FEDOROV A.S., MIRONYUK S.G., KLESHCHIN S.M.
OOO “PeterGas”, Russia

One of the main marine engineering survey problems is to discover and allocate the dangerous and negative sections of the sea bottom. They are the sections where slope failures occur, sections with gas-saturated and permafrost soils, sections with dangerous ice-hummocks and icebergs influences on the bottom.

At present time in the OOO “PeterGas” a considerable experience of geophysical methods usage while discovering different marine geological hazards on the sea bottom and in the Black Sea, the Baltic Sea, the Barents Sea, and the Kara Sea shorelines.

The geophysical methods complex, which we successfully use while solving different marine civil engineering problems is overviewed in the report.

This complex includes the next methods:
1. Continuous shooting.
2. Multibeam echolocation.
3. Side scan sonar.
4. Magnetometric survey.

Image acquisition (ROV) is planned to enlarge the mentioned geophysical complex, which is intended for visual object identification, which were recognized during the geophysical survey.

Geophysical survey is done in conjunction with other engineering-geological works such as drilling, ocean-bottom sampling, CPT etc. It is appropriate to carry out the geophysical survey first, and after that to carry out the geophysical survey. Round the clock continuous data quality control and data interpreting is an important marine geophysical survey technological peculiarity that is carried out in OOO “PeterGas”. It allows to interpret data and to carry out a preliminary profile and maps graphing.

The complex of potentially dangerous engineering-geological processes was recognized during the marine survey which was carried out on the Barents Sea shelf (Shtockman gas-condensate field and marine pipelines routes):
• Soft ground;
• Rugged relief;
• Submarine landslide processes;
• Gas-saturated soils and zones with gas accumulation;
• The gas kick effect out of the bottom sediments (pock-marks);
• Different faults.
The main part of the profile is composed of soft sediments in the top (silt or fluctuating clay), and that part of the profile is a zone of submarine production complex and ground stratum interaction. Their total thickness is 2-3 meters, sometimes it increases 10 meters on single sections.

The Barents Sea topography within the field area is rather rugged. The undulations of different nature are discovered on the sea bottom. There are well-defined extensive linear objects, which are considered to be of glacial genesis, on the color bitmapped bottom topography models, which are rendered using the multibeam echolocation data.

Additional investigations using sonar with a bathymetry function allowed specifying the bottom geomorphology. The main morphological characteristics of the submarine production complex and pipeline routes building sections were received.

We can see on the digital model image that bottom within the submarine production complex section is uniformly and strongly indented with ploughmarks. The ploughmarks characteristics are the following:
• average depth 3-4 meters;
• average width 60-100 meters;
• the cross-section form is vee;
• all furrows are chaotically directed.

Conditionally, we can mark out ploughmarks on three groups by size: large (with depth 5-7 meters, and width 120-210 meters), medium and small.
In the western section of the Shtockman gas-condensate field a depression with maximum depth 353 meters under-sea level is situated. Depression’s relief is flat, smooth, and practically without furrows.

Such specific morphostructures as pockmarks (of different diameter) are met in the depression. Earlier they’ve never been discovered on the territory of Shtockman gas-condensate field. Their formation is connected with powerful gas kick out of the bottom sediments and the following collapse of sediments. On single sections of the marine pipeline route bottom is covered with pock-marks. Their depth is 0.5-2/6 meters, diameter 0.5-34 meters. Rarely pock-marks are 6 or more meters depth, and 230-300 meters in diameter.

There is a definite regularity in pock-marks location. Lines of small pock-marks were discovered, and also lines were discovered in linear declines (furrows) of the sea bottom.
Examples of combined analysis of bathymetrical, seismoacoustic and side-scan sonar data are shown for one of the sections were pock-marks are widespread.

Our experience shows that magnetometry is an additional method of pock-marks investigation. There were discovered magnetic field anomalies, which are connected with pock-marks. Presumably they are conditioned with heightened electrochemical reactivity in soils, which will demand additional pipeline corrosion protection.

