Thursday, 19 May 2016

'INVISIBLE' WRECK.

Between the Agulhas Current and the coast there exists a zone of high current shear where cyclonic vortices can be induced, which may lead to contrary currents near the coast. These effects may be transient as the vortices advect downstream.

The direct effect of the wind (southwesterly) is its stress on the sea surface causing a transfer of momentum from air to sea near the surface such that the sea picks up a speed of 2-3% of the wind speed. This causes a convergence (sea piling up) at the shore and an alteration of the sea slope - currents sympathetic with the direction of the wind.

The dominance which the Agulhas Current imposes on the system south of Port Shepstone is sustained in the sector stretching to Morgan's Bay, except in the embayments at Cape Hermes where there is a local tendency for south going currents to be less frequent. At Cape Hermes there is a tendency (in winter) for a northeastward current close to shore which recirculates into the main southwestward Agulhas Current - see image below.

This implies that wreckage or bodies from Waratah off Poenskop would initially be carried northeastward, then by cyclonic circulation out to sea, rejoining the prevailing Agulhas Current southwestward. It can therefore not be assumed that wreckage or bodies would be washed ashore. Further to this, there is an extensive reef formation (barrier) off Poenskop which would prevent objects being washed ashore.

Note that there is a significant stretch of 'cliffs', rather than beaches, preceding Poenskop from the direction of Cape Hermes. This, combined with the reef system, would have made it very difficult for survivors to clamber safely ashore.

Harlow was coasting within 1 - 1.5 miles from shore, and the large steamer astern was significantly close to shore - observed from the port quarter (0.5 miles from shore).

There is also the possibility that she rolled over and floated for a time, drifting further northeastward until finally taking her complement and secrets to the bottom.

'Sidescan sonar is not, however, a foolproof method of geophysical prospection. Artefactual material may accumulate within crevasses of a reef, where it might be hidden by acoustic shadows or disguised by the otherwise complex nature of the background. Similarly, areas of wreckage can easily be buried periodically by highly mobile sediments such as fine sands and silts. The periodic exposure of several known wrecks within the Bay is well documented and can render wreckage all but invisible to side scan sonar. It is for these reasons that a complementary form of geophysical prospection was implemented during the Mount’s Bay project – the magnetometer survey.'

'The magnetometer is capable of identifying material hidden from view among rocks or buried under sediment, whereas the sidescan sonar can only detect exposed material.' 

'Deriving accurate positions for a target from observed magnetic anomalies is not possible. The position given for an anomaly is the position of the towfish at the peak of the relevant deviation. In effect we are recording the point of closest approach of the magnetometer to the target. The result is that, unlike sidescan sonar targets, magnetometer target positions are only approximations. If the same target is detected on adjacent run lines then a variation of the Hall equation can be used to iterate the most likely position of the target between the parallel search lines. Unfortunately in many instances the target is so small that it is only observable on the nearest survey track. However the method requires that the targets are indeed generated by the same object rather than two adjacent objects. In such instances this method of iteration cannot be used and the only position which is known for the target is the point when the magnetometer is at its closest to the target.'

'Bathymetric, or depth, data were collected to facilitate more accurate target mass prediction of magnetic anomalies. In order to use the Hall Equation, which relies principally upon input of target size and distance, we needed to know the water depth. This data was collected using a Garmin
narrow‐beam echo‐sounder mounted on a steel pole which was secured to the side of the survey and
tied off securely fore and aft to reduce vibrations induced by drag.'

http://www.cismas.org.uk/docs/Mounts_Bay_Survey_Report.pdf






X marks the spot - 3.247 nautical miles from Cape Hermes and 0.5 nautical miles offshore.



