Survey crewmember Sue Milstead scans the screens of the sonar and magnetometer equipment inside the Anomaly.

Scanning Underwater

Contour map of magnetic anomalies pinpointing the position of La Belle shipwreck in Matagorda Bay. This illustration, compiled by State Marine Archeologist Steve Hoyt, was developed by manipulating the original magnetometer data and overlaying the hull of the Belle. As such, it is only an approximation of the relationship between the hull and the anomaly.

The wreck of the Belle was discovered during a magnetometer survey conducted by the Texas Historical Commission. A magnetometer is an electronic device that measures the strength of the earth’s magnetic field. It is towed behind the survey boat and the data are collected on a computer during the survey. Iron and steel objects will cause a distortion of the magnetic field and that distortion is called an anomaly. The magnetic field within the survey area can be mapped to show anomalies as contour lines much as a topographic map on land shows hills and valleys as contour lines. A shipwreck, with many iron objects on board, will create a large anomaly that is easily detected. In this illustration, the contour map of the magnetic anomalies shows positive values in red and negative values in blue.

The approximate position of the Belle’s hull has been placed on the anomaly to show its relationship to the positive and negative magnetic poles generated by the wreck. With this information, researchers can estimate the position of the anomaly source before a diver ever enters the water. Unfortunately, other iron or steel objects generate anomalies as well. That is why the vast number of anomalies recorded during a survey are actually from discarded trash (washing machines, outboard motors, car engines, 50-gallon drums, cable, dredge pipe, etc.). A diver must always determine what is actually causing each anomaly.

Example of data collected by Chirp sonar, a method used to detect buried objects underwater. Although not used in the Belle project, it has been useful in other shipwreck surveys.
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