Thursday, November 11, 2004

Chapter I: Setting the Sensitivity on the Minelab Explorer (Update 08-03-2005)

Setting the sensitivity is the single most important adjustment you need to master on the Minelab Exlorer. It dictates the depth that you will achieve and ease of which you will be able to use this amazing machine. The techniques that follow are meant to guide you through the most neglected aspect of the machine.

First of all, realize that when you set sensitivity to a level where your threshold disappears for a second or two at a time, you have set it to a level that is detrimental to achieving maximum depth. When the threshold disappears, the detector is actually nulling. As the detector nulls, your visual ID actually hangs as the internal processor struggles to get things going again. Until you get the threshold back, you won’t be getting a new reading. Why? This is because when the explorer nulls excessively in a relatively clean site, the detector is actually recovering from picking up mineralization that is in the soil. In FERROUS mode, it’ll sound something like a Japanese Bon-dori drum concert, “bumbumbadambumbumbdabumbumbum”. Using CONDUCT, it'll be a virtual experience of the gunfight at the OK corral. Unless you are actually walking on a landfill, this is probably what is happening. At this point you are running the machine HOT enough to confuse the automatic ground balance. You can simulate this by running the explorer at sensitivity 30 with the stock coil.

The nulling I'm talking about here is similar to the silence that you hear right after you go over a legitimate target. However since the mineralization is ever present, it simulates the effect of a multitude of signals when there actually is a minimum of targets. Imagine that the threshold is the fine line between rejecting the ground and accepting a target. This is the essence of ground balancing. When the ground is balanced, this is actually the detector discriminating out ground mineralization.

To explain it fully, here is an excerpt of an explanation from Cody as posted in -

"Circuits in the detector look at the transmitter oscillator for a reference signal to establish the zero to know if a material is ferrite, metals, or iron oxides. The soil is neutral, positive or negative to the reference signal. Iron oxides are always negative so if there are enough in the soil matrix then the soil is negative. A VLF when tuned to a slight threshold with no offset will have the threshold go silent if lowered to soil that is negative or null. If the soil is positive the threshold will increase so we hear a loud sound. If the soil is neutral then nothing or very little change will be heard in the threshold. If the threshold changes, goes null or loud it will stay that way unless the user can do something (non-ground balancing).”

Setting your detector too HOT runs the risk of missing deep and faint targets as they may either be drowned by the nulls or averaged (MASKED) with the positive signals which the soil could be making. Set the explorer at manual 30 and listen. That is not nails you hear, that is the ground talking.

Learning how to set the sensitivity on the explorer begins with hunting in SEMI-AUTO mode and IM-16 (or clear screen in disc) at a relatively non-trashy site. Do this by pressing the button between up and down sensitivity to the left of the explorer front panel. You will know you are in SEMI-AUTO when a line starts circling the sensitivity level. Adjust the threshold sound to a comfortable level first on the explorer face panel or if you have the XS or the S, fine tune with controls on your earphones. For comfort and best depth, it is best to run the threshold tone barely audible but not silent, or else you miss the nulls. Set the tone to a frequency that is most comfortable to you in the advanced menu. The default threshold frequency is fine for 99% of users.

Note though that it is always best to keep the detector at full volume in order to hear faint targets. The exception is when you are using low impedance phones (32 to 60 ohms). With low impedance phones, you will usually get more sound volume than you will need at the risk of discomfort or worse hearing loss. Tune those down a bit.

Get some experience and learn what the threshold should be using semi-auto sensitivity. It should be smooth and much quieter than what you use to hear with sensitivity set at say, manual 28. The ground is not full of nails. I repeat, the “bumbumbadabumbum” that you use to hear is the detector picking up the ground’s mineralization.

It is very important that you note the recovery of the threshold after it passes over a target. It should be immediate. Much like “PING…hmmmmmmmmm” and unlike “PING………..PING(next target)”. This is true even if they are only inches apart. Continue hunting in semi-auto for a few hunts and listen carefully. What you need to master is the sound of a smooth and fast recovering threshold.

After learning how the threshold should sound and recover, use your normal settings and set the explorer to manual 28. Swing the coil and get a feel for it. Back off the sensitivity one or two clicks at a time and try to achieve that same smooth threshold. Keep backing off the sensitivity until you only get signals from actual targets. Back off another click if you hear radio interference. Radio interference can be described as a sort of warbling sound. What you are trying to get rid off is the constant drumming of mineralization and the pitter patter of interference.

When you are able to run the threshold smooth, then you will have achieved explorer nirvana. Hunting with the explorer will no longer be a constant barrage of sounds. No more finding that proverbial needle tone in a haystack of noise. If you get it just right, the tone should recover immediately back to the threshold just in time for the next target. This is the other advantage of setting the threshold perfectly. I have noticed that I am able to read more information in a tone especially on co-located targets. Besides tones taking on shapes, sometimes you will also hear differences by the way signals recover to the threshold.

One time, I found a deep faint target at a park which sounded like silver. I wiggled the coil over it to develop the digital ID. It read 00-31. “Iron” I said to myself. I noted that by using the sensitivity settings described above, I was also able to hear something after the tone. A sort of “PINGdum….hmmmmmm(threshold)”. By then I was sure it was iron but I dug anyway to confirm my reading. Eureka, an iron nail 10 inches deep using a Sunray X-8 coil. Depth hath not eluded me.

A disadvantage of monitoring and maintaining your threshold tone is that you will be thumbing the sensitivity switch a lot more. When you’re like me, I have the nasty habit of bumping it back up as much as I can when I notice that things are getting too quiet. Ground condition can vary up to 6 clicks from one corner of a site to another. Constantly listening and tuning the threshold will give you a sixth sense of sorts for mineralization and interference (i.e. you'll know your getting near wires even before you look up). Make sure that you compensate for noise as much as you can by clicking down or clicking up when the ground allows for it.

In my personal experience, hunting in manual is far superior to just being in semi-auto in most conditions. Most sites benefit greatly by maximizing sensitivity on manual with the exception of those that experience high radio interference.

If things get a bit confusing, a DFX user once put it to me that using the Explorer is like being in band practice, you can choose to dampen the sensitivity a bit more than needed with one or two more clicks down to avoid the constant tweaking. This added stability may marginally reduce depth but will be paid for by by the user being able to better discern good targets from noise and hunt more area!