Physical Contrast
Definition
The difference in a measurable soil property (density, susceptibility, resistivity, permittivity) between a target and its surrounding host material. No contrast means no signal, regardless of instrument sensitivity.
- Definition
- Difference in a measurable soil property, target vs host
- Example properties
- Density, susceptibility, resistivity, permittivity
- Field
- Forensic geophysical survey
- Key limit
- No contrast means no signal, regardless of sensitivity
Common questions
Why can a highly sensitive geophysical instrument still fail to find a buried target?+
Sensitivity only determines how small a difference the instrument can measure, not whether a difference exists. If a grave or buried object has essentially the same density, resistivity, or other property as the surrounding soil, there is nothing for even the most sensitive instrument to detect.
How do investigators choose which geophysical method to use based on physical contrast?+
They consider what property is likely to differ between the target and the local soil, for example disturbed backfill often has different density and resistivity than undisturbed ground, and select ground-penetrating radar, resistivity survey, or magnetometry accordingly, matched to the expected contrast.
Related terms
- Dielectric Permittivity
- A soil's capacity to store and transmit electric field energy. Higher water content raises permittivity and slows the radar wave. The contrast...
- Electrical Resistivity
- The resistance of a volume of soil to electrical current flow. Wet, clay-rich, or decomposing organic material has low resistivity. Dry sand...
- Magnetic Susceptibility
- A measure of how strongly a material is magnetised by an external field. Topsoil heated by fire or biological activity has elevated...
- Multi-Method Survey
- The deployment of two or more independent geophysical methods over the same search area. Anomalies confirmed by multiple methods carry higher confidence...
- Signal-to-Noise Ratio (SNR)
- The ratio of anomaly amplitude to background variation. Pipes, cables, rocks, roots, and instrument drift all add noise. A target is detectable...