Abstract

Current standards for reduction of observed gravity to a modeled Bouguer anomaly largely are unregulated and vary among geophysical textbooks, commercial software programs, and academic research spreadsheets available for download from the Internet. Using new standards established by the U.S. Geological Survey (USGS) and the North American Gravity Database Committee, we developed a spreadsheet for reduction of raw data to the Bouguer anomaly and, with the use of terrain correction, the complete Bouguer anomaly. The spreadsheet is available for free download from the Geological Society of America Data Repository. We view the spreadsheet as particularly useful for field data reduction and modeling where Internet access is limited or unavailable.

INTRODUCTION

With the use of the global positioning system (GPS) for surveying station locations and altitudes, availability of digital terrain models, and enhanced computational capability, gravity modeling is a cost-effective tool in subsurface analysis ranging from basin- to continental-scale studies. Existing gravity data for North America are archived and readily accessible via the Internet at the Pan-American Center for Earth and Environmental Studies Web site (http://paces.geo.utep.edu/). The North America gravity database provides principal facts and free-air and Bouguer anomalies calculated by a FORTRAN algorithm based on preferred correction and anomaly equations established by the Standards/Format Working Group of the North American Gravity Database Committee (Hinze et al., 2003).

To facilitate adoption of the standards established by the North American Gravity Database Committee (Hinze et al., 2003) and to provide an easy to use, portable gravity correction and anomaly computation platform, we developed a gravity spreadsheet. The spreadsheet is based on Microsoft Excel, which is a common software application used by government agencies, research institutions, and private companies. The equations used in the spreadsheet are derived from the FORTRAN code written by Mike Webring of the USGS and are the same as those used by the GeoNet Server accessible at the Pan-American Center for Earth and Environmental Studies (PACES, 2006).

STANDARDIZED GRAVITY REDUCTION

The equations described in this section are used in the gravity spreadsheet and conform to the new gravity standards set by the USGS (Hildenbrand et al., 2002) and the Standards/Format Working Group of the North American Gravity Database Committee (Hinze et al., 2003).

Ellipsoid Theoretical Gravity

The ellipsoid theoretical gravity calculation uses the Somigliana closed-form formula based on the 1980 Geodetic Reference System (GRS80) to predict the gravity at any height and any latitude (φ? north or south (Moritz, 1980; Hildenbrand et al., 2002).  
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where values for the GRS80 reference ellipsoid are: ge = 978032.67715 mGal, k = 0.001931851353 (a dimensionless coefficient), and e2 = 0.0066938002290 (a dimensionless coefficient).

Atmospheric Effect

The mass of the atmosphere is included in the theoretical gravity calculation and must be subtracted from the predicted gravity. Since the station is inside an approximately spherical shell, the portion of the atmosphere above the station has no gravity effect. The atmospheric correction uses the height h of the gravity station in meters above the GRS80 ellipsoid in the following equation (Hildenbrand et al., 2002):  
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Height Correction to the Theoretical Gravity

Measurements of observed gravity decrease with increasing distance from the center of Earth. In order to compare these values with the theoretical gravity at the same location, the height of the gravity station must be corrected to the reference ellipsoid (Li and Götze, 2001; Hildenbrand et al., 2002):  
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where h is the height of the gravity station in meters above the GRS80 ellipsoid and φ is the latitude of the gravity station.

Bouguer Spherical Cap

The Bouguer spherical cap models a simple mass from the ellipsoid to the station height. The density of the mass normally is the average continental density of 2.67 g/cm3 (Hinze, 2003) or a site-specific average density of the basement rock for local surveys. Older methods of reducing gravity data used a similar correction called the Bouguer slab, which was based on a flat Earth model. The Bouguer spherical cap correction is the new standard formula that accounts for the curvature of Earth (Hildenbrand et al., 2002),  
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where μ and λ are dimensionless coefficients (LaFehr, 1991), G is Newton's gravitational constant where G = 6.673 ± 0.001 × 10−11 m3 kg−1 s−2, ρ is the density of the spherical cap, usually 2670 kg m−3, h is the height of the gravity station above the GRS80 reference ellipsoid (km), and R is the combined height of the gravity station and average radius of Earth (km).

