depths on various metals

[Anonymous]

Hi Eric,
I was reading the specs on the Deepstar and noticed the ranges: US Nickel: 14 inches US Quarter: 13 inches
This seems strange, since a quarter is much bigger than a nickel, it would seem to have a much larger cross section and seemingly should be able to be detected much deeper than the nickel, not to mention it being clad and higher conductivity seems like that should help too.
Is it because the deepstar is tuned to smaller, low conductivity items, or is there something about the quarter or deepstar to behave this way?
Thanks,
-jeff

 

[Anonymous]

Hi Jeff,
It is the characteristics of both the object and the detector that gives this strange result. Dealing with the object first. One of the fundamental laws of electromagnetics is that any change in equilibrium is resisted. If you change a magnetic field, as happens at the end of the TX pulse, then eddy currents will flow to try and maintain the field at its original value. In the absence of a metal object, the coil will produce a very high voltage in an attempt to jump between the ends of the coil and maintain the field. Hence the back emf. If there is a metal object, then a small portion of the field is coupled to the object and on field removal, currents are set up in the object to try and maintain this local field. For a metal with a relatively high resistance (nickel) the emf necessary to set up the current to maintain the field is higher than for a lower resistance metal (quarter). This results in a higher signal initially for the nickel, but which decays quicker due to the resistive losses. For the quarter, the initial signal amplitude is lower but due to the lower resistance, carries on for longer. It