GNGTS 2015 - Atti del 34° Convegno Nazionale

We utilized the EdgeTech DF-1000 / DCI Digital Side Scan Sonar Sistem (100 and 500 kHz) controlled by CODA DA-50 (Coda Technologies). Data were acquired with the tow fish at the depth of 4 m with a cruise velocity of 4 knots. The investigation involved the generation of six sonograms NW-SE oriented (i.e, parallel to the TMT front) with a length of 500 m and a lateral range of 75 m. The tie line interval was about 50 m that permits a full coverage with overlapping of the study area. The acquired data were processed through Coda Octopus Geokit Mosaics software and graphically represented in a mosaic in order to locate objects on the sea floor and to define the morphological and sedimentological characteristics of the seabed that confim/complete the information coming from the bathymetric multibeam survey. In addition to SSS data acquisition, a magnetometer survey has been carried out to obtain a magnetic map in front of TMT. The survey was conducted from “Anthea” boat with NW-SE tie lines spaced 25 m interval. Magnetic data were acquired using a Marine Magnetics SeaSpy magnetometer towed at a distance of 20 m behind the boat. The sensor elevation was recorded with each magnetic measurement to allow for later correction of the water-depth related changes in magnetic intensity. The magnetometer was cycled at 4 Hz providing about one sample per metre. The magnetometer survey was preparatory to an UXO (Unexploded Ordinance) survey performed by certified personnel. Sub bottom profile. A dense grid of sub-bottom-profile (SBP) data was recorded to identify recent sediments up to the top of rock basement (Eocene Flysch). A total of about 11 km distributed along 30 lines (11 NW-SE direction and 19 NE-SW direction) was acquired by a Sub Bottom Profiler Edgetech SB-216S (tow fish) and Edgetech 3200-XS topside (control/ acquisition unit) with a cruise velocity of 3.5 knots. After several tests we acquire data with the following parameters: up-sweep 2-10 kHz, 20 ms length with a source rate of 2Hz, tow fish depth 1.5 m. SBP data penetration allows to obtain information of sediments/Flysch interface which in the studied area ranges from 30 to 50 m below the sea level. The two-way-time data were converted in depth below sea level using the velocity of 1530 m/s for the water column and 1610 m/s as an average for the sediments from sea-bottom to the Flysch top. This velocity was obtained by underwater refraction (see next paragraph) and by a check-shot measurement performed in a drilled hole. The SBP lines were analyzed more than once to avoid misinterpretation of multiple events and to test the repeatability of the interpretation. The sediments/bedrock interface were detected and correlated with to the borehole data and to tie-line crossings in correspondence of the line intersections. In the SW sector, where the bedrock is deeper, we compared SBP data with single channel reflection data acquired with boomer source in a previous unpublished study. Underwater seismic refraction. From a geotechnical perspective a more important problem is that it is very difficult to determine engineering properties from reflection records. Refraction seismic survey are routinely acquired on land for geotechnical purposes. S-wave data are preferred because are strictly connected with geotechnical parameters but the generation of shear wave at the sea bottom is not a trivial issue. To obtain information about the bedrock nature and alteration, an underwater P-wave seismic refraction was planned. Seismic survey has been carried out along seven profiles with hydrophone cables deployed on the sea-bottom. Acquisition geometry (i.e., trace interval and maximum offset) was defined after a feasibility study to estimate the minimum offset to detect refraction coming from the bedrock. This preliminary analysis was performed by the use of synthetic seismic data computed with different simplified velocity models. Two 24 channels hydrophone cables, 5 m interval, and a mud gun (Bolt DHS 5500, 7.6280 in³) with a pressure of 70 bar were used. Hydrophone cables was deployed by the boat starting from the TMT head along the planned lines. Five to eleven shot points was performed for each line lowering mud gun to the sea bottom. A precise time-break identification was obtained by 66 GNGTS 2015 S essione 3.2