USE OF DIFFERENT OBSERVATION TECHNIQUES IN HYDROGRAPHIC SURVEYING

Abstract

The basic objective of the present investigation is to layout the construction pipelines on the seabed. These pipelines and cables require precise underwater surveys. The instmments used to give the information about the seabed have been reviewed. These include GPS for horizontal coordinates, Echo-sounder for depth measurements, Side Scan Sonar for seabed object detection, Sub Bottom Profiler to check the seabed geological layer, and Magnetometer to detect any buried object.

Once a suitable line has been selected, geophysical surveys have to be conducted to discover the nature of underlying strata and to determine the suitability of the seabed sediment for burying the pipeline. Another advantage of such a survey is that, upon completion, secure holding ground can be selected for anchoring the lay-barge during construction.

After laying down the pipeline, the Side Scan Sonar has been used again to check the pipeline and determine whether it is still buried below the seabed or uncovered and as built survey has been made for comparison between . the proposed line and the actual line.

GPS kinematic mode of observation is extensively applied for position fixing throughout the current thesis. Hence, for future studies, for carrying out the necessary surveying works for updating or producing charts, the GPS differential surveying technique is recommended to be used, and possibly operated with single frequency receivers, without any practical need of using the much expensive dual frequency receivers. This might be done by maintaining only one fixed reference receiver, without any . practical need for using two or more reference receivers. The differential GPS technique should thus replace the traditional techniques such as Micro-fix, Trisponder and Syledis (offshore radio positioning system). The \ Echo sounder digital output feature ensures precise location of each depth \ measurement, passing the data on to the acquisition I recording system being already interfaced with the radio, navigation system. Seabed investigation using Side Scan Sonar (SSS) and Sub Bottom Profile (SBP), should be performed in good weather condition, wind speed should be less than 15 knot. When a magnetic survey is carried out to discover any buried targets the vessel speed should be less than 7 knot