The complexity of discovering buried shipwrecks has long-inspired archeologists to seek improved survey methods and data interpretation to increase detection of these archeological sites. Technological developments during the 1960s introduced the use of magnetometers into underwater environments, after successful demonstration in terrestrial settings. Towards this end, early underwater archeologists employed what is now considered more “analog” techniques often requiring great ingenuity in practice, as this work was commenced decades before the use of navigation software packages, positioning systems, and other computer programs. Since that time, technological advancements and increased understanding within the discipline have improved survey methodologies, led to changes in the graphic representation of shipwreck anomalies, and provided increased reliability of survey results and site discovery. This presentation serves as an introduction to the origins of underwater magnetic remote-sensing surveys and as a framework for better understanding session papers by early remote-sensing specialists.

The use of magnetometers in terrestrial archaeology began as early as post-Second World War. It wasn't until the 1970s that these instruments were increasingly adapted for use in underwater (UW) archaeology. The adoption of magnetometers for archaeological survey purposes was facilitated by State and Federal land/water management agencies who required their use in surveys mandated by cultural resource manangement (CRM) laws - Archaeological Resource Protection Act (1974); the Abandoned Shipwreck Act (1987), etc. This paper discusses illustrative historical uses of magnetometers in both riverine and marine settings primarily in Texas and the northern Gulf of Mexico. Texas was a principal location for the development of magnetic survey procedures and applications on high-profile shipwrecks sites such as the 1554 Flota wrecks off South Padre Island. The examples presented here are not as dramatic as those sites but they illustrate similar problems and solutions in the early use of magnetometers in UW archaeology.

As a consequence of professional acknowledgement of the historical importance and archaeological value of submerged cultural resources the role of magnetic remote sensing has increased significantly. Although initially utilized by firms organized for locating the remains of vessels carrying treasure, magnetometers were recognized as highly useful tools by archaeologists that recognized shipwrecks and inundated sites as valuable sources of closed context information about our past. In the decades that followed this recognition archaeologists focused considerable attention on the mechanics of magnetic data collection, data analysis and archaeological site association. That progress has led to the development of reasonably reliable criteria for identifying magnetic signatures that reflect the spectrum of anomalies that are potentially associated with shipwreck remains and other submerged cultural resources. While there may never be an absolute formula for identifying magnetic anomalies that represent potentially significant submerged cultural resources, it is important to define the spectrum of those signatures.

This paper traces the arc of the author’s experience from 1985 to the present, interpreting marine magnetometer surveys in search of shipwrecks. During that period, technological advancements in positioning systems, computers, and software improved the accuracy of close-order, in-site magnetic surveys over shipwreck sites. Close-order surveys led to a refined understanding of shipwreck anomalies and an updated model for archaeological interpretation of marine magnetic data. The North-Alignment Model, developed by the author, focuses on the polar alignment of wreck dipoles with magnetic north. Two research questions are suggested that might lead to further improvement of this model. First, what are appropriate minimum dimensions of potential wreck anomalies? The search continues for the smallest verified wreck anomalies. Second, how does a lower sensor height affect the appearance of wreck anomalies?

Magnetic sensors continue to be primary tools for the detection and mapping of near-surface manmade objects, as they have a unique ability to detect items that are buried, and that would otherwise be missed by acoustic or visual methods. Magnetic sensing technology and data processing have advanced rapidly in recent years. Smaller, lighter multi-sensor gradiometers make higher-resolution survey possible with smaller vessels. Underwater Autonomous Vehicles can cover area faster and with greater precision. Unmanned Aerial Vehicles can survey in difficult areas such as surf zones that were previously inaccessible. New data processing software makes magnetic mapping more easily accessible than ever. And a compact diver-held gradiometer device allows divers to locate targets that were previously invisible. Results are presented to illustrate how these new technologies work together to act as a start-to-finish solution in the search for manmade objects, such as archaeological remains, or Explosive Remnants of War (ERW).

Marine magnetometers are a powerful and proven tool for detecting shipwrecks during archaeological reconnaissance surveys. Marine gradiometers, composed of two or more magnetometer sensors in a towed array, have seen relatively limited use amongst maritime archaeologists during Phase I investigations, partially due to the increased cost and complexity relative to their single-sensor counterparts. However, gradiometers have the potential to increase detection rates and improve the characterization of magnetic sources during processing and analysis. Additionally, the usage of gradiometers can ease the removal of regional magnetic gradients and diurnal variation, as well as eliminate the selection of false positives related to geomagnetic storm sudden onset signatures (Carrier et al. 2016). This paper examines the interpretation of transverse gradiometer datasets from shipwreck sites, and identifies some useful processing techniques currently being employed.

An ongoing common practice in the development of offshore wind projects along the coastal areas of the United States includes the study and derisking of UneXploded Ordinance (UXO) present in the surrounding environment. As past military conflicts shaped and changed the world's imaginary borders, real borders of ferrous (and non-ferrous) explosives remain untouched in the aquatic world below. What this means for users of the maritime world is an added risk of encountering these explosive anthropogenic remains either by physical impact or accidental recovery. As the world of construction and engineering development becomes safer and more mature, the acceptance of risks such as UXO to employees and contractors working on these projects becomes more and more conservative. In this paper, we will explore UXO surveys in the United States, and the tools we use to conduct them.

An archaeological remote-sensing survey was conducted in search of the U.S.S. Firebrand, formerly known as the Dorada, situated on or near Square Handkerchief Shoal offshore Pass Christian, Mississippi. Prior to its sinking in 1819, the vessel was owned by the infamous pirate, Jean Lafitte before being seized and acquired by the US Navy in 1814/1815. In an attempt to locate this historic shipwreck, a state-of-the-art approach using a Sea Robotics Surveyor M1.8 Autonomous Surface Vehicle equipped with a towed Marine Magnetics Explorer magnetometer was employed. This innovative technology facilitated surveying at depths as shallow as one foot of water, opening new avenues for comprehensive investigations. The case study demonstrates that by integrating the ASV with a magnetometer, areas too shallow for traditional marine remote-sensing are now more accessible for archaeological surveys.

Commodore Joshua Barney’s Chesapeake Flotilla was composed of a collection of shallow drafted gunboats and barges designed to stave off British invasions into the Chesapeake during the American-British War of 1812. The vessels were ultimately scuttled to block the river and prevent British capture, and were swiftly buried under river sediment. The Navy excavated the suspected wreck of Barney’s flagship in 2010-11, and is now searching for the remaining scuttled vessels in the Patuxent River, employing magnetometers to find the buried wrecks. One area of interest, a large marshland, has necessitated the use of aerial magnetometers. This presentation details the methods and observations from the past few years of surveying with a drone and suspended magnetometer over difficult terrain in which marine magnetometers cannot be deployed. It also discusses the challenges of acquiring consistent data and plans for future applications and methodological refinement.