Currently, both the off shore survey vessel and the in shore survey vessel are conducting magnetometer surveys.  A magnetometer is a scientific instrument used to measure the strength and/or direction of magnetic fields within its vicinity.  The inshore magnetometer survey is searching for the location of other cables within the region, so we can map where PPC-1 will lie close to, or on top of, existing cables.

For the inshore survey, two existing cables within the survey corridor run approximately parallel to the survey centreline. This is in water depths from 30m to 50m.

The alignment of the magnetometer traverses will be surveyed obliquely to approximately perpendicular to the mapped alignment of the existing cables, to best suit sea conditions on site. The magnetometer traverses will be at intervals of 500m along the route, with one traverse at the position where there is a planned crossing between the existing APNG-2 cable and the main survey centreline.

At the inshore end the two existing cables are approximately 150m apart, and the magnetometer traverses will be extended to cover both cables in each pass. The existing cables diverge from one another towards the 50m depth contour, with a 1km separation as they exit the corridor. The inshore survey equipment used will be a Marine Magnetics SEASPY Magnetometer and a Magnetometer Logging computer.  Along the traverses, the magnetometer will be deployed within 20m of the seabed where it is safe to do so. Clearly this depends on seabed terrain and seawater currents.

The above map illustrates the proposed inshore survey.  The red line represents the proposed PPC-1 cable, and the purples lines represent other existing cables.  The shaded area highlights the proposed cable crossing with APNG-2.

These were the officials that processed the Gelendzhik’s entry into Papua New Guinean (PNG) waters.  There were representatives from immigration, police, customs, shipping, PNG Telikom and PNG NMSA (National Maritime Safety Authority). The Captain dealt with all the officials and the process was straightforward with the usual immigration formalities and personal declarations.

The Gelendzhik is currently surveying in the Coral Sea between Papua New Guinea and Australia. Yesterday the survey vessel came up against gale force 7 SE winds (as measured by the Beaufort Scale) which causes survey problems. These mainly centre on keeping the vessel stable in the high swells. When the vessel is pitching, the MBES transducers fixed underneath the hull can come out of the water and therefore do not provide accurate information.

To combat this a few things can be done: take on ballast to help stabilize the vessel, survey with the swell (in this case a northerly direction) or heave-to and wait for the storm to pass. The Gelendzhik was able to carry on surveying by moving with the swell but then had to transit south to start the next survey line. As the weather has been on our side up until now, we didn’t lose too much time and the forecast for the next few days is looking much more favourable.

More on MBES readings and transducers in a later post.

In this picture the crew are inspecting the water tight connections of the Towfish prior to launch. The yellow device on the right is a spare MAK-1M Towfish.

The towfish is deliberately buoyant so that should the cable break or get snagged and then break, it’s hoped the Towfish would become free and rise to the surface for retrieval.

This is the Side Scan Sonar Towfish (the “Towfish”) currently being used on the Gelendzhik to determine a suitable route for the cable to be installed.  It’s an ‘MAK-1M’ Russian built model.

The Towfish is towed behind the vessel at a slow speed, less than 4 knots but at a speed where the helmsman can maintain steerage in a straight line.   It’s ‘flown’ approximately 30m off the seabed controlled by cable payout, a weighted depressor and speed of vessel.

It records 300m of seabed data out to each side, and to a depth of about 2000m using its 100KHz transducers.  It is especially useful for detecting obstacles and features on the seabed. The dimensions of the obstacle can be measured from the record and height determined by measuring its shadow length.

In addition to the side scan transducers, the Towfish also has a sub-bottom profiler transducer which ‘pings’ the seabed to provide information on the type of seafloor, whether a hard or soft layer may be present. This is used to obtain possible depth of burial information. There’s also a ‘Transponder’ fitted, which is used to accurately position the Towfish.

The data is interfaced to the ship’s main navigation computer and a real time display of the Towfish’s location is continually displayed.  An Altimeter provides the operator with height ‘off-bottom’ information continually.