Archaeological Interpretation of Marine Magnetic Data

Abstract

Interpreting remote sensing data is one of the most important tasks of archaeologists working in submerged environments. Researchers rely on remote-sensing technologies to aid their search for historic shipwrecks of interest. Magnetometers are essential for detection of buried shipwrecks. The main goal of magnetic interpretation has been to distinguish shipwrecks from debris, usually resulting in an archaeological assessment of each anomaly concerning its potential for historic significance. The past two decades have seen improvement in archaeologists' abilities to detect shipwreck anomalies. This article provides a basic, nonmathematical summary of magnetism relevant to archaeological interpretation and the evolving perceptions of shipwreck anomalies. The basis for assessing magnetic anomaly significance must be firmly rooted in empiricism in order to improve the objectivity of data interpretation.

Marine Archaeological Survey for the Webster to Seadrift Pipeline Project in Calhoun and Jackson Counties, Texas

“…One unique magnetic characteristic of all known shipwrecks in the sample presented has to do with the magnetic orientation of the anomaly; on examination, it can be recognized that the negative component of a dipolar anomaly unfailingly resides to the geographic north. Additionally, it is recognized that the magnetic deviation of the graphically represented signature did not vary greater than 26° from magnetic north (Gearhart 2011). Thus, a dipolar anomaly with a positive gamma deflection to the north is not consistent with known shipwreck sites and therefore should not be considered a potential shipwreck.…”

“…The only way to know the source of every magnetic anomaly or sidescan image is to have a complete examination either by an archaeological diver or remotely operated vehicle. "Hands-on inspection of every buried anomaly source may not be an economic possibility, so researchers must trust their interpretive abilities" (Gearhart 2011). In the context of the present research, the environmental and historic considerations will be one of the factors considered while interpreting for potential significance of the sources of magnetic anomalies.…”

“…The magnetic anomaly for this site had a total magnetic deviation of 191 gammas made up of a positive and negative component and could be detected over an area of 1,580 square meters (0.4 acres) at a 5-gamma interval. The site, when examined by divers, measured roughly 7 by 16 meters (23 by 52 feet) and is thought to be the remains of a nineteenth century sailing vessel (Gearhart 2011).…”

“…For the present investigation, in the shallow bays of Texas where there has been considerable development and use, utilizing the abovementioned methods to filter anomalies to determine potential significance is considered prudent, as every anomaly is not a shipwreck. The main filter employed is the model developed by Gearhart (2011). Any anomaly that contains a positive magnetic deflection to magnetic north, in an overall dipole representation, was not considered potentially significant and thus removed from consideration of potential significance.…”