b. Remnant magnetism. This is the natural magnetic field that the ordnance 

 material contains. It is a function of the properties of the metal and the 

 casting procedure. Both of the above properties form the basis whereby 

 various sizes and types of ordnance may be detected. Currently, few 

 measurements have been made to determine what these values are for 

 WWII and earlier ordnance items. 



Instrumentation 



To accurately and rapidly detect the magnetic field variations produced by 

 ordnance, a much more precise magnetic sensor is used than commonly employed 

 in terrestrial and marine surveys. The instruments used for the Sandy Hook 

 investigation were state-of-the-art cesium vapor marine magnetic sensors 

 produced by Geometries of Sunnyvale, CA. These were fabricated and 

 configured expressly for this project in a development effort. The normal 

 precision of a standard marine magnetometer is about ±4 nT. (As a reference, 

 the Earth's magnetic field intensity is about 55,000 nT at this site.) For marine 

 use, this sensitivity level has been satisfactory in the location of larger objects 

 such as hulls, wrecks, etc. To pinpoint smaller items such as ordnance, it is 

 necessary to use cesium-vapor magnetic sensors or some other extremely precise 

 instrument which have a sensitivity of ±0.02 nT. This aids the discovery effort 

 in two ways: (a) a much smaller object can be detected, and (b) it is possible to 

 measure the local field using two or more closely spaced sensors and achieving 

 the gradient of the anomalous magnetic field. This measurement can be used to 

 effectively vector toward the object. From several locations, the target location 

 can be established by triangulation. In addition, by using the magnetic gradient to 

 detect the ordnance, a much more accurate and straightforward procedure is 

 achieved. In this investigation, two cesium-vapor magnetometers were towed 

 about 50 m behind a fiberglass-hulled research vessel at a height of 1 to 2 m off 

 the ocean bottom (Figure 10). These instruments were mounted 2 m apart, 

 transverse to the towed direction. The following data were collected every 2 sec: 

 (a) time, (b) ship's position, (c) instrument setback, (d) instrument altitude from 

 the sea bottom, (e) course over ground (COG), and (f) speed over ground (SOG). 

 The following were recorded every 0. 1 sec: (a) the magnetic field at both 

 sensors, and (b) the horizontal magnetic field gradient. As a consequence of 

 measuring the magnetic gradient, it was possible to immediately determine if an 

 ordnance type signature originated from the port or starboard side of the track 

 line. 



Test Site 



A test site was established offshore of the Sandy Hook Coast Guard Station. A 

 magnetic sweep of the site for any foreign iron objects was first conducted at low 

 tide confirming a magnetically clean test area. The magnetic gradiometer was 

 then towed over this calibration site after the inert ordnance targets had been 

 placed. In this test the magnetic sensors were approximately 1.3 m under water, 

 or 1 m above the bottom and the inert ordnance items. The individual and the 

 cluster inert ordnance targets were detected in various calibration passes over the 



Chapter 6 Magnetometer 23 



