1st IMEKO TC-4 International Workshop on Metrology for Geotechnics Benevento, Italy, March 17-18, 2016 Geophysical and geotechnical surveys for submarine cables installations: main applications and methods Giovanni de Alteriis 12 , Lucio Amato 2 , Giuseppe Guadagno 2 1 IAMC-CNR, Naples 2 Tecno In Geosolutions, Naples Abstract – The industry of submarine cables, both for power connection and for telecommunication (TLC) has experienced growing development in recent years and these assets have become strategic. We describe here the main methods and procedures that are followed during geophysical and geological investigations (site surveys) preparatory to the installation of submarine cables. I. INTRODUCTION The industry of sub-marine cables offers to geologists the opportunity of exploring very long corridors of the seafloor across a wide range of different, sometimes challenging, geo-morphological environments and settings. In Italy, for instance, several power and TLC cable connections crossing both continental shelves, slopes and deep basins in the Adriatic and Tyrrhenian seas have been recently designed (Fig.1). Fig. 1. SACOI (Sardegna-Corsica-Italia) powerline cable connection operated by Terna SpA, Italy. Geophysical and geological investigations are critical for the economy of such projects for several reasons: a) the optimal cable routing is selected in order to minimize the interference with geo-morphological constraints, geo- hazards and human activities; b) a detailed under- standing of the sub-seafloor stratigraphy and geo- technical properties is necessary in order to choose the best burial technology during the laying of the cable and reduce operational risks; c) the geo- morphological information, implemented with oceanographic and biological data are essential for habitat definition and thus for environmental analysis. Generally, the entire survey work includes a first, pre-engineering (mostly geophysical) survey covering a relatively wide corridor followed by a more detailed survey along first appraisal cable routes. This second pre-lay survey includes geophysics, geotechnical samplings and further visual and acoustical inspections through a Remotely Operated Vehicle (ROV). II. HYDROGRAPHIC AND GEOPHYSICAL SURVEYS Generally pre-lay surveys are carried out along already established routes and include swath bathymetry with Multibeam Echosounders (MBES), Side-Scan-Sonar (SSS), high-resolution seismics, sub-bottom profiling and magnetometry. Physical measurements of the water column and currentometry are usually carried out through Conductivity/Temperature/Depth (CTD) probes and (Acoustic Doppler Current Profiler (ADCP) systems. Positioning of each sensor is ensured by Differential Global Positioning System (DGPS) with Real Time Kinematic (RTK) correction in near- shore areas and accurate metrology of the vessel offsets. Hull-mounted transducers (e.g. MBES) provide a higher accuracy (in the order of dm in shallow water) with respect to towed sensors (e.g. SSS tow-fishes and magnetometers) which are positioned through Ultra Short Base Line (USBL) subsea methods. In shelf areas (< 200 m) swath bathymetry is acquired through medium-high frequency (200 - 400 kHz) MBES whose resolution is typically depth- dependent due to the geometrical spreading of the ISBN: 978-92-990075-0-1 322