Static vs Dynamic Acquisition

Posted 24 August 2020

When considering an asset inspection survey campaign, one of the most important factors is whether the site lends itself better for static or dynamic acquisition.

What is static acquisition?

To acquire the data from a static location, we mount the mechanical scanning sonar (MSS) on a tripod or suspend the system from a crane. For many, the most familiar context for this method of acquisition is Terrestrial Laser Scanner (TLS) surveys. 

For static acquisition subsea, the MSS is deployed in the same fashion, only underwater. The location of the sensor is then surveyed in using land surveying techniques such as total station dimensional control. The acquired data is typically in a local coordinate system, with each scan aligned only with the sensor's own frame of reference. 

As a result, these types of datasets are assigned a real-world location using a combination of traditional land surveying and LiDAR-style registration techniques.

What is dynamic acquisition?

Dynamic acquisition is where the MSS data is acquired from a moving object, i.e, a vessel. The Blueview data is typically obtained whilst the vessel sails parallel to the target, with 2-4 passes being optimal to achieve a high-resolution dataset. Using this technique, hit counts at a 5-10m range are typically between 2000-4000 per square metre. 

Positioning is governed by an Inertial Navigation System (INS), which combines GNSS and Motion data to assign a real-world co-ordinate to each sounding at the time of acquisition. Positioning can later be improved in post processing if necessary.  

Dynamic acquisition is only made possible by the type of MSS system owned by SEP Hydrographic; the vast majority of MSS systems only lend themselves to static acquisition techniques. Whilst conventional Multibeam systems have been used in this fashion for many years, are generally intended for resolving small targets at greater depths. Their lower frequency may inhibit damage detection at closer ranges.

What are the benefits of each acquisition method?

The benefit of acquiring the data from a vessel is primarily the efficiencies gained in comparison to the static acquisition technique. For straightforward surveys of rivers, inland waterways, port and harbour assets, the dynamic acquisition method is typically the most effective, allowing an entire dock system to be surveyed in a matter of hours.  However, for areas that are inaccessible by boat, crane or tripod deployment is preferable.

Another benefit of the vessel-based acquisition method is that it is possible to acquire other datasets simultaneously, for example, multibeam echosounder, laser-scan and photogrammetry data. These datasets can then be combined to give a full picture of the site. 

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Related Pages: Pulsar Static Acquisition Asset Inspection Survey