Gravity methods detect the differing densities of geological materials in the subsurface
Airborne gravity gradiometry data is an accepted cost effective exploration tool for oil and gas and mineral exploration. Yet until very recent times only a limited number of geothermal projects embraced the potential that this technology can provide.
The Relevance of Density in Geothermal Exploration
By mapping contact information generated by density contrasts resulting from stratigraphic and structural changes, it becomes possible to map and model the subsurface.
There are 4 common scenarios where measuring density will add value
- Basin Geometry : Gravity Gradiometry can highlight the density contrasts within a basin caused by burial history and compaction of sediments.
- Fault Offsets : Gravity Gradiometry is particuarly effective for mapping offsets on faults due to the differing densities of the rock types
- Intrusive Units : Igneous intrusives often have higher density than surrounding sediments
- Alteration or Silicification : Altered sediments may be depleted of specific minerals resulting in reduced density or be enriched in mineral precipitates which increases the density.
More Detail than Scalar Gravity Data
To put it simply, you literally see more with gravity gradiometry than you can with standard (or scalar) gravity data.
A gravimeter measures the vertical component of the Earth's gravity field as a scalar quantity (a magnitude described by a single number) whereas a gravity gradiometer measures the spatial rate of change in the gravity field as a vector quantity (both as magnitude and direction).
Why Gradiometry is Applicable to Geothermal Operations
1. Higher Resolution Data at the Near Surface
Gradient measurements resolve smaller wavelengths than scalar gravity. This results in a much higher near-surface resolution and a better image of geophysical targets.
This is a key advantage for geothermal operators who understand the geological relevance of the top 500 meters of the subsurface with regards to geothermal activities.
3. Multiple Independent Data Sets
Full Tensor Gradiometry offers multiple independent data sets. This automatically offers additional constraints for interpretations and inversions. This ovecomes the ambiguity associated with other potential fields methods.
This delivers geothermal exploration teams with a more reliable interpretation of data than would be feasible from a scalar ground gravity survey.
2. An Ability to Detect Edges
Scalar gravity identifies the center of mass of a target, whereas gravity gradiometry detects where the gravity gradient changes most rapidly at the edges of geophysical targets. The result is a better edge detection when it comes to interpreting faults, identifying geological boundaries and mapping structural features.
This is particuarly useful for geothermal exploration teams dealing with blind geothermal systems where the faults do not have any surface expression.
4. Cost Efficiency
Airborne methods provide the means for a quick, cost effective regional survey. Airborne acquisition overcomes the limitations of inaccessible land, cultural and community impact, vegetation and surface water.
Full Tensor Gravity Gradiometry will usually require double the budget of a standard gravity flight, easily justified by points 1 - 3.