Diver Evaluation of Unidentified Magnetic Anomalies and/or Side-Scan Sonar Targets: Search and Methodological Requirements


Magnetic anomalies are localized disturbances in the earth's magnetic field. The goal of the following test procedures is twofold:

  1. to determine if the magnetic anomaly and/or side-scan target is modern debris or a historic archaeological resource; and
  2. to determine if the magnetic anomaly and/or side-scan target possesses qualities that makes it a significant and unique archaeological property as defined by Federal law. Not all shipwrecks are considered "significant."

Testing Procedures

Unidentified Magnetic Anomalies

A professional marine archaeologist must be present to direct, observe and monitor the diving investigations and must direct the activities with the archaeologists at the Bureau of Ocean Energy Management (BOEM) who will coordinate the activities with the officials at the Bureau of Safety and Environmental Enforcement (BSEE) prior to going offshore. The following procedures must be followed:

  1. a Differential Global Positioning System (DGPS) must be used to relocate the unidentified magnetic anomalies listed in BOEM's archaeological analysis for this project;
  2. a marker buoy must be dropped at the precise location of each magnetic anomaly to be tested;
  3. at each marker buoy location, the diver will perform a series of 10 radial search sweeps around the position of the marker buoy. These search sweeps will be in 5-ft increments (i.e., 5 ft, 10 ft, 15 ft, etc.) out to a maximum radius of 50 ft; and
  4. anomalies detected by the diver shall be probed minimally to a depth of 5 feet to determine the depth of burial, approximate size of the target, and association with other features.

This methodology will provide a 100-ft diameter circle of investigation around each unidentified magnetic anomaly location. Each radial search sweep shall be conducted with a hand-held gradiometer, magnetometer, or metal detector to locate the anomalies on the seafloor. An alternate search strategy (e.g., linear grid, radials, etc.) may be employed with the approval of the field archaeologist.

If recorded anomalies on or near the pipeline centerline can not be located using the above search methodology, additional magnetometer surveying may be required to determine the precise location and limits of the anomaly or anomaly cluster. The specific survey methodology will be determined by the field archaeologist.

If the source of the anomaly is encountered by the probe, the anomaly must either be buoyed for avoidance by the proposed operations or evaluated by the field archaeologist to determine its historical significance. A determination of the anomaly's historical significance shall be made using the criteria specified in 36 CFR Part 60.4, "Determination of Eligibility for Inclusion in the National Register of Historic Places," and National Register Bulletin No. 20, Nominating Historic Vessels and Shipwrecks to the National Register of Historic Places."

If the anomaly can not be physically (i.e., hands-on) located by the 5-ft-probe testing procedure, it may be assumed that the source of the anomaly is buried deeper than 5 feet. Consequently, this anomaly location will not be affected by pipeline centerline construction. The effects of the proposed method of construction (i.e., lay-barge anchoring, jack-ups, etc.) on anomalies off the centerline also must be considered. The anomaly location is cleared when, in the opinion of the field archaeologist, construction will not adversely affect a potentially significant archaeological resource. Testing must extend to the depth of impact to the seafloor where possible. These search procedures shall continue until all anomalies either are cleared or evaluated by the field archaeologist.

Side-Scan Sonar Contacts

The diver must be able to conduct a hands-on assessment and evaluation of each sonar contact listed in BOEM's archaeological analysis. This assessment must include an analysis of the age, physical composition, and structural integrity of the object (i.e., wood or metal, intact or dispersed). Next, an evaluation shall be undertaken to determine if the object is of historical significance using the criteria specified in 36 CFR Part 60.4. Measured drawings and/or underwater video or still photographs of the sonar feature shall be made for documentation and submitted with the final "Report of Finding".

Required Coordination with BOEM and BSEE

A "Report of Findings" prepared in accordance with the report writing standards specified in Notice To Lessees (NTL) 2005-G07, Enclosure No. 2, must be submitted to the BSEE for approval within ten work days of the completion of fieldwork. BOEM and BSEE archaeologists must concur with the findings of the field archaeologist prior to the start of construction.

Alternatively, if the field archaeologist determines that the magnetic targets are caused by modern marine debris, or that the source of the anomalies can not be located by any of the means specified above, and that the proposed construction will have no effect on potentially significant archaeological properties, notice to proceed can be given by the BSEE Field Operations Pipeline Unit upon receipt of a telefaxed notification of these results signed by the field archaeologist. The proposed construction may proceed pending the mandatory 48-hour notification to the Pipeline Unit and fulfillment of any other permit requirements. A "Report of Findings" still must be submitted as indicated above.

If diving investigations discover a potentially significant archaeological property, the Field Operations Pipeline Unit must be notified immediately. Further operational measures will be developed in accordance with NTL 2005-G07 in consultation with the lessee or right-of-way holder, the field archaeologist, BOEM and BSEE.