question about metal detector capabilities

[leadweight]

Hey everybody, hows it going?
I've done lots of browsing on here, and now I have a question regarding the search radius of a metal detector. My knowledge is limited, so bear with me.

My understanding is that a standard PI metal detector can only really detect items directly below the search coil, maybe slightly to the side, right?
However, when I read the specifications for the JW fisher pulse 12 boat towed metal detector boasts a max 24ft wide search radius, despite carrying only a 24”W x 36”L search coil. Of course, I don't know about the actual functionality of those specifications, but I'm curious on the principle behind the matter.

How does this work?
Thanks for the help!

 

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[ROBOTCOP13]

Are you talking about the towed magnetometer (mag for short)?
If so, a mag detects the presence of iron only, no other metal. The presence of iron such as cannon, anchors or barrel hoops can suggest the presence of a shipwreck.

A mag operates differently than a metal detector. In a mag an electrical charge is transmitted rapidly thru a tube of for instance cesium gas. The gas molecular particles get excited and move around due to the electrical charge. When the electrical charge is cut off the particles begin to settle into their natural orbit which is controlled by the earth's magnetic field. If there is a presence of iron within say 20 feet of the tube it interferes with the particles settling into their normal orbit.
The mag notices this "interference" and records it as the presence of iron, a "magnetic" interference.

 

[TheGreenBoy]

A PI can detect an item about 1/4 to 1/2 of the coil diameter on a side. Depends on size and depth of the target, pulse power and sensitivity settings. Signal maximum is usualy in the middle of the coil (if the coil is round) or under the coil windings if the item is small and properly oriented, two maximums can be heard at a sinle pass in this case. Hence, the practically nonexistant target separation with this type of machines, specially for larger coils.
The "active" area of a VLF is sligthly more than the narrow band of Tx and Rx coils overlaping.
Two box can detect a larger target even more off the side, signal maximum is usually in between the Rx and Tx unit.