|
Articles
The
Magnetism of New Ground Reef
In a continuation
of our efforts to explore maritime history, the Mel Fisher
Maritime Heritage Society has begun conducting an extensive
archaeological survey effort at New Ground Reef, site of the wreck
of the Henrietta Marie. This work is being conducted with
sponsorship and extensive logistical support from the RPM Nautical
Foundation, an organization founded by MFMHS Director George Robb.
The intention of this work is to locate the remainder of the Henrietta
Marie, and inventory any other sites that might be found on
this remote, poorly investigated reef.
The primary tool
in this survey is the magnetometer. Man-made objects are
distributed on the sea floor by intentional dumping, or accidental
loss (i.e. dropped, snagged, or wrecked.) Many of the objects are
iron. Variations and disturbances in the earth’s natural,
ambient, magnetic field are result from these man-made ferrous
objects, and the magnetometer is specifically designed to measure
such variations.
Wooden ships
often used iron to fasten the hull, as components of the rigging,
in ground tackle, artillery, cargo, tools and many other items. When ships are sunk, these iron components cause variations
in the earth’s natural magnetic field at the position where they
lie. These are easily detectable through the use of a
magnetometer. Though a magnetometer will not find all the various
portions of a shipwreck, it does find a significant,
representative cross-section. As iron objects were distributed
throughout the vessels, they also are distributed across the
debris field of the ship’s wreck site, providing an adequate
sample for determining its location.
New Ground Reef
lies within a zone that straddles the historically important
Florida Straits and Gulf of Mexico. The region is one that has
seen maritime activities from 1500 to the present. It is an area
that lies close to the great “highway” of the sea, the Gulf
Stream current. This eastward flowing stream was, from 1513 on,
the preferred route from the Caribbean basin to Europe, and many
ships are recorded as having met their fate along the reefs that
border the Florida Straits. The New Ground Reef area was also
crossed by ships sailing from the Eastern seaboard of the United
States, Havana, and Key West to the ports of the northern Gulf of
Mexico.
Through sheer
geography, the potential is strong at New Ground for there to be
physical remains of past maritime activities, especially as found
in shipwrecks. When one considers the number of ships that sailed
through the area for nearly 500 years, combined with the tropical
storms and the uncharted reef, the odds are strong that many met
their doom at New Ground. They should be readily detectable via
the proposed sensing methods.
With roughly only
one-quarter of the Henrietta Marie’s remains currently
accounted for, a much deeper understanding of the nature of the
site should be revealed through continued fieldwork.
It is expected that the larger portion of the remainder
lies within the proposed area of study. It is known that the Henrietta
Marie carried eight cast iron cannon, of which two have been
located. Thirty-three tons of iron bars were put aboard the ship
when it left London in 1699, and though many were undoubtedly
traded, only 50 pounds of these bars have been recovered from the
site. It is thought that many more are likely to be somewhere on
the sea floor. Other significant features yet to be discovered are
the major part of the hull structure, an associated ballast pile,
and rigging elements. Many other general hardware items such as
galley wares, tools, and hand-weapons are also likely to still
remain on the site. These targets are all made of iron, or contain
significant iron components, and are good candidates for detection
by magnetometer. To unravel the mystery of how this wreck is
scattered would answer many questions, and not just about the
specific story of the ship, but also the slave trade in general.
New Ground Reef
has been the subject of previous efforts to locate cultural
materials. In 1972, Treasure Salvors, Inc., a Key West treasure
hunting organization conducted magnetometer surveys in the area
with the hope of locating the remains of the 1622 galleon Nuestra
Señora de Atocha. Though the primary target was not found,
this effort did discover Henrietta Marie. The logs of this
survey survive, but do not provide much detail as to the scope of
the work, or exactly where it was done. In 1998, the MFMHS and
RPM, Inc. conducted a magnetometer and sidescan sonar survey in
the area of the Henrietta Marie. This work was successful
in locating a large number of anomalies, but as has been learned
since, lacked the precision to produce irrefutable results.
The current
efforts got underway in June of 2001. So far, they have produced
excellent results. Of the twenty-seven square miles to be
investigated under the permit, 6.5, or approximately 25% of the
area was covered. Many significant anomalies have been recorded,
and some are thought to be related to the Henrietta Marie.
In addition, two 19th century shipwreck sites have been
discovered, along with many anchors and other, unrelated debris.
The design of the project has proven to be excellent at providing
the desired results.
In the field, the
50-foot, aluminum-hulled RPM Nautical R/V Lakota is used to
tow an array of three magnetometers. Two Geometrics model 881
cesium magnetometers are towed, from both the port and starboard
sides, while a larger model 880 is towed between the two. The
magnetometers record straight magnetic data. This data can then be
interpreted as is, or as a gradiometer, utilizing the differences
in readings between magnetometers. A 600 Hz Marine Sonics sidescan
sonar fish is attached to the center magnetometer. The sonar is
set to image either 25 or 50 meters to each side, depending on
water depth, and density of the reef coverage. In some
applications, where it is thought the bottom might be penetrable,
a 300Hz sonar is employed. Each of the towed devices is spaced 5
meters apart. The magnetometers sample the magnetic field at a
rate of 0.3 seconds, while traveling at a speed over ground of
between 3 and 6 knots. The survey vessel will travel in east-west
lanes spaced at 15 meters (approximately 1/100th of a
nautical mile), ensuring that one magnetic sensor passes over
every 5 meters of the survey area. Consistent and accurate lanes
are ensured through the use of Trimble “Hydrpro Navigator”
software, and Lakota’s combination of a bow-thruster
forward, and twin propellers aft. As the proposed survey area
varies in depth from 70 feet to 10 feet, the towed gear is
adjusted accordingly. In many shallow areas specially constructed
floats are attached to the fish to prevent them from colliding
into the bottom. These floats have been developed especially for
use with the magnetometers, and have seen much use in earlier
surveys. They have a very low “noise” factor, adding little
distortion to the data.
The collected
magnetic data is recorded via Geometrics’ “MagLog Lite”
software, which allows the information to be merged with that from
the Global Positioning System. Data is collected to three sets of
CD’s on a daily basis, as well as stored in the computers’
hard drives. The combined magnetic and geographic data provides
information relating to the distribution of shipwrecks, and other
iron debris. The magnetic data is interpreted via “Surfer 7.0”
software, which produces charts showing magnetic intensity
relative to the geographic position, via differential GPS, across
the surveyed areas. The sidescan sonar data is collected and
interpreted through “Sea Scan PC” software to provide
underwater, visual images of the surveyed tracks.
The next phase of
the remote sensing survey will be conducted from April 1 to August
1, 2002. During this period the waters of the Florida Keys are
free of spiny lobster traps. When lobster season is in effect,
each trap is buoyed with Styrofoam floats, making the navigation
of parallel lines, and thus the ability to gather accurate data,
impossible. Unfortunately for this proposal, the New Ground Reef
area is a very popular one for spiny lobster fishing.
The current data
has been assessed for its significance. Recorded “hits” are
looked at for their relationship to other known shipwrecks or
anomalies, and to some degree, magnetic intensity. From the first
phase of the investigation, seven significant areas have been
selected for further investigation. Divers been down on these
spots and found nothing, indicating their sources are buried.
Permission has been granted by the FKNMS for excavation at areas
of these anomalies to determine their origin.
As
this work progresses, many other shipwreck sites will most likely
be discovered, and they will provide a strong database for
continued field research. Most importantly, we are hoping we will
be able to report at sometime in the near future the discovery of
the rest of the Henrietta Marie.
Email
archaeology for more information.
Sign
Our Guestbook
|
