Contents:

1. Introduction

2, Method

3. Poster

Metadata:
24/06/2016.
Sidescan Survey: Started 1259. Finished 1337.
Weather: CU/CB 4/8. Light Rain.
Wind: F3/4 NW. Seastate: Slight.
Tides for Falmouth:
H0834 4.7m L1500 0.9m H2047 4.9m. UTC

Vessel: MTS Valonia.

Personnel: 9 Scientists, 1 Supervisor, 2 Crew.

Introduction:

On the MTS Valonia, on the 24th June, we went out to a Special Area of Conservation, approximately 1 kilometre East of Rosemullion Head in order to map the seabed and get samples in a hope to create a benthic habitat map of the area. Out to sea, there was little cloud cover and good visibility. We used a dual frequency sidescan sonar towfish with 410 and 100KHz frequencies to provide insight into the nature of the substrate and makeup of the bedforms using a swath range of 75m. Using the sidescan sonar allowed us to identify the most interesting areas on the seafloor where we then used a video feed to get a close up, live view of the different sediments and species of flora and fauna going perpendicularly across the 4 transects. After looking at the seabed, we chose where to take some grabs to get a hands-on look at the sediments and some of the possible benthic species. We then analysed this data and processed it to make a detailed habitat map.

       

Method:

Using the sidescan sonar towfish,we followed four transects parallel to Rosemullion, each two kilometres long and 100 metres apart. This was conducted aboard the MTS Valonia. The sidescan was towed at a constant shallow depth throughout the period of survey to allow for the greatest possible swath range of 75 metres. Upon analysis of the sidescan track output three areas of interest were identified, two consisting of a rocky and algal substrata and the last depicted a sandy substrate. A submersible video mounted on a stabilising board was lowered to just above the seabed; in order to confirm the habitat and identify the benthic flora and fauna. To identify sediment types of the seabed two Van Veen grabs were undertaken. The locations of these grabs are indicated on the map in the poster.

In the lab the following day, the start and finish times of each transect were used in conjunction with the time stamps on the sidescan trace. The sidescan trace was cut accordingly, and the boat turns omitted. The 4 transects were laid out next to each other, as a mosaic which was seen to show a distorted representation of the seafloor in the sample area. Following this, boundaries were drawn around significant features on the trace. Points were plotted on the boundaries, and the distance of these points from the towfish line was noted and using the trace scale in conjunction with the equations for slant range corrections, was converted to real horizontal distance.

A trackplot was created using the computer programme Surfer. The timestamp data was transferred to a map, where the 4 transects were displayed along with the times that each point was reached. The horizontal distances to each boundary point were then applied to this trackplot, and the points were joined up. The map was then colour coded depending on the sea floor terrain.

Back to Top