test site, Figures 11 and 12. In most tests the signature of adjacent items 

 overlapped, since the area of magnetic disturbance well exceeded 3 m. However, 

 it was still possible to distinguish the individual presence of seven to nine items 

 from the magnetic gradient data in every instrument pass through the test area. 



Small Site 



An offshore location near the northern part of borrow area 1 A where 

 previously recovered ordnance had been disposed was investigated 1 (Figure 6). 

 Multiple traverses were made over this site. The water depth at the time of the 

 investigation was nominally 10 to 12 m. This designated ordnance placement site 

 was about 75 by 100 m in size. Multiple passes over this and the immediate 

 adjacent area detected numerous ordnance-type magnetic signatures (Figures 13 to 

 17). During all of these short traverses, the cesium vapor magnetic sensors were 

 "flown" 1 to 2 m above the seafloor. All of these detected responses are 

 indicative of short magnetic dipole type targets, typical of the expected ordnance 

 that had been placed at the location. However, the magnetic responses of many of 

 the objects were suggestive of a dipole (i.e. an elongated object having a „ 

 distinctive north and south pole) in a rather random orientation. This would be 

 expected for ordnance items dropped on the site recently. In comparison, the 

 magnetic investigation of the borrow site using rather long traverses revealed that 

 for the most part, the ordnance items appear to have become aligned with the long 

 axis parallel to the shore. This preferred orientation has been observed in other 

 coastal environments (Pope, Lewis, and Welp 19%). 



Long Lines 



Five traverses, which stretched several miles in length, were collected in 

 north-south directions at separations of 60 m. These lines were immediately west 

 of borrow area 1 A (Figure 7). Adjoining track lines ran in opposite directions, 

 i.e, a north- to-south line was adjacent to a south-to-north line, etc. The 

 instrument package was located at a 54-m setback behind the vessel and was flown 

 at an elevation of 1 to 2 m above the seafloor. 



Significant concentrations of ordnance-sized objects were encountered 

 throughout these passes. The spatial distributions of magnetic responses along the 

 traverses are shown in Figure 18. Areas along the line where a magnetic 

 response was evident are darkened. This practice shows any two-dimensional 

 distribution of ferrous objects in the investigated area and allows for 

 discrimination of larger versus smaller objects. The transverse magnetic gradient 

 of each of the long track lines is displayed in Figures 19 through 27. A positive 

 gradient anomaly in these figures represents a magnetic object east of the line, 

 while a negative response indicates an object west of the line. A larger object will 

 have a longer segment of the line where a magnetic disturbance is recorded. 

 Evidence suggests that the density of magnetic objects diminishes at the southern 

 end of the surveyed area. 



24 



Personal Communication, James Mullens, USAE District, New York. 



Chapter 6 Magnetometer 



