Magnetic susceptibility
Definition
A measure of how strongly a material is magnetised by an external field. Topsoil heated by fire or biological activity has elevated susceptibility; digging mixes high-susceptibility topsoil into lower-susceptibility subsoil, creating a detectable contrast.
- Definition
- Measure of how strongly a material is magnetised by an external field.
- Forensic use
- Detects grave fill anomalies where topsoil is mixed into subsoil during burial site excavation.
- Why topsoil is susceptible
- Enriched by weathering and biological activity, making it more magnetizable than deep subsoil.
Common questions
Why does buried soil show up on a magnetometer?+
When a grave is dug, topsoil (which is magnetically enriched by weathering and biological activity) gets mixed down into the subsoil. This creates a contrast in magnetic susceptibility - the inverted topsoil is more magnetizable than the surrounding subsoil, so a magnetometer can detect the anomaly above the burial.
What makes topsoil more magnetically susceptible than subsoil?+
Topsoil accumulates magnetic minerals through weathering and biological activity over time. Subsoil, formed from deeper, fresher parent material, has not undergone the same weathering processes and contains fewer magnetically responsive minerals. This natural difference is what forensic surveyors rely on to spot disturbance.
How is magnetic susceptibility measured?+
It is the ratio of induced magnetisation to the applied magnetic field. A stronger response means the material is more easily magnetized. In soil, this property depends on the abundance and type of magnetic minerals present.
Related terms
- Fluxgate gradiometer
- An instrument that measures the vertical gradient of the Earth's magnetic field using two fluxgate sensors mounted on a staff at fixed...
- Wenner array
- A four-electrode resistivity configuration where all four electrodes are equally spaced. Depth of investigation is approximately equal to the electrode spacing. Provides...
- Apparent resistivity
- The resistivity value computed from a measurement assuming the ground is homogeneous. In practice the ground is layered and variable, so apparent...
- 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...
- Electrical resistivity tomography (ERT)
- A method using multiple electrodes on a line or grid to inject current and measure voltage, computing a two-dimensional or three-dimensional resistivity...
- EM conductivity (frequency-domain EM)
- A non-contact method using a transmitter coil to induce eddy currents in the ground and a receiver coil to measure the secondary...
- FEM (frequency-domain EM)
- A class of instruments that measure ground conductivity from above-surface, without electrode contact, by inducing eddy currents with an oscillating EM field....
- 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...
- Physical contrast
- The difference in a measurable soil property (density, susceptibility, resistivity, permittivity) between a target and its surrounding host material. No contrast means...
- 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...
- Thermoremanent magnetisation (TRM)
- Magnetic alignment locked into iron-bearing minerals as they cool through the Curie temperature after being heated. Burning soil can transform weakly magnetic...
Explained in these topics
- Geophysical Survey Principles in Forensic ContextsA measure of how strongly a material is magnetised by an external field. Topsoil is generally more susceptible than subsoil, so inverted grave fill (topsoil mi...
- Magnetometry and Soil Magnetic SusceptibilityThe ratio of induced magnetisation to the applied magnetic field. Higher in topsoil (magnetically enriched by weathering and biological activity) than in deep...
- Magnetometry and Earth ResistanceA measure of how strongly a material is magnetised by an external field. Topsoil heated by fire or biological activity has elevated susceptibility; digging mix...