Marine non-intrusive survey Merseyside

SafeLane's marine team conducted a non-intrusive survey using magnetometer, side scan survey and 3D (seismic) chirp sub bottom profiling for a client in Merseyside.

The principal objective of the marine and land non-intrusive surveys requested of SafeLane at the Twelve Quays ferry terminal site was to identify any magnetic anomalies that could have been commensurate with potential UXO.  These would then need to be highlighted for later investigation and removal/disposal.

The objective of the client was to ensure a safe working environment for new piling works and working areas for jack up construction vessels.

The following techniques were employed to achieve the following principle objectives:

  • Land drilling and magnetic data collection to identify any potential UXO-like objects on two shore-based pile locations
  • High frequency side scan sonar (SSS) survey, with the aim of identifying any objects on or protruding from the bed surface.  The data was also used to see if there was any correlation with magnetic targets to add knowledge and site intelligence to the interpretation of potential UXO.
  • Magnetometer survey, with the aim of detecting magnetic anomalies that might indicate the presence of UXO.

When discussing the scope of works with the client, two solutions were offered: magnetometer and side scan survey, the standard approach by most UXO survey providers to conducting non-intrusive marine survey.

This was a process that the client was familiar and felt comfortable with.

However, it was clear from the outset that the site posed many challenges, not least existing piled structures, sheet piling and a working ferry terminal.  In addition to all that, there were the issues of an extensive tidal range and a very fast current.

The second solution suggested was the state-of-the-art 3D seismic chirp system.

This system allows the data processor to build up a detailed picture of each potential UXO target, and has the potential to significantly reduce false positives whilst being unaffected by existing ferrous structures such as sheet and tube piles.

The client initially opted for the magnetometer and side scan survey method.  This was conducted – and as expected and advised – the results left many areas of the site where the magnetometer could not differentiate between targets.

This left a situation where extensive diving investigations would be required to reduce the UXO risk to a tolerable level.

Further discussions were conducted with the client and the 3D chirp option was once more proposed.

SafeLane worked with the client to look at the costs and benefits associated with the 3D chirp option versus  extensive diving operations.

It was proven that the cost of potential diving investigations could be significantly reduced, or potentially even eliminated, by conducting a second survey using the 3D chirp approach…and so the client gave the go-ahead.

The survey results spoke for themselves; after processing and quality control, only two targets over the whole construction footprint could not be discounted as UXO.

By putting in a safety exclusion zone around each, the risk was reduced to as low as reasonably practicable, (ALARP), saving the client a great deal of time and money whilst providing a safe working environment for construction activities.

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