White Ribbon

BGS own and operate a small 9 m survey catamaran called the White Ribbon. Its name refers to the data gap within the coastal zone where offshore data collection has been difficult. The vessel is being used to fill this gap, as its small size and draft allow survey operations to be undertaken in very shallow water depths. The boat has also been used in various lochs and lakes, with the aim to increase our understanding of what lies beneath the waves.

Equipment

Figure 1: mounting of EM3002D sonar heads using moon pool in hull of the <em>White Ribbon</em>.

The White Ribbon is equipped with a Kongsberg EM3002 multibeam echosounder. This uses a fan or swathe of narrow acoustic beams to completely cover the sea floor, to enable very accurate seafloor mapping. We can hull mount a variety of seismic systems such as the Innomar SES-2000, Knudsen, Edgetech 3100 216S and tow an applied acoustics surface tow boomer. The boat can also be used to collect drop camera/video and sampling (light weight van veen, small gravity corer).

The Kongsberg EM3002D is a dual head, multibeam echosounder system (Figure 1). The EM3002D is capable of producing up to 508 dynamically focussed beams, which produce a swathe width of up to 200° operable in water depths of up to 150 m. The system operates in the 300 kHz frequency range, which ensures narrow beams and a small acoustic footprint. The EM3002D uses a powerful processing unit that applies sophisticated algorithms for beam forming, beam stabilisation and bottom detection.

Positioning

Figure 2: RTK radio repeater using Ford Ranger for Kyle Rhea area.

Vessel position is obtained from a Trimble SPS461 receiver with precise RTK-DGPS corrections provided via the Trimble VRSnow network. These corrections are obtained over any available global system for mobile communication (GSM) network via a simple Nokia mobile phone, which is transmitted to the SPS461 using Bluetooth. When the mobile phone signal is problematic a RTK radio repeater is set up in a suitable location to ensure no drop outs (Figure 2).

Vessel motion/attitude values for pitch, roll and heave are obtained from a Kongsberg-Seatex 200 motion reference unit (MRU). This is an inertial navigation system that can provide accurate real time attitude, heading, heave, position and velocity data. This device is housed in a sea bottle directly above the sonar heads and is used as the common reference point (CRP) for all sensors.

The RTK position solution allows a sufficiently accurate resolution of vessel height relative to the ellipsoid to allow tidal reduction, removing the need to apply tide gauge data. The United Kingdom Hydrographic Office's vertical offshore reference frame (VORF) model is used to define the relationship between the obtained reference ellipsoid height and chart datum. A tidal correction is then applied in specialised post processing software (CARIS). A Valeport TideMaster tide gauge is always installed at a suitable location to allow us to validate the VORF model in the survey area and to also act as a back up.

Data collection

The EM3002D has a sound velocity sensor mounted at the heads providing a real time indication of sound velocity at the transducer face. Sound velocity profiles (SVPs) are then collected using a ValePort mini sound velocity probe. SVPs are taken regularly throughout the day; the interval is primarily determined by the conditions experienced within the survey area.

Information from the EM3002D processing unit, Trimble and Seapath are sent to an operator PC running the Kongsberg seafloor information system (SIS) acquisition system. The hydrographic surveyor can monitor data quality and adjust settings accordingly within SIS. Sound velocity corrections are applied in real time. All data is logged within a Kongsberg .ALL raw file format ready for subsequent data processing.

A range of bathymetric and backscatter outputs are then produced to meet the clients'/marine geoscientists' needs. The software we primarily use for this is CARIS, ESRI ArcGIS and QPS Fledermaus.

Gallery

Contact

Contact Rhys Cooper or BGS Enquiries for further information.