Accurate Marine Magnetic Survey of Shallow Water Littoral Area

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Accurate Marine Magnetic Survey of Shallow Water
Littoral Area
B. Ginzburg 1, E. Weiss 1, T. Ram Cohen 1, H.
Zafrir 1, R. Alimi 1,N. Salomonski 1, J. Sharvit
2 1 RD Integrated System Section, Propulsion
Division, Soreq NRC, Yavne 81800, Israel 2
Israel Antiquities Authority, Marine Archaeology
Unit, P.O. box 272, Or-Akiva 30600, Israel
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Introduction
Integrated magnetic system for locating
ferro-metallic objects masked by sea floor
sediments or buried under the seabed in shallow
water areas.
Shallow water depths 1 10 meters.
Numerous examples of such sites are found in the
field of underwater archeology. Reason - rise in
sea level over the shallow continental shelf
flooding prehistoric settlements and eventually
burying them under sand.
In the case of complex bottom topography there is
a danger the tow fish can be caught by prominent
rocks or debris. Solution floating platform.
One of the primary purposes of the current survey
was to search for the wreck of a T-6 Harvard
airplane that crashed in 1960.

• Integrated system for precise magnetic
  measurements, data acquisition, and
  navigation.
• Target .
• Survey area characterization.
• Survey results.

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Non-magnetic katamaran
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• Careful non-magnetic design
• Choice of proper materials and components
• Compensation of stray fields

Magnetic interference of the platform has been
reduced down to about 50 pT level.
Electronics box
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Tow boat
Towed catamaran
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Target - wreck of a T-6 Harvard airplane that
crashed in 1960.
Engine of radial internal-combustion type - model
Pratt and Whitney R-1340 "Wasp".
Measurement of engine magnetic moments.
Engine magnetism (8.5 9.0 11.0) Am2 - induced
components for 0.3 Oe external field (5.5 2.0
4.5) Am2 remanent components
Estimated anomaly is about 0.2 nT _at_ 25 m (max
depth of survey area) in dipole approximation.
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Survey area

Trapeze shaped - roughly 1.8 km wide and 2 km
long. It is located about 1 km northwest of the
Atlit Crusader fortress promontory off the
Israeli Mediterranean coast at a water depth
ranging from 7 to 25 meters.
Château Pèlerin, also known as Atlit Castle and
Castle Pilgrim
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Magnetic features of the survey area

• The Seabed coastal plain is composed of
  underlying sandstone bedrock with partial sand
  overburden up to 1 meter thickness. Rock magnetic
  properties may feature significant spatial
  changes depending on concentration rates of
  iron-rich paleosol layers.
• Because of the proximity to the shore line, the
  seabed sediments comprise materials swept away
  from the shore in addition to sand overburden.
  These sediments, especially iron-rich "humra
  (cmass7110-6 cgs) and sandstone bedrock
  "Calcareous" ((?mass410-6 cgs) that are exposed
  along the shoreline, cause an extra complication
  of magnetic relief.
• Survey area is located in a zone where thousands
  of years of intense seafaring have left numerous
  ancient relics on the sea floor. Therefore,
  considerable contamination with ferro-metallic
  debris and archeological remains was expected.
• It is likely that shore-based iron structures
  also contributed to magnetic picture of the area.

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Survey area with the actual survey lines.
The survey was carried out along NorthSouth
parallel lines with 10 m spacing between them as
well as a few EastWest tie lines
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Band pass filtered magnetic map of survey area
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Detailed analysis of the survey map revealed 59
target-like anomalies in an area of approximately
3.5 km2. The anomalies were ranked according to
their probability of being anomalies created by
the airplane wreckage.
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