GSadjust: a graphical user interface for processing combined relative- and absolute-gravity surveys

Release Date:
Screenshot showing example of GSadjust data hierarchy shown in tree view

Screenshot showing example of GSadjust data hierarchy shown in tree view.

Overview

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. 

Screenshot showing example of GSadjust data selection window

Screenshot showing example of GSadjust data selection window.

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).

Data Selection

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. 

Screenshot showing example of GSadjust drift correction window

Screenshot showing example of GSadjust drift correction window.

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

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
Screenshot showing example of the GSadjust network adjustment window

GSadjust screenshot showing the network adjustment window.

Network Adjustment

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.
 

Screenshot showing example of GSadjust time-series plot of gravity change

Screenshot showing example of GSadjust time-series plot of gravity change.

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. 

Screenshot showing example of GSadjust gravity-change map

Screenshot showing example of GSadjust gravity-change map. with one point per measurement location.

Download Current Version of GSadjust

The current version of GSadjust is version 1.0, released December 20, 2020.

Software repository
GSadjust v1.0 for Microsoft Windows; distribution includes Python code and environment, test data, and documentation [286MB ZIP] 
GSadjust v1.0 Documentation [3.4MB PDF]

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.

Cost
This software is freely distributed. There is no fee to download and (or) use this software.

License
Users do not need a license or permission from the USGS to use this software. Users can download and install as many copies of the software as they need.

Public Domain
As a work of the United States Government, this USGS product is in the public domain within the United States. You can copy, modify, distribute, and perform the work, even for commercial purposes, all without asking permission. Additionally, USGS waives copyright and related rights in the work worldwide through CC0 1.0 Universal Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/ ).
 
SOFTWARE USER RIGHTS NOTICE
This software has been approved for release by the U.S. Geological Survey (USGS). Although the software has been subjected to rigorous review, the USGS reserves the right to update the software as needed pursuant to further analysis and review. No warranty, expressed or implied, is made by the USGS or the U.S. Government as to the functionality of the software and related material nor shall the fact of release constitute any such warranty. Furthermore, the software is released on condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from its authorized or unauthorized use. Also refer to the USGS Water Resources Software User Rights Notice for complete use, copyright, and distribution information.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.