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Estimates of various UXO (unexploded ordnance) maximum detection distances using magnetometers

Alexey Dobrovolskiy
CEO, CTO @ SPH Engineering
Matiss Brants
Geophysicist @ SPH Engineering
Integrated Systems
July 25, 2024

In recent decades magnetometers (terrestrial, airborne, and towed systems) have become widespread tools to detect various types of unexploded ordnance (UXO) and explosive remnants of war (ERW). Despite the vast experience collected by field practitioners and organizations participating in UXO and landmine clearance, there is still a lack of public information about the real capabilities of magnetometer systems in terms of detection range for particular types of ordnance.

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.

The noise floor is the noise level below which useful signals cannot be detected. Magnetometer sensors have their own internal noises, plus the system consists of a magnetometer and its platform adds many sources of additional noises. Magnetometer’s manufacturers in their specifications often claim very low values for noise floor - sometimes fractions of nT (nanotesla - unit of magnetic flux density, main characteristic of magnetic field). That noise floor is valid only in laboratory conditions and for magnetometer sensors only, and doesn’t have any relation to the field conditions.

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

No. Ordnance type Distance*, m Estimation method** Sources***
1 20 mm M55 projectile 0.4 lab 1
2 20 mm UXO Surrogate 0.7 field 3
3 37 mm UXO Surrogate 0.9 lab 4
4 40 mm MKII projectile 0.9 lab 1
5 45 mm shell (Germany, WW2) 0.8 field 2
6 50 mm shell (Germany, WW2) 0.8 field 2
7 57 mm M86 APC projectile 1.2 lab 1
8 60 mm M49A3 mortar 1.2 lab 1
9 60 mm UXO Surrogate 1.5 field 3
10 70 mm (2.75") M230 rocket 2.4 lab 4
11 70 mm (2.75") M230 warhead 2.0 lab 1
12 75 mm shell (Germany, WW2) 0.9 field 2
13 76 mm AP projectile 1.5 lab 1
14 76 mm HE grenade (Germany, WW2) 1.5 field 2
15 81 mm M374 mortar 1.5 lab 1
16 88 mm shell (Germany, WW2) 2.0 field 2
17 90 mm AP projectile 1.8 lab 1
18 105 mm M456 HR 2.1 lab 1
19 105 mm M60 AP projectile 2.6 lab 1
20 105 mm UXO Surrogate 4.0 field 3
21 125 mm HEAT warhead (USSR, WW2) 2.4 field 2
22 150 mm HE shell (Germany, WW2) 2.8 field 2
23 150 mm HE shell (Germany, WW2) 3.2 lab 1
24 215 mm shell (Germany, WW2) 5.0 field 2
25 320 mm rocket warhead (Germany, WW2) 3.8 field 2
26 500lb MC aerial bomb (GB, WW2) >7.0 field 2
27 BDU-26 submunition 0.5 lab 1
28 BDU-28 submunition 0.5 lab 1
29 Flam C-250 aerial bomb (Germany, WW2) >7.0 field 2
30 M-31-UK rocket (USSR, WW2) >7.0 field 2
31 M42 submunition 0.5 lab 1
32 OZM-72 anti-personnel mine (USSR) 1.3 field 2
33 RGD-5 grenade (USSR) 0.5 field 2
34 RPG-18 rocket-propelled grenade (USSR) 0.9 field 2
35 SD 70 aerial bomb (Germany, WW2) 7.0 field 2
36 TM-62M anti-tank mine (USSR) 2.8 field 2
37 Type 100 rocket-propelled grenade (Germany, WW2) 2.0 field 2
38 Type 100 rocket-propelled grenade launcher (Germany, WW2) 3.0 field 2
39 Type 100 rocket-propelled grenade warhead (Germany, WW2) 1.1 field 2

*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.
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