Method
Introduction
A gravity survey consists of taking measurements of Earth’s gravitational field, which varies based on the density of the material below each measurement point on the surface. As the density of the material increases, so does the acceleration due to gravity at that location. Zonge produces the Complete Bouger Anomaly value for each station. A gravity survey may be designed as a grid, profiles, or sparsely populated stations across an area of interest using 1-2 people, a gravimeter, and a base station.
Applications
Gravity surveys are a common tool for identifying density contrasts in the subsurface.
This can be applied to:
identifying structures (hanging vs foot wall zones, shear zones)
May pick up lithogic differnces in density if there is enough contrast
Detection of voids (karst terrain)
Alteration zones
Basin characterization/depth to bedrock
In the right circumstances, such as massive sulfides, may provide direct detection
Monitoring of CCUS sites has been a proposed application
Survey Design
| Parameter | Description |
|---|---|
| Depth | The depth is inferred based on the wavelength of the anomaly |
| Station Spacing | Optimal station spacings should not be greater than the lateral extent of the target, ideally 1-2x smaller than the target dimension. |
| Scale | Local to Regional scale survey type |
| Cost | Low-cost relative to other geophysical methods. |
| Timing | Based on station spacing and terrain, multiple stations acquired per day. |
Instrumentation
Lacoste & Romberg G Meters
Scintrex CG5 when higher precision measurements (typical repeatability of 0.01 mGal) are required.
It is required to use geodetic-grade GPS receivers to obtain accurate position information (most importantly elevation)
Deliverables
- Color contoured plan-view maps of Complete Bouger Anomaly (CBA) made at the selected reduction density
- Contoured plan-view maps of the horizontal gradient and calculated 1st derivative
Survey Life Cycle
- A local base station is deployed to provide an absolute gravity measurement, taking a measurement at the start and end of the day which accounts for drift.
- A crew of 1-2 people per receiver will take measurements (2 stacks measurements) at each station.
- Data undergoes QA/QC procedures including evaluation of repeat measurements and loop closures to meet specifications.
- The Complete Bouger Anomaly measurement (CBA) is produced through the data-reduction process, which includes correcting for the Free-air (elevation), Bouger (terrain), latitude (Earth’s curvature), and tidal variations. Zonge routinely delivers the first vertical derivative and horizontal gradient of the dataset.
- Further modeling and interpretation may include completing inversions and trend removal.
Case Studies and Resources
- [Link to Scott’s Info]
- [Tucson Gravity Case Study]
- [Additional Gravity Survey References]