GRAVITY SPREADSHEET

The gravity spreadsheet calculates the corrections for instrument drift, height above the GRS80 reference ellipsoid, atmospheric effects, and the Bouguer spherical cap, as well as the DC shift (i.e., constant value added or subtracted to observed gravity values) for multiple-day gravity surveys. The meter-specific calibration table in the spreadsheet will convert gravimeter counter readings to corrected gravity measurements. Tide and terrain corrections are not calculated in the spreadsheet, but users can enter values from other programs, such as InnerTC (Cogbill, 1990) in order to reduce gravity data to complete Bouguer anomalies.

Prior to the standards set by the USGS (Hildenbrand et al., 2002), gravity reduction typically used orthometric heights (i.e., elevation with respect to mean sea level or the geoid) to calculate free-air and Bouguer slab corrections. In this spreadsheet, we conform to the USGS standards and employ ellipsoidal height corrections. The revised method eliminates the need to include an estimate of the indirect effect caused by the difference between the ellipsoidal and geoidal heights in the Bouguer anomaly, as described by Hinze et al. (2003) and Hildenbrand et al. (2002).

Input Parameters

The four basic input parameters needed to calculate a complete Bouguer anomaly are: (1) the height of the gravity station above the GRS80 reference ellipsoid, (2) the latitude of the station in WGS84 coordinates, (3) the drift- and tide-corrected observed gravity readings tied to an absolute gravity base station, and (4) the terrain correction for the location of the gravity station. Additional utilities built into the spreadsheet are: (1) the instrument drift correction, (2) gravity meter dial conversions, (3) DC shift, and (4) conversion of local observed gravity to absolute gravity readings. Refer to Appendix A for a detailed description of gravity spreadsheet use.

Error Analysis

We referenced the output from the spreadsheet to the calculated complete Bouguer anomalies determined by the USGS FORTRAN code used as the source for spreadsheet equations. Using the same values for gravity, altitude, and latitude, the calculations produced differences of 2–14 μGal (i.e., 0.002–0.014 mGal). There are no systematic correlations among calculated gravity, altitude, and latitude (Fig. 1), and the discrepancy ostensibly arises from a truncation error within the spreadsheet algorithms.

Combining Spreadsheet Results with Legacy Data

Care must be used in studies that incorporate legacy gravity values downloaded from the GeoNet server and those determined from the spreadsheet. Although the same equations were used for both sets of calculations, the GeoNet server values were determined using orthometric versus ellipsoidal heights. This difference in datum results in a discrepancy of as much as ±7 mGal and arises from the geoidal separation from the ellipsoid, which in North America can be as great as ±20 m.

Reconciliation of this discrepancy is important if existing data are to be combined with the results of the spreadsheet calculations. For a given gravity survey, possibly the simplest way of calibrating the two sets of values is to reoccupy several sites taken from the GeoNet database. In this method, the co-located sites are used to determine a conversion factor between legacy and new values via linear regression. With dual-frequency GPS positioning, accurate station reoccupation at the subdecimeter scale is straightforward.

Alternatively, reconciliation of legacy and new gravity values requires recalculation of GeoNet gravity data using ellipsoidal heights. The GeoNet gravity data were supplied by authors in an original vertical datum that typically was not explicitly defined. Although ambiguous, the presumed GeoNet datum is the National Geodetic Vertical Datum of 1929 (NGVD29), and conversion to World Geodetic System of 1984 (WGS84) is achieved by a series of transformations using programs that are available on the National Geodetic Survey Web site. The NGVD29 elevations reported in GeoNet are first converted to the North American Vertical Datum of 1988 (NAVD88) using the North American Vertical Datum Conversion Utility (VERTCON). Next, the NAVD88 elevations are converted to the North American Datum of 1983 (NAD83) by using the GEOID03 model for the conterminous United States in the Geoid Interpolation Program (INTG). The final step involves converting the NAD83 position to the WGS84 by using the National Geodetic Survey program Horizontal Time Dependent Positioning (HTDP). Once the station heights are referenced to the ellipsoid, the converted station heights (i.e., ellipsoidal heights) can be entered into the spreadsheet for computation of the Bouguer anomaly.

CONCLUSIONS

The gravity spreadsheet is free and provides a simple tool for the reduction of raw gravity data to Bouguer anomalies, all in conformity with the standards set by the U.S. Geological Survey and the North American Gravity Database Committee. The spreadsheet eliminates the need for Inter-net access by allowing the user to calculate the Bouguer anomaly of a gravity station in the field.