The upper boundary of gas-saturated sediments was recognized with fine precision by OOO “PeterGas” specialists on depths from 25 to 55 meters under sea bottom. Gas saturated sediments can be hazardous for submarine buildings because such sediments may rapidly loose their lift cuttings ability while additional static and dynamic loads are applied, because of partial or complete degassing.

Relief highs mezzo and microforms are also can be factor risks in evaluation of submarine building construction and exploitation conditions. Sections with hummocky surface, with relative excess over bottom which is several meters, with angle of slope near 10°can be unsuitable for extensive basements and templates. Besides, different gravitational processes may occur on the areas with such slope angles. Hazardous zones with unstable bottom sediments were singled out on the territory of Shtockman gas-condensate field. Blacked-out relief contours with slope angle more than 5° are singled out.

Faults can be met rather often on the field territory, and as a rule they have a north-north-west (sometimes meridional) and north-east orientation. In the surface part of the geological profile (~150 meters) they are from 1,5-2 to 4-5 km length, sometimes 8-11 km, and as a rule faults concentrate in two zones 7 km width each. Faults were discovered in area extent according to continuous shooting data and less using the geomorphologic signs, which can be seen on a minute bottom topography map, based on multibeam echolocation data.

We should carry out additional geophysical and seismological research to specify the nature of marine lineaments, to trace the active faults, to determine the faults zone width and the fault surface inclination.
The Kara Sea (Bovanenkovo-Uhta gas-main pipeline route across the Baidarackaya gulf) drilling data and geophysical survey data analysis showed that engineering-geological conditions of the pipeline construction region are complex because of the relict permafrost, gas-saturated sediments and bottom grounded hummock exaration.

Relict permafrost is in degradation phase and it was discovered that on several pipeline construction sections relict permafrost is situated in the depth interval from 10-15 meters to 100-150 meters under sea bottom. Its distribution is dissimilar and it is characterized like insular permafrost. Towards the sea the intermittence and the depth of upper boundary burial increases and thickness of the relict permafrost decreases. Modeling of the pipeline influence on the ground showed that thermal influence will reach the permafrost upper boundary in 5-7 years and will result its melting.

According to seismoacoustic data it was recognized that along the pipeline route the gas-saturated sediments are widely spread under the marine Holocene sediments. Using the seismoacoustic survey data it was discovered that the upper boundary depth of the gas-saturated soils is situated on the depths from 1,5-2 to 10-15 meters under the sea bottom, sometimes it reaches 18-22 meters.

As a rule gas-saturated soils are sands, which lay under the clay stratum on the depths from 1-2 to 10-15 meters.
There was carried out an analysis of bottom microforms along the pipeline route which was based on side scan sonar and bathymetrical data. Investigations showed that the furrows are present at the sea bottom in the surveyed area of different width and shape. The furrows depth in the sand and loam is not more than 1.5-2.2 meters and width is 10-40 meters, length can reach several kilometers. The ice influence can be tracked to 25-30 meters depth.

Analyzing the geophysical methods from the investigating geological hazards point of view we can say the following:
1. The best result in investigating the topography mezzo and microforms can be achieved using the combined analysis of bathymetrical, seismoacoustic and side scan sonar data. And we should mention that multibeam bathymetric sonar is better than the single-beam one.
2. Continuous shooting data and drilling data taken together are the best methods to investigate the lateral soil, rock, profile itself variability, and also in investigations of gas-saturated sediments and permafrost.
3. To achieve the optimal correlation between the resolution and the depth of the survey, the seismoacoustic measurements should carried out using three systems simultaneously with the following frequencies: 250-300 Hz, 1500-2000 Hz, 2000-12000 Hz.
4. The best result in detecting the local objects on the sea bottom showed the integrated system BENTHOS C3D+SPB, which includes the profilograph CHIRP-III and a side scan sonar with C3D bathymetry function. Side scan sonar C3D which is possessed in the towed body allow to get a high-definition sonar image and a bathymetry data in a wide area of survey. Using the side scan sonar C3D allow to determine the topography peculiarities with definition of 0.1-0.2 meters.

OOil and gas of Arctic shelf 2008