Proceedings of the Open source GIS - GRASS users conference 2002 - Trento, Italy, 11-13 September 2002

Sea-bed classification and sea-bottom mappingwith GRASS in the Natural Marine Reserve of Miramare

(Gulf of Trieste, Italy).

Bonacito Clizia*, Ciriaco Saul*, Costantini Marco** & Spoto Maurizio*

*Miramare Natural Marine Reserve, viale miramare 349 34014 Grignano Triestetel +39 040 224396, fax +39 040 224636 e-mail info@riservamarinamiramare.it

** University of Trieste Department of Biology

1 Introduction

The use of underwater sound to measure depth or locate fish and other targets has beenreferred to the field of hydroacoustic, whose techniques range to collect targets number orbiomass of targets in an area, density and distribution of biomass, acoustic size, and, inrecent years, bottom reflectivity and typing. All of these data comes from the analysis ofecho waves reflected by targets. To obtain echoes from targets sound pulses should begenerated in the water column by an echosounder, a sonar which sends out verticallyfrom the surface to the seabed a beam of sound pulses through a transducer, a devicewhich converts electrical energy into acoustic energy (sound pulses) and vice versa,converting echoes in electrical signals, afterwards digitized and processed by suitablehardware and software.The information about bottom type (bottom sediments e.g. sand, mud,…) and depth isencoded in the echo signal, that can be acquired and stored together with GPS data.Results from echoes, collected in suitable surveys, can be processed with date, time,latitude and longitude and projected on a digital chart.

2 Study area

The WWF-Miramare Marine Reserve is located at about 8 km from Trieste (Italy) and itextends for 1700 m coastline and for 120 hectares at sea; area is divided in a core (30hectares) and in a buffer zone (90 hectares). The maximum depth reaches 18 m. Thegeographic position and the geo-morphological structure of this coastal tract makeMiramare an unique environment that alone represents the peculiarities of the entire Gulfof Trieste.

3 Acoustic device, survey and techniques

Acoustic data were collected with a 200 kHz Biosonics digital echosounder DT6000 splitbeam mounted on a fin, towed alongside an outboard boat. The transducer was connectedto a laptop computer.First of all 2000 echoes were collected from each of the 5 different bottom sedimentspresents in the area (sand, mud, small angular rocks, mixed bottom, seaweed bottomsediments). Bottom reflectivity calibration surveys (n=5) were conducted the 23th ofMarch, 2002 in 5 different locations, chosen locking the map of sediments of theprotected area, developed by GEOS in 1991.Five bottom classification classes, using Fractal dimension method (DT User Manual,Biosonics Inc.), were obtained in order to classify the echoes collected during asubsequent zigzag survey (n=1), which covers the entire protected area, conducted the 8th

Sea-bed classification and sea-bottom mappingwith GRASS in the Natural Marine Reserve of Miramare

(Gulf of Trieste, Italy)

2

of April, 2002. It was formed by 27 perpendicular legs to the protected coastline.The distance between legs is 50 m, through the GRASS analysis grid is a 50 m squaregrid.During all surveys pulse width (0.4 ms), pulse per second (4 pps), depth range (25 m)were constant parameters, sound velocity was calculated by Biosonic Visual Acquisition4.02 software programme, giving the requested salinity and temperature values collectedby a probe.

Acoustic data were processed with Biosonic VBT 4.02 software programme, using fractaldimension techniques (DT User Manual, Biosonics Inc.).The acoustic instrument was calibrated before surveys with an iron standard target sphere,as suggested by the manufacturer.

All collecting computer devices where Windows98 licensed due to the acquisitionsoftware; mapping was conducted initially with a WinNT PC with CYGWIN and GRASSand finally with an Asus LK8400 REDHAT 7.3.Input devices where connected through serial port to a DGPS TRIMBLE in order to havemore precision.

Cartographic basic layer was imported from a DXF CTRN 1:5000 chart through the“v.in.dxf” interactive command .Ascii files produced by DT Biosonics Visual Bottom Analyser were merged and exportedto an ascii file with a “;” delimiterGeographical co-ordinates were in decimal degrees, bottom types as integer and depth inmeters.

Through GRASS “s.in.ascii” ascii file was imported in a site file; then converted in avector file

Route and sample points

Bonacito Clizia* , Ciriaco Saul*, Costantini Marco** & Spoto Maurizio* 3

Finally we rasterized at 50 m resolution.Region was pointed to the raster.

Bottom type

4 Conclusion

This first preliminary map is going to be update through a more fine survey which will beconducted during next year in order to realise a more detailed sea-bed bottom map.Depth data collected will be also analysed to produce a 3d model.