Yes Terra is right, I have heard that so many times before, if your soil in high mineralization forget using the X-70(5) for gold nuggets or specimens as it picks up iron ground mineralization, it cannot cope to cancel it out. But if your ground is medium to mild mineralization, especially low on iron then you will have no trouble and it should be fine to use then do not worry. It totally depends on what your ground is like in your area. If you live in a glaciated area with lots of glacial till it is usually low in iron mineralization and hardly no volcanic hot rocks there will be likely be no problem. But if you live in Arizona or Oregon-Washington state there might be problems in areas. All VLF's have problems in All-Metal Mode and a PI is the way to go then. It all depends on the area you are in. I have heard in some places in Alaska like Ganes Creek a VLF is great to use, but in other areas it is useless.
Something of interest from Jack Lange(the same applies to the 705):
MINELAB XTERRA 70 by Jack Lange
This article mainly discusses the top of the range Xterra 70 but briefly touches on the mid range Xterra 50 and the cheaper Xterra 30
X TERRA 70 VERSATILITY
This is easily the best goldfields detector on the market under $2,500 when used with the optional DD coil. It is also the best beach/coin machine under $1200. So it is a true all-purpose machine. The standard coil, while great on the dry beach sand and park soils, cannot handle the mineralized goldfields. Put a DD coil on it and it will handle the goldfields, even the noisy spots, provided you turn the sensitivity control down.
I did some depth tests in relatively quiet goldfield soil and was amazed at its depth ability--only down about 20% to 30% on the Gp3500! However this quickly shrank to about 60% less in mineralized, noisy soils. This is fine for the hobbyist but the serious gold hunter will need to fork out more for top of the range machine.
However, the Xterra 70 has some advantages over the top of the range GPs. It discriminates small junk targets out much better and it beeps on small gold specimens far better. By small specimens I mean fine veins and small rough pieces of gold in quartz rock as are commonly found on reef- gold mullock heaps or quartz paddocks. In fact some cagey professionals use the XTerra 70 especially for specimen hunting and for searching high-junk goldfield camp sites.
When in Prospecting mode, the character of this little hi tech machine changes. It becomes sensitive to tiny gold nuggets that it wont even look at in the treasure hunting-coin modes. Its discriminator will only reject iron at half depth, but thats enough to get rid of most junk targets. When detecting goldfield areas thick with junk, its best to sacrifice the tiny nuggets and select a coin mode where the discriminator will reject all iron and aluminum foil. At least it will pick up bits of gold as big as this letter O or bigger.
GROUND BALANCE
It allows you to chose between manual or auto ground balance or to fix (lock) it in. Auto ground balance, means the detector reads the ground and tunes itself to the varied soil types. This means, you dont have the hassle of self tuning at every few steps.
DEPTH
This detector goes as deep as the expensive models (Explorer Quattro Sovereign) in dry beach sand, all soils, but does not go nearly as deep in damp beach sand because of the salt effect. Especially when set in prospecting mode, it will easily pick up very fine jewelry and tiny nuggets that the others wont even beep on. On the goldfield, it will detect deeper than any of the above, especially on small nuggets.
DISCRIMINATION
Is adequate for 99% of users. Only the Explorer series will discriminate better but the Xterra 70 is much simpler to use.
It has 3 discrimination modes or patterns that can be used factory pr-set or customized by the user to set their needs.
HANDLING
All the Xterra series may look light, small and unimpressive compared to their big Minelab brothers but thats what makes them a featherweight that any little lady or kid could swing for hours on end.
SUMMARY
If you can only afford one budget detector and want to prospect for gold as well as coins relics, jewelry etc, this would have to be the best choice for you. If you can afford a few grand for a gold detector then get a GPX 4000 or an earlier GP model and get the Xterra 70 for the little quartz-gold specimens and general treasure hunting.
Happy Hunting!
Jack
Also for depth and size of coils, Randy(Digger) wrote:
http://www.findmall.com/read.php?55,1127217
Re: messin' with my 505, 705 and a few coils.....
Posted by: Digger
Date: January 14, 2010 08:57AM Moderator
I have put all the coils through quite an extensive testing process. I could say those test results were the reason I was willing to set some of them aside, and continue using the coils I chose. But with a bit more explanation, I found the 6 X 10 (HF DD) Elliptical to do what it was designed to do (in my opinion). At 18.75 kHz, it is a "longer" version of the 6 inch (HF) DD at the same frequency. Frankly, when comparing these two in my moderate soil, I found very little (if any) difference in the depth of detection. There is a little more difference in airtests. But I believe the effects the soil has on the larger "footprint" of the elliptical cancels out any increase in actual hunt depth. They are both DD coils operating at 18.75 kHz. As such, it seems the "width" of the coil is a very important factor when comparing depths. Since both are nearly identical in "width" the main difference I found is in the amount of turf covered with each sweep. Therefore, if I am needing a coil that separates better than the concentrics (which the DD design does), my preference is for the smaller of the two (comparing the elliptical 6 X 10 with the 6-inch DD) simply because the 6-inch round DD does not have to analyze as much soil at any one time. (again, footprint) "Left to right" target separation is nearly equal between these two. But we must consider that adjacent targets are not always to either side of the primary target. Sometimes they are toward the tip or heel of the coil, depending on your direction of sweep. And it those instances, the 6 inch out performs the elliptical. (once again, due to the smaller footprint)
As to the round 10.5 DD coils...... since the depth of detection is somewhat dependent on the width of the coil, the "wider" 10.5 round coils will be deeper than an elliptical coil of the same "length". From my observations, the large round DD coils have +/- 40% more depth than the elliptical. >I've found the 10.5 DD at 18.75 kHz to be more sensitive than the 10.5 DD at 7.5 kHz. As such, I usually have to lower the sensitivity setting to operate it without falsing. But with all those things considered, both are similar in performance, depth wise.< I'm still convinced that I get more "raw depth" out of my 9-inch concentrics than I can either of the two 10.5 inch DD's. But my soil is moderately mineralized, with a typical ground phase reading in the 40's.
Hope this helps!
Jim.