Analogue vs. Digital Geophysics for Unexploded Ordnance (UXO) Detection - Why cheaper isn't always better
Countries all over the world are affected by explosive remnants of war (ERW) or unexploded ordnance (UXO). These ERW/UXO impact people's health and safety, and prevent communities from using productive agricultural land or building important infrastructure. Non-governmental organizations (NGOs) assist communities with mine action programs to detect and remove ERW/UXO so that they can live safely and develop their land without risking harm.
While traditional analogue surveys (also known as "mag and flag") can be fast and cheap methods to detect ERW/UXO, factors such as magnetic soils, inexperienced operators, instrument malfunctions, and inclement weather can cause targets to be missed and left in the ground or underwater, leaving people vulnerable to harm.
It has been documented that a much smaller percentage of UXO are left in the ground using digital geophysics for UXO surveys. Digital geophysical investigations provide a detailed and accurate record of survey coverage, as well as a historical archive which can be reviewed and reprocessed to ensure the data has been properly collected, or can be corrected for common survey problems. Digital surveys also provide more information about targets which can be analyzed for depth, size and other properties to help objectively distinguish UXO from non-UXO (eg. harmless scrap metal or magnetic rocks).
Q1. What are the options for areas without good GPS such as under tree cover?
A1. In areas where GPS systems are not able to have a clear view of the sky, such as under tree cover, an alternative positioning technology that has proven successful is robotic total station (RTS).
Q2. Why are landmines harder to detect? Can you recommend methods for detecting landmines?
A2. Detecting landmines depends somewhat on the nature of the landmines themselves – some are designed to be difficult to detect and may have little to no metallic content.
Geophysical methods that are used to detect landmines include magnetics, electromagnetics (EM) and ground penetrating radar (GPR). Magnetics and EM depend on the landmine having a detectable metal content. GPR does not share this limitation but it can be limited by soil conditions and the difficulty in distinguishing landmines from other buried objects like rocks. Keep in mind for digital geophysics, the operator must be able to safely cross the survey area, which may not be possible in the case of anti-personnel mines. Improvised Explosive Devices (IEDs) are often made with metal and so these may be easier to find.
This Wikipedia article (https://en.wikipedia.org/wiki/Demining) provides some suggested methods for landmine detection and clearance. Drone technology, in combination with improved and smaller geophysical sensors, offers some exciting possibilities, especially as the drone payloads also increase.
Q3: How long is the DGM Data generally considered valid if intrusive work is planned for a later date?
A3. The recorded digital data and the analysis of those data are valid forever. The only challenge is if the site conditions / landscape is changed or the targets are moved. In a terrestrial environment, you would usually be able to return to the site at any time. As the data are recorded with real-world GPS coordinates you can go back to the exact location and begin your intrusive work, unless there have been human activities affecting the site (such as landfill or excavation work), or natural shifts such as landslides or dune movement. In a shallow marine environment the situation is different as UXO can be moved around in currents and storms, and the sea bottom itself can be modified by the same forces. In this case it is important to attend to intrusive work immediately.
Q4: At what anomaly density (items/acre) does the DGM data become cluttered for identifying single point anomalies?
A4: The anomaly density at which it becomes difficult to identify single anomalies in DGM data depends on the nature (size and depth) of the target bodies, the type of sensor being used, and with line and station spacing of the survey. When using advanced EM sensor data and advanced processing, anecdotally we have been able to resolve 4 to 5 (small) objects per square metre.
Q5: Which of these systems is available commercially?
A5: Magnetometers and basic electromagnetic (EM) sensors have been commercially available from a number of manufactures for many years. They can be purchased as individual sensors or in a small selection of pre-fabricated arrays, or can be built into more complex arrays by individual survey contractors to meet their specific needs. Providers include Geometrics, Geonics, Ebinger, Marine Magnetics, and Vallon. Advanced EM sensors capable of collecting data to facilitate classification of UXO targets, such as the MetalMapper 2x2 (from Geometrics) are just now becoming commercially available.
Q6: How is interference removed in high density metallic areas to determine which are UXO vs debris? At my sites we've had no success with this method especially around buildings.
A6: All sensors are subject to some degree of interference from buildings, fences and other nearby cultural effects. In general, the EM sensors are less impacted than magnetic sensors by buildings and fences. With the advanced EM sensors, it can also be possible to use only portions of the data (use the "late time" data) to reduce the impact that a thin surface layer of metallic debris will have on the final interpretations.
Q7: Can you give a short description or a reference on the characteristic magnetic signal of a UXO?
A7: The magnetic signal or response from a UXO can vary widely depending on the type of UXO, how it is buried, the location in the world, any magnetic remanence in the manufactured object itself, and other factors. While it's difficult to fully describe here, we can say that as some UXO are relatively long compared to their width and if oriented horizontally, these will appear as dipoles in the data (negative and positive anomaly). Where others, or ones oriented vertically will appear as monopole in the data (positive),in northern latitudes. Often the anomaly is not centered above the target creating the anomaly. This underlines the importance of processing the data effectively. Tools in Geosoft's workflow provide ways to handle these data including calculation of the analytic signal, which creates a positive anomaly centered over the target for magnetic datasets - very useful for target picking.