Chapter III: Archeological Record
(continued)

PEARL HARBOR SURVEY 1988

Instrumentation and Methodology

Side-scan sonar -- a Klein 52 IT with a 100KHz towfish coupled with a Motorola Falcon electronic positioning system -- was the basic remote-sensing tool for all operations. A Geometrics 866 magnetometer was towed along with the side scan within the harbor. A Mesotech color video was also utilized during the survey.

Sonar survey procedures followed standardized Navy mine and obstruction detection procedures. Microwave positioning transponder stations were established in the target area vicinity. A search grid was constructed with arbitrary (0-0) geographic coordinates that could be post-plotted by hand into latitude longitude, if necessary. A survey block widely encompassing priority crash-site target areas was plotted and divided into 50-meter transects offset from a base line. Each lane was surveyed twice (designated on records as "normal" -- east to west, and "reverse" -- west to east) at a sonar scale of 50 meters and a vessel speed of 2-3 knots, giving 400-percent coverage that provided two directional views of each contact from two adjoining lanes.

Side-scan records were notated by observers and reviewed after completion of all preplotted runs within a survey block. The positioning system and magnetometer computer clocks were coordinated, and automatic event marks representing positioning system updates were recorded on the side-scan record. When a magnetic anomaly was observed, it was noted on the side-scan record, which allowed data record cross-referencing. The combined data allowed a determination of whether a side-scan contact was ferrous metal. In addition, side-scan record areas -- where a magnetic anomaly was present but no obvious contact was visible -- were scrutinized for small contacts that may have been missed by the observer. Contacts selected for diver evaluation were based on signature attributes of size, shape, density and shadow presence. Contacts were selected by the commanding officer of the EOD survey team and an NPS underwater archeologist. A range of targets, including many not fitting expected attributes of structural remains, were selected for diver survey.

Computed plots were generated for all contacts potentially representing items of interest. All necessary corrections for accurate contact location were computed, including towfish layback and slant correction. The Falcon system was used to relocate the contacts so that numbered buoys could be set for in-water evaluation. All selected contacts were bouyed and investigated by a Navy EOD dive team experienced in target location. Diver investigation included systematic grid or circle search and target location with diver-held sonar. Most targets were located within 2 meters of the buoy.

Deepwater side-scan search of the harbor mouth's defensive perimeter was done to locate a Japanese Kohyoteki submarine reported sunk by the USS WARD shortly before the 1941 air attack. The water depth was between 750-1,100 feet (337m), which necessitated deploying the towfish from approximately 3,000 feet of cable. TWR 6 FERRETT, under the command of BMC Allan Connelly, was used for the deep water search. Two searches were at tempted. Deep tows were run at 3.5-3.8 knots, the idle speed of the vessel, with positioning by the Falcon system. The 100m side-scan scale was used. Deployment of the side-scan sensor was difficult because of the length of cable necessary for the depth and the irregularity of the survey area seabottom. It took a lot of complicated coordination between the survey team and the helmsman to keep the sonar sensor from hitting bottom.

Figure 3.35. Monitor of Mesotech sonar used by team for sector, polar and side-scan operations. USS ARIZONA image is emerging from left. Note 14-inch guns of forward turret. (NPS photo by Larry Murphy)

Unpositioned side-scan survey utilizing both the Klein and Mesotech systems was done in pier areas and also offshore the ARIZONA and UTAH. The objective was to discover materials above the bottom remaining from post-1941 salvage operations. Divers investigated contacts located during this survey.

Visual surveys were conducted directly offshore historic piers and mooring quays within the harbor. MDSU divers were primarily responsible for diver surveys, although an EOD team searched some areas. Diver surveys employed running jackstays to ensure systematic coverage. Jackstay searches utilized two rope base lines between buoy weights. A third rope was incrementally moved between the base lines. A dive team moved back and forth along the third line searching visually and by feel. Any material discovered was buoyed for later documentation and examination by archeologists.

An ROV was also provided by the Navy for the survey. However, equipment failures precluded its effective deployment.

<<< PREVIOUS

TABLE OF CONTENTS

CONTINUE >>>