From here, you can submit data files and set options for processing gravity data. Results will be emailed back to you at the email address associated with your username. The result email will include a zip archive of the output files and plots for inspection.
Note that with the current system, exploration campaign processing can include terrain corrections. The terrain correction algorithm is valid globally, including ocean areas. Please be aware that the global elevation model for terrain corrections does not (currently) account for ice at the poles! Use WGS84 or EGM96 geoid heights for stations for terrain corrections. Available global DEMs are referenced to the WGS84 or EGM96 geoid, not the ellipsoid! Geoid and ellipsoid heights can differ by >20 m in North America!
Terrain corrections can be carried out for any number of stations spread over any amount of the world. Please note that each terrain correction takes ~20 sec to run; data sets with 100s of stations can take hours to compute.
PROCESSING RAW DATA
Upload raw data files (ASCII TEXT, NOT BINARY!) from CG-3, CG-5, or Aliod gravity meters. You will also need a file with the station ID (number) and station name mapping (one per line), and a file with station name, latitude (dd), longitude (dd), elevation (m), and elevation change (m) for all station names. Optional files are a skip file with station IDs or names to be skipped in processing (one per line) and a meter calibration file for Aliod gravimeters. The calibration file needs dial reading to mGal numbers, and tilt correction constants.
If exploration options are chosen, the processed raw data will be automatically run through the exploration processing steps outlined below.
EXPLORATION PROCESSING
Processed gravity data can be converted to anomaly values,
including a Complete Bouguer Gravity Anomaly (CBGA). Processed data
can be from the raw processing steps above, or an uploaded file
directly. If the "Raw gravity data?" checkbox is cleared, the uploaded
"Gravity Data File" is assumed to be processed gravity data in the
exploration format:
name lon(dec. deg) lat(dd) elevation(m) gravity(mGal) grav_sigma(mGal) iztc(mGal) total_tc(mGal)
The required information for the exploration processing includes inner-zone terrain corrections (if desired), and a list of CBGA anomaly densities (in kg/m^3). As would be expected, excluding terrain corrections computes a Simple Bouguer Anomaly rather than complete, and also choosing a reduction density of 0 will provide a Free-Air Gravity Anomaly.
The inner-zone terrain correction file is a set of "name iztc" lines, with the names being the station names in the gravity data, and the iztc is the inner-zone terrain correction in mGal.
There are a lot of options for the ways to process raw data from the gravity meter to observed gravity values at a station. The full description of the options, and when you want to use various ones, is in the High-Precision Gravity Manual. The manual also includes the development of the algorithms used in the raw data processing.
Processed raw data (a.k.a. observed gravity at stations) can be converted to anomaly values with the exploration processing option. See any typical gravity textbook or Hinze et al (2005) for the basis of the calculations for Free-Air Gravity Anomaly, Simple Bouguer, and Complete Bouguer Gravity Anomalies.
Hinze et al (2005): Hinze, William J., Carlos Aiken, John Brozena, Bernard Coakley, David Dater, Guy Flanagan, René Forsberg, Thomas Hildenbrand, G. Ranmdy Keller, James Kellog, Robert Kucks, Xiong Li, Andre Mainville, Robert Morin, Mark Pilkington, Donald Plouff, Dhananjay Ravat, Danial Roman, Jamie Urrutia-Fucugauchi, Marc Véronneau, Michael Webring, and Daniel Winester. New standards for reducing gravity data: The North American gravity database. GEOPHYSICS, vol. 70, no. 4 (July-August 2005); P. J25 — J32. DOI 10.1190/1.1988183
This code uses the International Gravity Formula for theoretical gravity, a second-order free-air correction, an atmospheric correction, and Bouguer slab, spherical cap, and terrain corrections scaled by reduction density. All corrections are taken from Hinze et al (2005) and references therein.
The terrain corrections computed by this service are based on integrating spherical diamonds that cover the Earth with a spacing of approximately 500 m. The coverage is global, and the terrain correction algorithm will compute corrections for anywhere on the globe.
Elevations and bathymetry were taken from global down-sampled SRTM data set where available (about 60 S to 60 N latitudes), then GLOBE 1-km land data, then ETOPO1 for the ocean bathymetry. Elevations are WGS84 orthometric heights; convert from ellipsiodal to geoid/ortho heights before submitting data!
Pay particular attention to the terrain correction part of the output log sent back with results. Most problems with a terrain correction will show up with warning messages about polygons with large individual terrain corrections. Note that a common problem is a station that is significantly below the elevation of the enclosing polygon; look for a warning message about a polygon with large terrain correction with distance <500 m away. Check station elevations or tweak for the terrain correction calculations.
All terrain corrections are computed assuming 2670 kg/m3. Scale by the desired density when computing anomaly values; the exploration code will do this automatically. Terrain correction values are stored in the data files to allow this.
Also note that if doing an airborne survey, the terrain correction can be computed referenced to DEM elevation, not the station elevation (which is in the air, not on the ground). Set the airborne flag in that case.
Airborne gravity surveys can be processed using this exploration code. This service does not have facilities for processing the raw airborne readings, removing the airframe accelerations, etc. If you really have to do this using the current system, start with raw data, Thiele extrapolation, and go from there.
Once you have observed gravity at lon, lat, and elevation above ellipsoid, use the exploration processing portion, but set the airborne flag.
Airborne processing has a different flow: theoretical gravity is from a confocal ellipsoidal code from NGS (replaces theoretical gravity formula and free-air correction), there is no atmospheric or FA correction (already included in NGS algorithm), and the Bouguer slab, spherical cap, and terrain corrections are done in the terrain code as one step; terrain corrections will be hundreds of mGal, instead of tens, because it includes the other corrections as well! The anomaly values are still a CBGA anomaly at altitude, but are computed very differently.