This article was originally written and published on a previous version of rasstech.net on 30th of May 2019.

Detecting Bombs, Mines and Pipelines

Figure 1: Disposal explosion of a large UXO found in an offshore geophysical survey

Introduction

There is an industry in Europe’s North Sea where survey companies are contracted to detect potentially dangerous un-exploded ordnance (UXO’s) on and below the seafloor. These UXO’s are remnants from previous wars and pose a serious danger to both equipment and to the lives of people on ships who do work on the sea floor. Historically, this has affected mostly fishermen, but recently, the surge in renewable energy has increased wind turbines and seafloor cable production. This article seeks to outline the fundamentals of operation, and make a prediction for future success within this industry.

Here Be Dragons

Production of wind turbines in the North Sea is a growing industry. With more than 4,543 total offshore turbines as of 2018, there are now over 105 offshore wind farms operating from 11 different European countries. Production continues with 15 new wind farms added in 2018. These turbines and farms are interconnected via sub-seabed power cables. Each of these cables must be laid in a trench and then back filled by a special dredging machine operated from a ship. These dredging machines are massive remote operated vehicles, and cost hundreds of thousands of dollars. Due to the intrusive nature of cable laying work, these machines are susceptible to making unwanted contact with UXO’s. Making contact with a UXO can range from a small incident to a massive loss time accident. It is commonplace for energy companies to contract surveys for projected work areas to identify and remove any UXO’s prior to seafloor work. This is in the spirit that mitigating unwanted UXO accidents is much more cost effective and safe than losing a piece of dredging equipment or loss of life. These UXO’s have many forms, they arrive on the seafloor in many ways, and they all pose a high risk of explosion. This is why a UXO desktop study is commissioned by the client, to determine risk areas, and if a full survey investigation is needed.

Figure 2: Three small dense UXO’s of various sizes. Used to calibrate geophysical equipment, these had been picked up in a previous investigation and disarmed

Behind the Scenes

The first step in identifying risk areas begins with research from senior experts in the UXO industry. The main client contracts these professionals to perform a desktop study which assesses risk and to make recommendations. They use various resources, not limited to extensive research in national libraries, military logs, and the internet. Their main purpose is to determine likely hood and type of UXO in the designated areas. They do this by understanding the routes of wartime planes, known offshore mine fields, or other potential hazards such as munition dumping grounds in the offshore. Upon completion of the desktop study, a recommendation of what targets to look for is made with example pictures and diagrams. Survey ships are then outfitted with cutting edge scientific equipment to investigate high risk work areas to ensure that they are cleared of danger.

Figure 3: Off the coast of the Netherlands, near a sub-sea cable corridor, a common place for geophysical surveys

Hunting for Explosives

There are a number of ways to conduct an offshore survey, it is usually dependent on the capabilities of the company and what type of equipment is available. A piece of survey equipment can be mounted on the vessel itself, an ROV, a towed array, or even an automated underwater vehicle (AUV). There are many variations of implementing the survey equipment, but the most common and robust method is an ROV.

Electromagnetic

The survey equipment itself is a combination of cutting edge and older techniques that have been refined over many years in the industry. The most common method for finding UXO’s is using Electromagnetic waves to detect ferrous (magnetic) material. Electrical current is run through coils which create a magnetic field around them. These coils are brought near the seafloor where the magnetic field creates feedback in any magnetic objects through the principles of Faraday’s law. This response is presented as a dipole pattern in the data, with a north and south end. Professionals will interpret this data and determine if the anomalies need further investigation.

Figure 4: Deployment of a working class ROV with electromagnet geophysical equipment attached

Non Penetrating Acoustic

Multi-beam sonar is a method of survey that delivers a precise 3D model of the sea floor topology. It emits high frequency acoustic sonar beams and measures the time it takes for the beams to return to the emitter. It is useful in UXO operations because it shows the interpreters any visual clues present on the sea floor, and it can be used to help isolate surface anomalies taken from other methods. Side-Scan sonar is a similar technique to multi-beam but it covers a much smaller survey area. It is typically associated with areas of known targets that require higher resolution imaging such as a ship wrecks or other surface investigations.

Penetrating Acoustic

More recently, the use of a specialized acoustic sonar called the Sub Bottom Imager has broken into the market. Analogous to conventional acoustic methods, its resolution and depth reside between seismic surveying and side scan sonar; this intermediate frequency fills a useful range for UXO detection. Acoustic pings are sent into the sea floor which penetrate a depth where UXO’s are typically located. This sound reflects off of any objects that have a high acoustic impedance contrast with the surroundings. UXO’s are typically made from dense metals, and as such provide a strong acoustic impedance contrasts. This method is an excellent complement to the Electromagnetic method as it can provide an accurate shape and depth to the magnetic anomalies. The acoustic sonar can detect non magnetic anomalies buried below the surface, which is useful for detecting nonferrous UXO’s such as the German LMB.

Figure 5: German aluminum coated LMB mines. Typically dropped by helicopters and considered the deadliest of all mines due to their extensive detection countermeasures, and versatility

Carrots for the Stew

At this stage, the acquired data for the investigation area is compiled and combined. The UXO experts take reports from the data processors and make interpretations on the data. Interpretations are done in real time on the vessel so that if an anomaly indicates a UXO then direction may be given to investigate further. Ultimately, when a UXO is confirmed found, the proper authorities are notified, in most cases this is the bomb disposal unit of the host countries military. Measures are taken to remove or explode the UXO to make the area safe for future operations.

Figure 6: Buoyant mines, held down with an anchor to the sea floor, left mine was found in Wales in 2017

Final Thoughts

The limitations of working on the seafloor pose severe restrictions on what geophysical methods can be used to detect buried UXO’s. Most electrical methods are ruled out, and stationary methods are inefficient to implement. Ground Penetrating Radar is a great method on land, but due to the high salinity content of salt water, it is only feasible in fresh water. Electromagnetic methods are currently the most common method, but they have challenges associated with shape, orientation, depth, and false positives of anomalies. The Multibeam is useful for some surface contacts but it fails if anomalies are buried. Side-Scan Sonar and the Sub Bottom Imager begin to fill the voids that the Electromagnetic methods lack, but can be expensive to implement, and the technology is proprietary. It remains to be discovered if other geophysical methods can penetrate deep enough for UXO’s while dealing with the challenges of a saline (highly conductive) environment. Regardless of new tech, one thing has been whispered among some industry professionals, that the future of these surveys belongs to a system which fully integrates Electromagnetic, Multibeam, and sub-seafloor acoustic penetration efficiently.

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