A
Anonymous
Guest
In the thread "Salt Water Interference in PI" which began 20 Dec 01, Mr. Bill's post indicated that the problem tended to be more in evidence if the sand was mineralized. Here is an explanation of that phenomenon.
Iron minerals in beach sands are almost exclusively magnetite and titanomagnetites, which usually have multidomain crystal structures free enough of crystal lattice defects to exhibit low magnetic loss.
The presence of these minerals lengthens slightly the flyback time, due to increased coil inductance. Ordinarily the damping and the delay of receiver turn-on are sufficient to keep this from having any appreciable effect on received signals.
The presence of salt water or other conductive materials changes that. The salt water puts a "tail" after the flyback, and some of that "tail" sneaks into the receiver. So we get a little salt pickup maybe. No big deal.
Now add magnetite. The salt "tail" is steep, so pushing it into the receiver on-time even a little bit further greatly increases the area of the tail which is now detected.
So, the consequence is that the combination of salt and magnetite is a lot worse than just the arithmetic sum of the two effects.
On a mineralized beach in the shallow stuff, this could easily get interpreted as having to do with water velocity. When the water is moving fast, it's shallow, the coil is right close to the sand, and the sand is rapidly eroding around it.
I believe the best technical solution to this problem is to make the receiver on-time track the end of the flyback pulse, rather than being fixed in time relative to the termination of the transmit pulse.
--Dave J.
Iron minerals in beach sands are almost exclusively magnetite and titanomagnetites, which usually have multidomain crystal structures free enough of crystal lattice defects to exhibit low magnetic loss.
The presence of these minerals lengthens slightly the flyback time, due to increased coil inductance. Ordinarily the damping and the delay of receiver turn-on are sufficient to keep this from having any appreciable effect on received signals.
The presence of salt water or other conductive materials changes that. The salt water puts a "tail" after the flyback, and some of that "tail" sneaks into the receiver. So we get a little salt pickup maybe. No big deal.
Now add magnetite. The salt "tail" is steep, so pushing it into the receiver on-time even a little bit further greatly increases the area of the tail which is now detected.
So, the consequence is that the combination of salt and magnetite is a lot worse than just the arithmetic sum of the two effects.
On a mineralized beach in the shallow stuff, this could easily get interpreted as having to do with water velocity. When the water is moving fast, it's shallow, the coil is right close to the sand, and the sand is rapidly eroding around it.
I believe the best technical solution to this problem is to make the receiver on-time track the end of the flyback pulse, rather than being fixed in time relative to the termination of the transmit pulse.
--Dave J.