High Resolution Marine Magnetic Survey of Shallow Water (original) (raw)
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High Resolution Marine Magnetic Survey of Shallow Water Littoral Area
Sensors, 2007
The purpose of this paper is to present a system developed for detection and accurate mapping of ferro-metallic objects buried below the seabed in shallow waters. The system comprises a precise magnetic gradiometer and navigation subsystem, both installed on a non-magnetic catamaran towed by a low-magnetic interfering boat. In addition we present the results of a marine survey of a near-shore area in the vicinity of Atlit, a town situated on the Mediterranean coast of Israel, about 15 km south of Haifa. The primary purpose of the survey was to search for a Harvard airplane that crashed into the sea in 1960. A magnetic map of the survey area (3.5 km 2 on a 0.5 m grid) was created revealing the anomalies at sub-meter accuracy. For each investigated target location a corresponding ferro-metallic item was dug out, one of which turned to be very similar to a part of the crashed airplane. The accuracy of location was confirmed by matching the position of the actual dug artifacts with the magnetic map within a range of ± 1 m, in a water depth of 9 m.
Mapping marine ferrous targets using real-time 3-D analytic signal data
SEG
The analytic signal is a well-known magnetic data form that provides a high frequency, symmetric response that is centered over source bodies regardless of the effects of permanent (remnant) magnetization or the inclination of the ambient field. This, and its immunity to the effects of diurnal variation make it an ideal data form when searching for ferrous objects such as pipelines, unexploded ordnance (UXO), mines, or archaeological remains. A recent marine magnetic site clearance survey conducted by the U.S. Naval Underwater Warfare Center (NUWC), Keyport division, is presented in which SeaQuest, a new 3- axis marine gradiometer was employed. Using highly accurate total-field Overhauser sensors, this gradiometer system simultaneously measures all three components of the total-field magnetic gradient vector, thus allowing the calculation of the 3-D analytic signal in real time. The presented results highlight the advantages of interpreting measured analytic signal data when attempting to quickly locate the position and depth of ferro-metallic objects. The results also demonstrate the gradiometer system’s sharp resolution of small ferro-metallic signatures, particularly in the presence of strong geologically-sourced anomalies.
Ferromagnetic Underwater Target Detection Based on a Magnetic Anomaly Map
Geomatics and Environmental Engineering
A magnetic anomaly map of an underwater area indicates the places where the distortion of a magnetic field has occurred. Through the interpretation procedures, a hydrographer can easily indicate the places where the ferromagnetic objects are, then calculate the level of each distortion – by the value of total anomaly – and initially, based on their own knowledge, try to classify the sources of distortion. Objects that induce micro anomaly changes (>30 nT) – like industrial infrastructure, such as pipelines and cables; to unintendingly located targets with ferromagnetic characteristics: wrecks (vessels, planes, cars), military mines, UXO, lost anchors and chains. Interpretation of such a map with the attempt to identify the source of magnetic field distortion, requires a specific knowledge as well as experience.In this article the author presents the research results of dimensioning and location of potential ferromagnetic underwater objects based on a magnetic anomaly map. For fur...
Developing magnetometer techniques to identify submerged archaeological sites
Camidge, K., P. Holt, C. Johns, L. Randall & A. Schmidt 2010. Developing magnetometer techniques to identify submerged archaeological sites (5671 DT). Historic Environment Service, Environment and Heritage, Cornwall County Council Report No: 2010R012. , 2010
Marine magnetic surveying has become a standard technique for mapping the location of ferrous material on the seabed. Existing guidance documents are concerned principally with data collection methods. The aim of the project was to acquire a better understanding of magnetic data and thus develop our ability to interpret these data with increased confidence.
Brazilian Journal of Geophysics
The detection of buried clandestine objects challenges forensic and archeologic search group teams on varying terrains, and variable scales of research. Therefore, the study of controlled buried objects is useful for trainings in geophysical acquisition and processing. In this study, we applied ground survey data for testing the magnetic method at controlled geophysical sites for the location of ordinary objects and firearms. We used data filtering techniques in order to facilitate the location of magnetic targets. Also, we experienced the 3D inversion of analytic signal of the vertically integrated magnetic field (ASVI), for the location of targets in depth. As a result, the study determined the location of four magnetic targets, and a three-dimensional view was constructed from the estimated magnetic susceptibility. We concluded that modeling transformed magnetic data is an affordable technique for application in near-surface investigations. Also, this experiment exemplifies the r...
