Unfortunately, stakeholders and donors of clearance activities often overestimate the capabilities of modern detection equipment and are looking for silver bullets to reduce the risks and costs of landmines and UXO clearance. That’s especially true for post-war land release activities with vast contaminated areas and limited budgets.

The reality is that the laws of physics limit the detection range for most “popular” ordnance types between dozens of centimeters and a few meters. In this short article, we provide an estimated detection range for some ordnance types to help build the right expectations for all stakeholders and participants in land release and clearance activities.

Methodology

Ordnance (ferrous metallic item) to be detectable using a magnetometer must create a strong enough anomaly to not only break through the noise floor of the magnetometer system but also through a set ceiling (signal-to-noise ratio or SNR) to count as a valid signal.

Based on our experience, we presumed that the magnetometer system consists of a modern drone and professional-grade magnetometer has a noise floor of around 2.5 nT (which seems about right or even cautious), but the signal must have an SNR of 6 dB, which means: noise floor multiplied by 2. So we considered a signal to be valid if it generates at least 5 nT anomaly. There are some caveats with this method, but estimations should be fine for most types of high-quality systems available on the market.

We combined data available from external sources and collected by SPH Engineering and our partners to calculate the distance at which a magnetic anomaly from different types of ordnance will have amplitude 5nT. 

Disclaimers

1. We tried to be conservative in our estimates, but we can’t guarantee that the same or similar ordnance type will be detected at the distance taken from our estimates.
2. Detection distance in our estimates should be considered as “maximum” detection range. It is very unlikely that the same type or similar ordnance can be detected at a bigger range.
3. Information is provided “AS IS” and SPH Engineering doesn’t accept any liability for the accuracy of information.
4. The distance was estimated for particular ordnance items used in tests or measurements. Other items depending on their state, and age, manufactured in different times in different factories may have different detection distances.

Note on landmine detection

While magnetometers can detect some types of landmines (for example, M15, M6, TM-62M anti-tank mines; M16, PROM-1, OZM-3, OZM-4, OZM-72 anti-personnel mines and similar types with considerable amounts of ferrous metal), landmines search is not a direct application of magnetometers as they can’t detect most types of modern landmines. That means that the system should never be used to confirm the absence of landmines (and small ordnance) in a certain area, but it can be a valuable asset during a Non-Technical Survey (NTS) or Technical Survey (TS) to confirm the presence of UXO/landmines with a considerable amount of ferrous metals.

Detection range

*Distance:

• detection distance = sensor altitude + target depth, if we suppose a noise floor of 2.5 nT, and a signal-to-noise ratio of 6 dB (that is, the signal amplitude is at least 5 nT)

**Methods of estimation:

• ‍Field - the value is estimated from very limited (in measurement spacing, target orientation, noise control, etc.) field measurements of active or deactivated ordnance
• ‍Lab - the value is estimated from detailed laboratory measurements of active or deactivated ordnance in controlled conditions

***Sources:

1. Billings et al. 2006. Magnetic models of unexploded ordnance. DOI: https://doi.org/10.1109/TGRS.2006.872905.
   Estimates are based on the orientation that causes maximum magnetic intensity (axial orientation).
   Link: https://ieeexplore.ieee.org/document/1661800
2. SPH Engineering & SENSYS. UXO Detection Test Results using various types of magnetometers (SPH Engineering’s MagNIMBUS, SENSYS MagDrone R1, R2, R4) over SENSYS GeoMil test range. Link: https://app.dronegis.sphengineering.com/api/v1/shared_urls/656c9c959a6dff6e20f7de42
3. SPH Engineering. UAV-based magnetometer comparison: UXO test.
   Link: https://www.sphengineering.com/news/uav-based-magnetometer-comparison-uxo-test
4. Seequent Oasis Montaj UXO Extension Forward Model tool. The model supposes an external magnetic field of 50000 nT, inc = 65 deg, dec = 0 deg, while the targets inc, dec = 0 deg.