To access Spreadsheet 1, please visit http://dx.doi.org/10.1130/GES00060.S1.

To access Spreadsheet 2, please visit http://dx.doi.org/10.1130/GES00060.S2.

To access Spreadsheet 3, please visit http://dx.doi.org/10.1130/GES00060.S3.

To access Spreadsheet 3, please visit http://dx.doi.org/10.1130/GES00060.S4.

APPENDIX A: GRAVITY SPREADSHEET V. 1.0 INSTRUCTIONS

The Gravity Spreadsheet v. 1.0 (Spreadsheet 11) is a Microsoft Excel workbook that is divided into several worksheets. The primary worksheet, entitled Gravity, uses equations and values from four secondary worksheets (Bullard B Table, Meters, Calib. Table, Absolute Base) and inputs from Earth tide-correction and terrain-correction programs not supplied to calculate the complete Bouguer anomaly. The Bullard B Table worksheet is a list of constants used to calculate the Bouguer spherical cap correction (LaFehr, 1991) and should not be modified. The Meters worksheet is a list of the three meters available for selection from the drop-down menu in the spreadsheet and should not be modified. The Calib. Table worksheet is a conversion table for counter values to gravity values for LaCoste-Romberg gravimeters and should be modified for the specific gravimeter used. The Absolute Base worksheet references relative gravity measurements to an absolute value; the user must enter a drift- and tide-corrected measurement taken at the absolute base station. Information for absolute base stations is available at PACES (http://paces.geo.utep.edu/).

Each column in the Gravity worksheet is described next. We also include a brief outline of the purpose of each column and the associated spreadsheet formula.

Column A: Gravity Station

Gravity station identification.

Column B: Date

Enter the date as MM/DD/YYYY. Calculations for time durations use this format.

Columns C and D: Time—Hours/Minutes

Enter hours in military time. It is important use the 0–24 h time scale because subsequent calculations depend on this format.

Column E: Duration (h)

This equation calculates the time, in decimal hours, that has elapsed from the initial basestation reading. These calculations are used to determine the drift correction.  
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Columns F–K: Latitude and Longitude

There are three columns under both headings: d—degrees; m—minutes; sec—seconds. If your coordinates are already in decimal degrees, skip this data input section.

Column L and M: Latitude and Longitude (DD)

DD stands for decimal degrees. These columns convert coordinates that are in degrees, minutes, and seconds into decimal degrees.  
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Column N: Ellipsoid Height (m)

The altitudes of gravity stations are entered as ellipsoidal height.

Columns O and P: LaCoste-Romberg Meter, Counter and Calibrated (mGal)

These two columns are used to convert counter values to gravity for a LaCoste-Romberg gravimeter. Each gravimeter has a calibration table that the user must manually input into the Calib. Table worksheet. If the table for a specific gravimeter has more or fewer rows than that presented in the worksheet, then the cell range used in the equation under column R must be modified. For example, if the calibration sheet supplied with the LaCoste-Romberg gravity meter has one more row than the table in the Calib. Table worksheet, then change the following equation in the Gravity worksheet from:  
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to:  
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This modifies cell P3 of the Gravity worksheet. Drag the equation down for the remaining cells in the column. (Hint: Select cell P3; place cursor over the small black box at the lower right-hand corner of the cell; cursor will change to a black cross-hair; left click and drag the box down for the full range of cells.)

Column Q: LaCoste-Romberg, Measured (mGal)

For a digital gravity reading from a LaCoste-Romberg gravimeter, enter the values in this column (mGal).

Column R: Worden Meter, Counter (Dial Reading)

This column is for Worden gravimeter users. The counter values from the dial readings are entered into this column.

Column S: Calibrated (mGal)

In cell S1, enter the meter-specific constant for the Worden gravimeter. The values entered in column S are calibrated using this value.

Column T: Scintrex Meter, Drift and Tide Corrected (mGal)

For a Scintrex meter, enter the values in this column (mGal).

Column U: Raw Observed Gravity (mGal)

Select the appropriate gravimeter from the drop-down menu. The data entered for the specific meter will be transferred to this column.

Column V: Tide (mGal)

The user must make Earth tide corrections using an external program because they are not calculated in this spreadsheet. Enter the Earth tide values for the specific time and location of each gravity measurement.

