burris meter, or 60-90 second interval for Scintrex meters).
GSadjust reads data files output by relative-gravity meters manufactured by ZLS Corp. and Scintrex Ltd., and by absolute-gravity meters manufactured my Micro-g LaCoste, Inc. Both types of data can also be read from .csv files.
The data tab provides shows the samples at each station occupation, plots of the most relevant data, and tools for selecting/deselecting samples based on quality criteria. The average gravity value for each station is based on the selected samples.
Drift correction is a fundamental step for processing relative-gravity meter surveys. Even when a drift correction is applied internally in the meter data-collection computer, there may be a residual nonlinear drift component that requires further correction. Four types of drift correction are included in GSadjust:
- No drift correction
- Network-adjustment drift correction
- Roman method drift correction
- Continuous-model correction
Least-squares network adjustment is the process for solving the system of equations that represent all of the survey observations along with any additional parameters that describe relative-gravity meter drift and calibration.
GSadjust solves the system of equations using either the least squares routine in the Python Numpy package (using the linalg.inv method), or using the program Gravnet (Hwang and others, 2002). Extensive options are available for setting and modifying the measurement standard deviations (and therefore their weight in the adjustment), an important consideration in weighted least squares. After adjustment, there are several tools available for evaluating results, including least-squares statistics (a posterior standard deviation, chi-square statistic and test results, standard deviation statistics), residual histogram plots, and network maps.
Calculating gravity change
When two or more surveys exist in a project, GSadjust can calculate the difference in gravity between surveys. This is particularly useful for repeat microgravity, a method for measuring change in gravity over time to identify changes in subsurface mass (Kennedy and others, 2021). Gravity change at each station can be shown in presentation-ready plots either as time series or in map view. Furthermore, the linear trend in gravity over all observations can be calculated and plotted.
GSadjust builds on the software PyGrav (Hector and Hinderer, 2016). Compared to PyGrav, GSadjust offers enhanced plotting and analysis of relative-gravity meter drift, additional methods for drift correction (beyond inclusion as a parameter in the adjustment), and inclusion of absolute-gravity observations. GSadjust also offers additional functionality for network adjustment, including plotting results, enabling/disabling gravity differences, and calculating the change in gravity over time.
Download Current Version of GSadjust
The current version of GSadjust is version 1.0, released December 20, 2020.
How to Cite GSadjust
Kennedy, J., 2020, GSadjust v1.0: U.S. Geological Survey Software Release, 20 December 2020, https://doi.org/10.5066/P9YEIOU8
Software License and Purchase Information
This software is a product of the U.S. Geological Survey, which is part of the U.S. Government.
This software is freely distributed. There is no fee to download and (or) use this software.
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