Abstract

Waterborne swath acoustic and airborne laser systems are the main methods used to detect and investigate fully submerged shipwreck sites. In the nearshore, waterborne techniques are compromised as search tools as their effective swath is a function of water depth, necessitating very close survey line spacing in shallow water, increasing cost accordingly. Additionally, in turbid coastal waters bathymetric LiDAR is ineffective as it relies on clear non-turbid water. Therefore, the nearshore turbid zone represents a challenging area for archaeologists in the search for fully submerged archaeological sites. In this study, we describe a new methodology to detect the presence of submerged shipwrecks using ocean colour satellite imagery in turbid waters. We demonstrate that wrecks generate Suspended Particulate Matter (SPM) concentration signals that can be detected by high-resolution ocean colour satellite data such as Landsat-8. Surface SPM plumes extend downstream for up to 4 km from wrecks, with measured concentrations ranging between 15 and 95 mg/l. The overall ratio between the plume and background SPM concentrations is about 1.4. During slack tidal phases sediments in suspension settle to create fluffy mud deposits near the seabed. Scour pits developed around wrecks act as sinks where fine-grained suspended material is preferentially deposited at slacks. The scour pits subsequently act as sources for suspended material when the bottom current increases after slacks. SPM plumes develop immediately before maximum ebb or flood current is reached, during maximum current and immediately after. Particulate matter is suspended in sufficient concentrations to be detected in ocean colour data. The ability to detect submerged shipwrecks from satellite remote sensors is of benefit to archaeological scientists and resource managers interesting in locating wrecks and investigating processes driving their evolution.

http://www.sciencedirect.com/science/article/pii/S0305440315003088

Since the launch of the first Landsat satellite in 1972, imaging sensor technology has undergone rapid advancements that have enabled explorers to collect increasingly more useful data. When the technology was in its primitive stages, geologists used the sensors to collect simple data, such as surface features, and used this data to provide clues to a potential mineral deposit beneath the surface. This surface data was also used as a tool in mapping. Now, satellites fitted with “more advanced” sensors use the spectral properties of materials (what wavelengths of materials they absorb/reflect) to identify the materials without having to view them “in person.” This spectral data can be collected by sensors mounted on aircraft and/or satellites, and these sensors use infrared, near infrared, thermal infrared and short-wave technology to collect the data.


The sensors interpret the electromagnetic data in wavelengths that the human eye is unable to distinguish. The differences in absorption and reflection are analyzed and translated into assigned colors that are differentiated for each type of rock and each group of wavelengths. Other wavelengths can identify certain minerals of interest- such as clays and sulfides based on their absorption and reflection qualities. Geologists use data interpreted from satellite images to pick out rock units and seek surface clues such as alteration and other signs of mineralization to subsurface deposits of ore mineralsoil and gas, and groundwater.

The very first sensors used on satellites were problematic, mainly because of their poor spectral resolution, and inadequate spectral coverage. These limitations were rapidly changed in the early 1980’s with the launch of Landsat 4 and 5. These new satellites carried the TM (thematic mapper) scanner. The TM system added coverage in the short-wave infrared and mid-infrared regions of the spectrum. Collecting data from these regions enabled the collection of data that could be used as a tool for identifying alteration mineralogy on the earth’s surface potentially indicative of economic ore deposits. The thematic mapper is still routinely used as an exploration tool, however, since its introduction, other satellites were launched that had higher spectral resolution and could, therefore, provide more accurate data in determining surface mineralogy. The technology in satellite systems has advanced to the point where not only individual mineral species can be mapped, but chemical variations within the molecular structure of the crystal lattice of the mineral can also be detected. The resolution of the sensors on satellites can’t compare to aircraft spectral remote sensors, however, satellite systems have the other advantages compared to aircraft, including the ability to collect more data, from greater areas, without having to fly an aircraft over the land in interest.
With the ability to determine texture and petrology from miles above the Earth’s surface, locating, analyzing, identifying and mapping the structure and composition of the Earth’s surface is now greatly advanced. Using remote sensing enables explorers to collect data without having to visit the actual location, exponentially increasing the amount of data that can be collected in a given time, and enabling the collection of data from areas that would be inaccessible by foot, due to climate, topography, or location. In addition, the advancement of remote sensing, has enabled exploration companies to combine a variety of data sets, obtained from both remote sensors and surface exploration activities, such as historic drilling to pin-point, with a greater amount of accuracy, the location of potentially viable deposits of minerals.
http://investingnews.com/daily/resource-investing/precious-metals-investing/gold-investing/satellite-imagery-and-gold-exploration/

Very tricky business locating a wreck close to shore, reefs, in turbid water. The above technology, however, would make the identification of lead and copper in one specific location possible.

31 36 30.29 S; 29 36 18.51 E

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