Everybody is looking for that one detector that will solve all of our problems and find treasure, no matter where it is or how deep it is. The truth of the matter is no such animal exists. Metal detectors are like any other tool; there is usually a specific tool for a specific job. When it comes to deep seeking metal detectors this is almost always the case.
If you are cache hunting you need to have a good two-box detector. Several companies make these including Whites, Fisher, Discovery, and Garrett. Personally, when it comes to a two-box unit I prefer a dedicated machine and not an add-on like the Garrett. The dedicated machines seem to get better depth and are easier to work with.
Everybody has heard the hype about a two-box unit picking up an object fifteen feet in the ground. This is possible but only under ideal conditions, which rarely exist in the field and the object you are looking for would have to be the size of a car. I don’t recall any stories about Jesse James filling the trunk of a car with gold and burying it so this won’t do you much good. It’s been my experience that a good two-box detector will give you somewhere between four and six feet in depth in the field. This is good for the outlaw cache hunters in the crowd because most outlaw caches will rarely be more than four feet deep. You can find most two-box detectors starting at a price around $600 new. You can find them used for one to two hundred less if you look around.
For the Spanish cache hunters out there you may need something that has a little more reach. A good pulse induction unit with a 40-inch coil is another choice for looking for the bigger caches. A pulse induction unit will work better in highly mineralized soil than a two-box and will generally get more depth. Field experience says a pulse induction unit will pick up a small object, lets say something the size of an iron pot or teakettle, at six feet with very little problems. It would be my opinion that if you are looking for something the size of a car battery or bigger then you will probably get between 5 and 10 feet in depth with a pulse induction unit, depending on the soil and terrain. Some of the manufactures of these units will claim depths up to thirty feet but I haven’t heard of any recoveries at that depth using a pulse unit, junk or treasure. The bad part about the pulse induction units is the cost. Unless you are smart enough to build your own (which leaves me out) then you’re probably going to have to shell out at least two or three grand to get a good one.
As with the two-box units, there are several manufactures of pulse induction units. If you are interested in looking into a PI unit Bob Fitzgerald at http://treasurenow.com/ has two different PI units he sells that are less expensive than others but don’t have a lot of the bells and whistles either. You will want to look over his descriptions closely because the least expensive unit comes with the wire for the coil but you have to buy your own PVC pipe and make the actual coil to put the wire in. He also seems to exaggerate the depths some but you can at least see the difference in the cheaper models as compared to the more expensive ones. His prices start around $700.00. Kellyco Electronics sells a few different PI units also but their prices start around $4,000.00. Ouch!
You need to remember when using a two-box or PI unit that the coils need to stay parallel with the terrain you are searching or you will get false signals. A lot of flase signals will make for a really long day.
For those of you that have more money than you know what to do with you can look into an electromagnet induction unit. These sell for around $16,000 but are very handy for finding underground tunnels or sealed caves. For us normal folks that can’t afford to buy one of these until we hit that big one, they can be rented, some for as little as $50.00 a day.
Two of these units that I have been researching are the Geonics EM31 (http://www.geophysicsrentals.com/geonics_em31.htm) and the GSSI Profiler (http://www.geophysical.com/Profiler.htm). Both of these machines work on the same principals. They work along the same lines as the older resistivity machines however you don’t have to stick any probes in the ground. They measure the resistivity of the soil from one point to another showing different colors for the different amounts of resistivity in your survey. In other words, if you are walking a grid across solid ground your picture will stay about the same. If you cross over a void within the ground the air in the void is less resistive to signals therefore the signal will travel through the void faster, giving a different reading at the machine in the form of different colors and shapes on the screen. This of course is a very simplified explanation of how they work but it should give you the right idea. These machines are routinely used for ground water surveys, archeological investigations and geological assessments. The drawback for the amateur treasure hunter in using these machines has always been the cost involved. Now that there are companies that will rent the machines it has made it possible for others to get full use of today’s technology.
The EM31 can be rented for about $50.00 per day and the Profiler goes for about $800.00 per week. The Profiler is the newest machine and includes a wireless PDA that gives you a real time color picture of what you are seeing. The EM31 requires a laptop computer to be carried around at the same time to see real time results or you can do your scan and then go to a computer and download the results. I’m more of a real time kind of guy, no reason to make two trips when one will do!
The depths that both of these machines will “see” are measured in meters with the minimum being three meters and the maximum around eight meters in ideal conditions. Ideal conditions aren’t what you would think. In speaking with a techie at one of the companies I was told that working in granite mountains would give me about the same depth as working on the flatlands of Kansas. Apparently these machines don’t like pockets of clay any more than they like large buried granite boulders. Of the two machines the Profiler is the lightest and easiest to use. Of course, it’s also the most expensive one. The Profiler weighs less than ten pounds compared to the almost twenty pounds of the EM31. The Profiler is only six feet long compared to the sixteen feet of the EM31.
There are other machines available to individuals such as ground penetrating radar and seismic machines that work something like sonar but in most cases treasure hunters aren’t looking for the kinds of things these machines will find or they won’t work in the areas we are looking in. Didn’t anybody bury anything on flat open ground that you can just drive up to? That would be nice, wouldn’t it?