156 oz gold nugget
found by Chris Ralph with a metal detector.

Metal Detectors

There are three major classes of metal detectors available.

The first type of metal detector is the PI, which stands for Pulse Induction. Pulse induction means that you are creating an electromagnetic pulse that exits the coil, bounces off a target, then returns to the coil giving a report on the target it hit.

Pulse induction detectors are usually more sensitive to smaller and deeper targets. The down side is they also pick up more trash targets and have a difficult time telling good targets apart from bad targets.

Next is the VLF metal detector, which stands for Very Low Frequency. This kind of detector is versatile and commonly used. It uses phase demodulation to discriminate between types of metallic objects like iron and gold. It does a good job at telling the difference between good objects, like gold and treasure, and trash such as nails, bottle caps etc.

Another class of metal detector is the BFO, or Beat Frequency Oscillator. The beat frequency oscillator is a low cost circuit design. It has two coils, one at the end of the arm of the detector and one in the control box on the handle. These coils form a pair of tuned oscillators. The oscillators are tuned so that the difference in their frequencies is an exact multiple of each other. The presence of any metallic object will produce a change in either oscillator and thus produce an audible beat frequency harmonic tone.

Each of these technologies has trade-offs, usually between cost and their ability to sense small valuable objects. Newer advances are coming every day. Most of these are in the area of using special digital signal processing of the return signal. Every conductive object has a peculiar signature spectrum that tells a bit more about the object and can also be filtered out to separate junk (iron & aluminum) from treasure. As technologies advance, the cost of better equipment drops. This enables people with smaller budgets to have access to better metal detectors.

Remember to fill your holes, and have fun metal detecting!