New Techniques for a New Bridge
Following Nicholson’s recent land and overwater investigation for the proposed new crossing of the Forth at Kincardine, the Land and Marine Survey Group were awarded a separate contract to carry out a series of hydrographic, geophysical and oceanographic surveys for the same project.
Due to the generally shallow water over the site, the Group’s own launch The Surveyor was utilised. The Surveyor is a 6.5 metre Shetland Alaska which is racked out to allow rapid installation of survey equipment and is powered by twin 40 hp outboard engines, essential when working in areas of high tidal flows. The vessel underwent an extensive re-fit in early 2000 and now meets the requirements for MCA Category 6 allowing work up to 3 miles from land or protected waters.
The contract involved obtaining bed levels across a series of lines and within two corridors, mapping the distribution of surface materials and sub-surface geology along with current velocity and direction data and in-situ salinity measurements. Though somewhat standard requests for the Survey Group, the contract provided the opportunity to utilise new techniques and equipment.
A new seismic reflection source known as the Squid was utilised to increase penetration through the sediments and rock. The Squid is a sparker source which generates a seismic signal by passing a short duration high voltage spark across between its tips. Traditionally sparker systems are up to 3 metres in length and weigh 100 kilograms, the Squid is 0.5 metres long and weighs only 2 kilograms, a distinct improvement in design, particularly for small boat work. In fact, the Survey Group field tested the prototype in a survey off Great Yarmouth in 1998.
The current measurement data was collected over periods of both spring and neap tides utilising an Acoustic Doppler Current Profiler (ADCP). The relatively new instrument was selected for the survey as it provided quick, accurate readings of current velocity at a whole series of depths along lines defined by the Client. The ADCP software included a bottom track feature which allowed measurements to be taken whilst the vessel was underway. The ADCP obtains current velocity by measuring the Doppler shift (the 'mee-maw' often heard from passing sirens) between adjacent packets of water.
The survey specification required use of fixed station current monitoring techniques which would have taken some 24 days on site over a four week period. Using this new efficient methodology, this portion of the survey was completed in eight days on site and we provided the client with ten times more data than would have been possible with fixed station current monitoring. |
