UXO Survey, Magnetic Processing and Target Modelling. A Case Study from Cable Survey, Offshore Netherlands

In the last decade UXO geophysical survey identification and clearance have largely widespread in the undersea cable industry and offshore windfarms. The need to detect metallic, potentially UXO, objects with the maximum accuracy achievable has led to a general improvement of the performances of magnetometer/gradiometer arrays and of underwater positioning techniques.

Here we report a case study based on an extensive UXO survey carried out off Ijmuiden (Netherlands) in the frame of a windfarm installation project (Hollandse Kust Noord and Hollandse Kust West Alpha for TenneT company). The entire workflow has included: surrogate trials and system calibration; high resolution surveys with a towed magnetometer array flying at low altitude (2–3m) above the seabed along densely spaced run lines; nav processing; magnetic processing and target modelling; target inspection and digging with ROV and possible removal. The complete set of geophysical data, coupled with the results of targets inspection gives an unprecedented opportunity of verifying the validity of the magnetic processing and of inverse modelling algorithms that are commonly used in the offshore industry.