Sea Technology

JUL 2017

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www.sea-technology.com July 2017 / st 15 With the increasing en- ergy demand and the need for modern fiber-optic com- munications, Lestcon Engen- haria, a Brazilian company specializing in submarine construction, was assigned to build and trench a new cable between the island and the mainland. The proposed route position list (RPL) com- prises a 4.7-km-long, 300-m- wide corridor with a maxi- mum water depth of 35 m. The objective of the sur- vey was to map the seabed in order to define the detailed route of the cable, avoiding rock outcrops and subcrops (presence of rock within the burial target depth), debris and the existing cables in the corridor. Survey Methodology A Kongsberg GeoAcous- tics 250-kHz interferometer was employed to acquire bathymetric data and de- tect seabed texture changes through backscatter analysis. Backscatter was also used to correct OOS positions mapped with the side scan sonar (SSS), once a pole-mounted inter- ferometer had better position accuracy compared to SSS layback. SSS data were acquired with an EdgeTech 4125 sonar operating with 400 and 900 kHz at 75-m range. A single-pass multifrequency (MF) high-resolution seis- mic signal was obtained by simultaneously operating a 10- to 20-kHz chirp, 2- to 8-kHz chirp and a 75-J boomer system. A 75-J sparker was acquired only in the centerline along the proposed RPL. The dual-frequency chirp sub-bot- tom profiler is manufactured by Oy Meridata Finland Ltd., and the boomer, sparker and hydrophone are manufactured by SIG. The post-lay survey was conducted with a Kongs- berg GeoPulse sub-bottom profiler with a central frequency of 3.5 kHz and a Teledyne Odom Hydrographic M1 multi- beam system. The seismic systems were integrated using Meridata MDCS data acquisition software, which relies on intelli- gent trigger management employing advanced temporal and spectral multiplexing techniques to enable single-pass operations using multiple sound sources at the same time, without interfering crosstalk between the different acoustic subsystems. All culture, vector and processed data were loaded into IHS Kingdom Software for interpretation. Seismic data were processed with the Seismic Unix package. Results The interferometric bathymetry and side scan data have covered 100 percent of the survey corridor, allowing not only mapping of the seabed morphology with a digital eleva- tion model (DEM), but also showing route obstructions and providing a reliable seabed composition assess- ment through the backscat- ter intensity values. Seabed composition could be inferred by the comparison of sonograph- ic reflection patterns with five grab samples. In some cases, because of sediment similarities, the contact between different types of seabed was not well de- fined either by interferome- ter backscatter or side scan acoustic images. These dif- ferences must be consid- ered and charted for cable installation because of the methods of cable burial (jetting and trenching). The SSS data were also used to identify debris, such as a disposal area and an 8-m metal pipe, and ar- eas where seabed scars are evident. These scars indi- cate trawling fishing gear, which must be considered when planning the cable route. The sonograms also made it possible to map previously laid cables at the route corridor and provided an idea of the present conditions of these cables. The four previously laid cables, three OOS and one IS, were placed on the seabed without any burial or seabed assessment and were found gathered by one side of the route corridor, commonly pass- ing above rock outcrops, which very likely is the source of some of the cable faults throughout the years. Also, some pieces of cable could be linked directly to seabed scars, indicating that the effects of fishing on cable must be con- sidered. The general recommendation is to perform clearance of OOS cables in burial areas prior to the cable lay operation. All of the OOS cables inside the burial sections should be located and cut before the plowing and trenching opera- tions. However, the difficulty in this case is that due to the lack of proper planning and installation of the previous cables, one cannot tell which cable is the IS with only this survey approach. The key benefits of performing a multifrequency seismic survey for cable installation is that sub-bottom geological features can be defined in different scales. As a common practice in the industry, only one seismic system is required for route surveys, most commonly utilizing one operating system with a lower frequency range able to provide pen- etration in sandy sediments. This is normally an impulsive system like a boomer or sparker, which has a frequency spectrum initiating below 2 kHz with lower levels of resolu- tion (typically around 30 to 50 centimeters). For a cable burial of 1 to 1.5 m, it is important to recog- Slight distinction in soil composition detected by the high-frequen- cy chirp.

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