Application of the Magnetic Method to Explore for Shipwrecks on the South African Coast
Many ships that foundered on the South African coast have yet to be discovered. In most cases of previously discovered wrecks, incorrect published coordinates necessitate relocation of wrecks to facilitate recreational or salvage activities. Some of these wrecks present beautiful dive sites and offer interesting discoveries ranging from rum bottles to perfectly preserved ammunition dating back to before the Anglo-Zulu war. Other wrecks offer incredible stories that enrich our South African cultural heritage. And some offer rich treasure. Magnetic surveys effectively detect shipwrecks but require proper design. A standard airborne survey might suffice for detecting an old steam ship but older wooden wrecks, such as the Britannia (1826), require hi-res airborne surveys or better. Boat-borne surveys are used for searching for wrecks in shallow waters and may be complimented by cheaply available side-scan sonar equipment. In areas where hazards prevent boat access a small drone equipped with a fluxgate magnetometer offers good value. Where storms have washed up wrecks onto the beach a ground-magnetic survey offers potential success as well as some other attractions. Such is the case with the current search for the Haarlem, a Dutch ship that foundered in Table Bay in 1647, an event that was directly responsible for Jan van Riebeeck's relocation to the Cape some years later.
Over 400,000 shells of different calibers were dumped in Burgas Bay after the end of WWI. The operation, under the coordination of the British forces, was completed without the dumping areas to be clearly mentioned and mapped, allowing us today to assume that they can be anywhere in Burgas Bay. Many other shells were also dumped in the sea at the end of WWII. During the first phase of expansion of Burgas harbor, carried out between 2001 and 2006, a great deal of unexploded ammunition, originating from both WWI and WWII, was discovered and recovered by the dredging operations. Prior to the start of the second expansion phase, the Burgas port authorities ordered a detailed geophysical search to be carried out and cover a 90,000 sq.m surface where dredging activities are planned, aiming to identify sites where items such as UXO, wrecks, pipes, anchors and other metal objects lying on the sea floor and/or buried in shallow sediments could disturb and even endanger the dredging. The high-resolution marine magnetic mapping carried out in August 2016 by a team comprising both Romanian and Bulgarian specialists entirely covered the area of interest. The search was conducted in a totally unfriendly magnetic environment, dominated by huge variations of the total geomagnetic field due to the numerous outcropping or shallow intrusions of Paleozoic and Late Cretaceous ages, to lens of “black sands” with high contents of titanomagnetite minerals, resulted from the physical decomposition of the magmatites and not least to the large pile of steel pipeline sections stored on the neighboring quay. Still, the careful analysis of the magnetic analytic signal as well as the individual analysis of each marine magnetic line were able to highlight the presence of over 75 potential target areas where the presence of UXO-type items, possibly dumped or lost ferrous objects, accumulations of ammunition, etc. on the seabed or shallowly buried in sediments was highly likely. The indexed target sites became subsequently subject for scuba diving inspection which confirmed the existence of ferrous objects within all selected sites and brought to surface over 2,000 artillery shells and other items.
Study of marine magnetic field
2007
Measurement of the Earth’s magnetic field in the marine areas (marine magnetic field) is one of the most widely used marine geophysical techniques. Data about the variation of the marine magnetic field has various applications. The conventional uses are to determine depth of burial and trend of magnetized basement rocks, which in turn yields overburden thickness, basin demarcation and information about the underlying structures. Marine magnetic data are used as one of the tool to differentiate areas underlain by continental and oceanic crusts. Other usages are in the fields of marine archaeological investigations or search of sunken ferrous objects (such as ships). However, the most important use of marine magnetic data is to address questions pertaining to geodynamics.
Small, low cost Autonomous underwater vehicles (AUVs) provide ideal platforms for shallow water survey, as they are capable of unmanned navigation and can be programmed to acquire data at constant depth, or constant altitude above the seabed. AUVs can be deployed under most sea states and are unaffected by vessel motions that often degrade sonar and magnetometer data quality. The integration of sonar and magnetometer sensors on AUV’s is challenging, however, due to limited payload and strong magnetic fields produced by the vehicle motor. In this study, a Marine Magnetics Explorer Overhauser magnetometer was mated to a portable AUV (OceanServer Iver2) creating the first practical AUV- deployed magnetic survey system. To eliminate magnetic interference from the AUV, the magnetometer was tethered to the AUV with a 5 m tow cable, as determined by static and dynamic instrument testing. The results of the magnetic tests are presented, along with field data from a shallow water test area in Lake Ontario near Toronto, Canada. AUV-acquired magnetic survey data were compared directly with a conventional boat-towed magnetic survey of the same area. The AUV magnetic data were of superior quality despite being collected in rough weather conditions that would have made conventional survey impossible. The resulting high-resolution total magnetic intensity and analytic signal maps clearly identify several buried and surface ferrometallic targets that were verified in 500-kHz side- scan sonar imaging and visual inspection by divers.