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X-TERRA... Controlling the search coil

January 04, 2011 10:10am

Randy Horton
Randy Digger Horton

When it comes to sweep speed, the X-TERRA is a very forgiving detector. You can operate with a relatively quick sweep in wide open areas, and slow down when targets are abundant. But to gain a better understanding of how sweep technique may affect the quantity of our finds, let's first analyze the field of detection for both the X-TERRA Concentric coils and the Double-D coils.

Based on my field analysis, with the center of the coil as the "hot spot", I find the field of detection for a Concentric coil will go "straight down" until you reach approximately 55 - 60% of the maximum depth for a specific target. In other words, if you are capable of detecting a target at maximum of 10 inches, you will find the circular field of detection begins to narrow at the 5.5 to 6 inch depth level (see Concentric diagram). The deeper the target, the more closely centered (under the coil) it must be, to provide a target response. As such, if you are not overlapping your Concentric coil by 1/2 the width of the coil, you risk missing those deeper targets that are not directly centered under the coil as you pass over them. By overlapping your swath by 1/2 the width of the coil, you've increased the field of detection “coverage” at any given point, maximizing the opportunity for a solid target response.

Illustration of a metal detectors Concentric Coil

Double-D coils are designed to help neutralize the effects of mineralization. Part of those design characteristics place the "hot spot" of the coil directly down the center, from the front tip to the rear heel. Don't be misled into thinking that the field of detection goes straight down, from front to back, until the maximum depth is achieved. Although not to the degree of Concentric coils, I’ve found the field of detection from a Double-D coil also gets "more narrow" (front to back) as target depth increases (see Double-D diagram). To ensure that you are not missing those deeper targets, I recommend overlapping the swath of a Double-D coil by 1/3. Again, this allows you to maintain a maximum field of detection during your hunt.

Illustration of a metal detectors Double-D Coil

As to sweep speed… the majority of us have a tendency to sweep more quickly over a wide-open, smooth surface than when working the coil in and around vegetation. Another thing I've noticed is that, when walking uphill or downhill, our swaths don't get overlapped as consistently as when we are walking on flat ground. It's almost as if our arm feels compelled to keep up with our feet in both speed and distance covered! Regardless of whether you are in a field or on a beach, walking uphill, downhill or on level ground, I encourage you to maintain a consistent, even sweep when you work the coil. If you notice that your X-TERRA is chirping as you sweep the coil, and your X-TERRA has been properly set up, you are likely sweeping too quickly. When that happens, the first instinct of many would be to adjust the Threshold, lower the Sensitivity or set the ground phase a bit more negative. If they were set properly for the site, don't change them. Instead of overcompensating with electronic adjustments, simply slow down your sweep speed and concentrate on working the coil. By maintaining the proper electronic settings and modifying your sweep speed, you'll find those deep and partially masked targets that others have missed.

If your X-TERRA makes false signals when you change the direction of your sweep, you’re probably tipping the edge of your coil. Again, slow down the sweep speed and keep the bottom of the coil parallel to the surface of the ground at all times. As I near the end of each sweep, I'll make a wide, deliberate turn with the coil, to maintain my rhythm. This not only reduces the chance of me tipping the edge of the coil, but allows me to line up with the proper "overlap" for the return sweep (see sweep path diagram).

Covering ground with a metal detector illustration

Each of us have different detecting styles and hunting techniques. Being 6 feet tall, the width of my typical sweep is around 42 inches. Yours may vary, dependent on your stature and physical ability. Regardless, my recommendation is to sweep comfortably, without over-reaching and without lifting the coil from the surface of the ground. When I’m searching an old homestead site, I'll usually begin my hunt with my 3kHz Concentric coil and pace myself at 2 - 2.5 seconds for each swath. When I get into an abundance of targets, indicating I've reached an area with past activity, I'll switch to the 6-inch Double-D for superior target separation and slow my pace, according to the number of targets per sweep. A fast sweep speed in an area with multiple targets can be overwhelming (and under-achieving). I’ve hunted some areas that were so nail infested, proper target separation required a sweep speed of no more than 6 inches per second. As someone who hunts "by ear", using multiple tones and minimal discrimination, I find the slower sweep speed allows me to process the audio response of every single target. And by using a controlled sweep, I am able to maximize my hunt time by minimizing the number of false signals. 


Randy Horton


thank you Randy, this is some of the most useful advice I've heard yet.
Posted By: jetserg on February 03, 2011 09:02am
Thanks for the comments. Glad you enjoyed reading the blog post. HH Randy
Posted By: Digger on February 16, 2011 12:02pm
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