Forensic geophysics

Forensic geophysics is a specialized branch of forensic science. It involves the study, search, and mapping of objects hidden beneath soil or water. Experts use geophysical tools to find these items for legal purposes. These targets can vary greatly in size and type. They include small weapons or metallic barrels. They can also include large human burials or hidden bunkers. This field is an evolving technique. It is gaining more popularity and prestige within law enforcement agencies worldwide.

This science works by identifying a contrast in physical properties. A contrast occurs when a target differs from the material surrounding it. When this difference exists, experts can precisely define the location of the hidden object. Geophysics allows for non-destructive investigations. This means experts can scan large areas without digging everything up. It is a rapid way to investigate the subsoil. Scientists can also detect evidence of human soil occupation. This includes finding signs of recent excavations or much older ones.

Before starting a survey, investigators must prepare carefully. They begin with a thorough desk study. This often includes looking at historical maps of the area. They also perform utility surveys and site reconnaissance. Control studies are also necessary before the actual geophysical surveys begin. Investigators often use other search techniques to prioritize specific areas. For example, they might use cadaver dogs or forensic geomorphologists. Once the site is ready, they move into phased investigations with full surveys.

Ground-penetrating radar, or GPR, is the most popular technique for forensic searches. It can detect objects at a maximum depth of 10 meters below ground level. The depth depends on the antenna frequencies used, which typically range from 50 MHz to 1.2 GHz. There is a trade-off when choosing a frequency. Higher frequencies allow operators to resolve smaller objects. However, higher frequencies also result in decreased penetration depths. Operators must choose carefully and often use trial surveys at known depths.

While GPR is common, it is not suitable for every environment. It struggles in clay-rich soils or coastal areas with salt-rich soil. In these cases, electrical resistivity methods are used. These methods can detect objects even in clay where GPR cannot penetrate. The dipole-dipole method is a common configuration. It traverses an area to measure resistivity variations at a set depth. Other slower methods include Electrical Resistivity Imaging, or ERI. This involves placing many probes to collect data both horizontally and vertically. Multiple 2D profiles are known as electrical resistivity tomography, or ERT.

Magnetometry is another vital tool for finding buried items. It can detect buried metal or fired objects like bricks. It can even find areas where surface fires once occurred. Experts use different tools depending on the required accuracy and cost. These range from simple total field magnetometers to fluxgate gradiometers. High-end alkali vapour gradiometers are also used for precise work. Additionally, surface magnetic susceptibility has shown recent promise in forensic searches. For very large objects, seismic surveys may be used. However, these only have a 2m vertical resolution and are mostly used to find bedrock.

Forensic geophysics is also expanding into water-based searches. This is becoming more common for investigating ponds, lakes, and rivers. Specialists use marine magnetometers and side-scan sonar. They also use acoustic methods and water-penetrating radar. These tools allow for rapid scanning of river bottoms and near-shore environments. These techniques help investigators find targets in depositional environments where objects might settle.

To improve these methods, researchers conduct controlled studies. They use simulated forensic targets to find the best equipment configurations. A common method involves burying porcine cadavers for long-term monitoring. Scientists also study how seasonal effects impact electrical resistivity surveys. Some research looks at burials inside walls or beneath concrete. These studies take place in the UK, the US, and Latin America. Researchers even study old graves in graveyards. They want to see how geophysical responses change as burials increase in age.