Marine GeoPhysical Survey

EXECUTIVE SUMMARY

The Tuticorin Port Trust has entrusted the National Institute of Ocean Technology with a task of carrying out Marine Geophysical Surveys for the proposed Sethusamudram Navigation Channel north and south of the Adams Bridge near Dhanushkodi. They requested NIOT to carry out seabed surveys with subbottom profiling along with bathymetry to assess the nature of the subsurface below the seabed and to choose proper alignment for the proposed 168 Km long Sethusamudram Channel. The alignment for the channel is proposed in such a way that it cuts through and open up the natural barrier, i.e., the Adams Bridge, and make it a navigable channel for higher draft vessels connecting Gulf of Mannar and Palk Straits.

The proposed length of the line channel is approximately 168 Km of which about 5.0 Km falls south of Adams Bridge, about 6.0 Km of the length crosses the barrier to north and falls in the shallower region with 5.0m or less water depths. The alignment length north of Adams Bridge and in to the Palkstraits is approximately 157Km.

After detailed discussions with the Sethusamudram project authorities, it was decided that detailed seabed investigations be carried out in a corridor spanning 500m on both sides of the proposed channel alignment consisting of 8 lines, including the proposed central line, spaced 150m apart. According NIOT mounted operation in the quickest possible time. Most of the surveys are of Adams Bridge. The surveys  were carried out by deploying the department coastal research vessel CRV Sagar Paschimi. The surveys were carried out between  6^th Nov ’04 and 23^rd Nov ’04  and 16^th and 17^th December 2004 and completed a total of 1379 line Km, of survey. For correcting the today effect, in all 3 tidal stations were stations were established at the following places and tide was monitored for 30 days.

1.       Nagapatnam

2.       Pamban Pass

3.       Sethubavachatram

The tide data indicated that it is a complicated tidal regime in the area and is influenced by atleast the three following factors.

1. South bound currents from Northern Bay of Bengal, north of  Adams Bridge,

2. the tidal buildup from south, i.e., from Gulf of Mannar, and

3. the over all tidal effect from the Bay of Bengal from the east.

The net result due to the three different tidal regimes showed a complicated tidal scenario, not anticipated. NIOT carried out several tidal modeling programmes for the area to arrive at the proper model that is suitable for the area. The model that was formulated by NIOT is found to be perfectly matching the results and same was applied for tidal corrections.

For accurate positioning, Leica DGPS with RTCM corrections ‘OMNISTAR’ was used, which gives very accurate position of the vessel. The bathymetry surveys were carried out by ODOM make Echotrack(MK-II). Care was taken for calibrating the echosounder data on day to day basis. The subbottom profiler surveys were carried out by deploying high resolution Sparker & Boomer System. Very good penetration is achieved in spite of the sandy strata. On an average, the penetration obtained was of the order of 40m below the seabed, with 70m penetration at places.

Based on the over all analysis and detailed interpretation of seismic section for the alignment, the following broad conclusions were be arrived at:

In Section E-E4 there is a fairly uniform alternating layer sequence, likely to consist of silty sands, clays and calcarious sands. Quite a few pockmark like features were located in the surveys area which are likely to be gas emission from the deeper surface. This may not have relevance with the present study. In order to test the subsurface strata to the depth of about 3 m, it is recommended that alteast 11 vibro corers are taken in this area. Since thick and high reflecting layers are noticed around 10m below the seabed surface, bore holes have to be drilled based on interpretation of the vibrocore lithology.

In section D-E the seismic records generally indicate uniform alternating layer sequence like the earlier sector. Here also there are pockmark like features. In this sector a total of 11 locations are identified for carrying vibrocore sampling. The details of the locations are given in the body of the main report.

In section C-D the seismic records point to a uniform and alternating sequence of strata likely to be composed of silty sands, clays, calcarious sands etc. Quite a few pockmark like features were also observed. A total of 8 vibrocore locations are identified in this sector and are given in the main body of the report.

In section B-C, like other sectors the sequence of alternating layered strata were inferred which contain pockmark like features. A sedimentary basinal like feature with a width of 350 m and at depth of 24m below the seabed was located between fixes 386 and 391. An erosional unconformity at a depth of 30m is inferred with the fixes 369 and 379. A total of 6 vibrocore locations are recommended in this sector, details of which are given in the main report.

In section A-B several prominent reflective layers are deciphered within a dredge depth of 15.0m. These appear to be composed of silty clay layer, shell or Oyster bed, etc. These prominent reflective layers are sandwitched between thick and what appears to be sandy formations, and the thickness of these, varying between 0.1 to 4.0 m The area at the end of the survey towards Adams Bridge has a basinal fill features, which is seen in almost all the records with the depth of the fill more than 3m and the arerial extent up to several hundred meters. In order to identity the typeof material present a total of 4 vibro corer locations are recommended.

The totals of 40 vibrocore locations have been identified. In addition, based on the lithological aspects of the vibrocores and the interpretation there of, four bore hole are recommended to be drilled in the area to test deeper depths falling within the dredgeable limits. These are also to be tested for their insitu geotechnical parameters. These locations for boreholes to test the insitu geotechnical and lithological nature will be decided after the interpretation of the samples from vibrocores. The survey pertaining to shallower areas to the north and south of the Adams bridge will be carried out shortly and the report prepared to the completion that surveys.

 Top