Magnetometry and Earth Resistance

Every soil has a magnetic susceptibility determined by the iron minerals it contains and the thermal history of those minerals. Topsoil, which has been exposed to biological activity, burning, and repeated wetting and drying, often has higher susceptibility than the deeper subsoil beneath it. A grave cut mixes this magnetically distinct topsoil into the deeper horizons and brings lower-susceptibility subsoil to the surface. The refilled grave pit is therefore a zone of mixed susceptibility, sitting in a matrix of more uniform soil, and the boundary between them generates a dipolar or diffuse magnetic anomaly.

The fluxgate gradiometer detects this anomaly by measuring the vertical gradient of the total field. Measuring the gradient rather than the absolute field is a practical choice: it reduces noise from diurnal (daily) variations in the Earth's field, which can be larger in amplitude than the grave signal itself. The two sensors, typically 0.5 m apart on a vertical staff, subtract background variation and leave only the anomalies generated by the local soil.

Earth-resistance measurement injects a small alternating electrical current into the ground through contact electrodes and reads the voltage drop across a second pair. The ratio of voltage to current gives resistance, and this is converted to apparent resistivity by applying a geometric factor that accounts for electrode spacing.

For forensic grave searches, the signal source is the moisture-content contrast between the grave fill and the surrounding soil. The specific direction of the contrast depends on conditions at the time of survey. In dry weather, the loosely packed grave fill retains more moisture than the compacted surroundings, producing a zone of relatively lower resistance. After heavy rain, the loose fill drains faster, and the grave may appear as a higher-resistance zone. This behaviour means the interpreter needs to know the recent weather history when assigning sign (high or low) to anomalies.

The twin-electrode configuration has become the standard for forensic fieldwork in the UK and elsewhere because a single operator can walk the grid with a Frame meter (a T-shaped frame with two mobile probes) relatively quickly. The English Heritage/Historic England archaeological geophysics survey guidelines, which many forensic practitioners adopt, specify 0.5 m mobile electrode spacing and 1 m traverse spacing as a standard starting configuration.

The resistivity signal from a grave is not constant. It changes with rainfall, evapotranspiration, temperature, and the progressive settlement and compaction of the grave fill. Understanding these seasonal effects helps investigators choose survey timing and interpret the polarity of anomalies correctly.

• Dry summer conditions: the loose grave fill holds more moisture than the cracked, dry surrounding soil. The grave appears as a low-resistance (high-conductance) anomaly. This is generally the best survey condition for resistivity.
• Saturated winter conditions: if the entire survey area is uniformly saturated, the moisture contrast between grave and surroundings may collapse and the grave may become invisible. Conversely, a grave with coarser fill in clay soil may drain faster and appear as a high-resistance anomaly.
• Seasonally frozen ground: frozen soil has very high resistance; electrode contact fails or gives noisy data. Resistivity surveys should not be conducted with frost in the ground unless specialist low-frequency impedance techniques are used.
• Older graves: over years, the grave fill consolidates and its moisture characteristics approach those of the surrounding soil. The resistivity contrast decreases. This is one reason magnetometry sometimes performs better than resistivity for older burials.

Frequency-domain EM instruments (often called ground conductivity meters or by their trade name, EM38 or EM31) measure apparent ground conductivity by transmitting an oscillating magnetic field from a transmitter coil and measuring the secondary field induced in the ground from a receiver coil at a fixed spacing. No electrodes are required: the instrument is carried above the surface, making survey very fast.

The depth of investigation depends on the coil orientation and spacing. The EM38 in vertical dipole mode investigates to approximately 1.5 m, making it appropriate for typical grave depth ranges. The EM31 with its 3.66 m coil spacing reaches to about 6 m, more appropriate for deep or geological targets.

In forensic practice, FEM is most valuable as a rapid first-pass triage over large search areas. An operator can walk a hectare in a day and map conductivity variations. High-conductance anomalies (possible disturbed wet fill) or low-conductance anomalies (possible coarse disturbed fill in a clay matrix) are then flagged for follow-up with GPR or contact resistivity. FEM is not a primary grave-detection method: its depth resolution is too poor to confidently attribute an anomaly to a grave versus a geological feature. But as a filter over a large, undifferentiated search area, it saves time.

The modern standard for systematic forensic grave searches is to use at least two complementary geophysical methods. The combination is selected based on a pre-survey soil assessment and the operational constraints (area size, time, access). The most common forensic pairings are GPR + magnetometry and GPR + resistivity.

Case datasets from English Heritage (now Historic England), the UK Forensic Science Service, and academic comparative studies suggest a general principle: anomalies confirmed by two independent methods have a substantially higher excavation priority than those seen in only one. When GPR and resistivity both flag the same location, the chance of a false positive is much lower than either method alone.

Survey documentation is as important as the survey itself. Line positions, instrument settings, calibration readings, weather conditions, and notes on surface features (trees, paths, disturbed areas) should all be recorded and submitted alongside the radargrams and resistance grids. This documentation forms part of the chain of custody for the geophysical evidence and allows an independent expert to assess the quality of the survey if the anomaly findings are challenged in court.

The most systematic UK datasets come from controlled burial trials, archaeological surveys, and the few peer-reviewed forensic case reports in the literature. English Heritage's Geophysics team (now Historic England) has published extensive methodological guidance based on thousands of surveys across varied soil types. While most of this work targets archaeological features rather than single forensic graves, the principles transfer directly.

For forensic single graves specifically, resistivity surveys have shown mixed results in published case comparisons. In one controlled-burial study at Cranfield, resistivity detected approximately 50–60% of burials in loamy soil, compared to over 70% for GPR. However, in the clay-dominant test beds, resistivity outperformed GPR because it was not defeated by the conductive soil in the same way. Magnetometry showed the lowest detection rates for recent single burials in most published trials, though it remains useful for older graves where soil mixing has had time to alter magnetic properties.

The evidential value of a geophysical anomaly in court depends on how rigorously the survey was conducted, documented, and interpreted. An expert who can explain the physics, demonstrate calibration, acknowledge limitations, and show that the excavation result confirmed the prediction will be more persuasive than one who simply presents a colour plot. Courts in the UK, US, and Australia have accepted geophysical evidence from qualified experts when those conditions are met.