Column W: Tide Corrected (mGal)

This column calculates the tide-corrected gravity.  
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Column X: Meter Drift (mGal)

The drift correction uses the base-station gravity measurements at the beginning and end of the day or each survey loop to calculate the rate of drift for the gravity measurements during a survey. For surveys that involve multiple days or several loops, the drift correction is calculated for each day/loop separately. The following is an example of a daily survey using a LaCoste-Romberg gravimeter given in the LR_Example_ Spreadsheet (Spreadsheet 22).

Day 1 (9/12/2004)

Day 2 (9/19/2004)

Day 3 (10/16/2004)

When using a Worden meter, the base station is typically reoccupied every 2–3 h; one reoccupation of the base station is considered a loop. The following is an example of the drift corrections for three loops given in the Worden_ Example_Spreadsheet (Spreadsheet 33).

Loop 1 (From Base Station 1 to Base Station 2)

Loop 2 (From Example 012 to Base Station 3)

Loop 3 (From Example 019 to Base Station 4)

Column Y: Drift and Tide Corrected (mGal)

This column is the sum of column W (tide corrected) and column X (meter drift), and it produces a drift- and tide-corrected value of gravity. For Scintrex and LaCoste-Romberg meter users, the formula in the LR_Example_Spreadsheet (Spreadsheet 2) and Scintrex_Example_Spread-sheet (Spreadsheet 44) is:  
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Worden meter users must add the sum of the previous base-station readings to the drift correction for each loop. For example, the drift correction to be applied to all gravity stations measured within Loop 3 must add the sum of the base-station drift-correction factors that precede Loop 3. The following example is given in Spreadsheet 3:

Loop 1 (Base Station 1 to Base Station 2)

Loop 2 (Example 012 to Base Station 3)

Loop 3 (Example 019 to Base Station 4)

Column Z: DC Shift (mGal)

This column supplies the values used in the DC shift calculation in column AF (observed absolute gravity). The DC shift is determined for the same relative base station among multiple-day surveys and amongst multiple surveys. The first occupation arbitrarily is used as the reference value. Subsequent relative observations are differenced from the reference value after the values are corrected for drift and tide. The DC shift value is then entered manually for each individual day or survey.

Column AA: Theoretical Gravity (mGal)

The Somigliana closed-form formula (Hildenbrand et al., 2002) is used to calculate the theoretical gravity for each gravity station based on the GRS80 reference ellipsoid  
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Column AB: Height Correction (mGal)

This column corrects for the height of the gravity station relative to the GRS80 ellipsoid.  
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Column AC: Atm Effect (mGal)

This correction accounts for the mass of the atmosphere above the reference ellipsoid.  
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Column AD: Bouguer Spherical Cap (mGal)

The Bouguer spherical cap correction is the new standard method that accounts for the average mass and curvature of Earth with respect to the ellipsoid. This calculation uses the Bullard B Table. The default density of the Bouguer spherical cap is 2.67 g/cm3 (LaFehr 1991).  
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Column AE: Terrain Correction (mGal)

Terrain corrections are not supported by the spreadsheet and need to be calculated using another software program. Only enter the terrain correction value, not the sum of the observed gravity and terrain correction into column AF.

Column AF: Observed Absolute Gravity (mGal)

This column converts relative gravity observations to absolute gravity values. The relative gravity value in column Y (drift and tide corrected) is referenced to an absolute gravity basestation reading. To complete this calculation, enter the absolute base-station information in the Absolute Base worksheet.

Absolute Base Worksheet

Cell B6 is the absolute gravity value in mGal from the National Geodetic Survey information sheet.

Cell B12 is the observed relative gravity value in mGal.

Cell B13 is the tide correction in mGal.

Cell B14 is the DC shift in mGal.

Cell B15 is the meter drift correction in mGal.

Cell B16 is the reading of the gravity meter after it has been corrected for tide and drift in mGal.  
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Column AG: Complete Bouguer Anomaly (mGal)

The complete Bouguer anomaly is the difference in observed gravity from the corrected theoretical gravity.  
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We wish to thank Mike Webring and Carlos Aiken for their detailed reviews and helpful suggestions that clarified and improved the text. This work was partially funded by a National Science Foundation grant (EAR-0225421) to Oldow.