reply
There's a few bugs in going through asphalt with a detector:
a) First, the gravel rocks used in asphalt composition are mineralized more than normal soil. So it's going to depend on what type detector you're using, what your settings are, etc... You might need to lower your sensitivity.
b) Asphalt is not laid right on top of virgin dirt. There is a compaction bed made first of DG (decomposed granite). That's a type fill dirt that can be many inches thick. It is rolled and compacted, and THEN the asphalt is laid on top of THAT. So when you see them grind out old asphalt (to prepare for a fresh "lift"), they usually only grind out down to that DG layer, NOT the native soil. So you can see, that if the asphalt is 1 or 2" thick, and the compaction layer is several more inches thick beyond that, you can see that native soil is going to be fairly deep. This can be a foot of depth for public streets/roads, as they are engineered to handle more weight of continual use. But for a private driveway or lot, less thick d/t it's only handling slow traffic and parking only.
c) My experience in hunting any medium, where it changes mid-way down, is that the machine tends to track/tune/balance to the topmost "soil" minerals type. So if, in your case, the "soil" type changes 3 times (asphalt, DG, then native soil), it seems to confuse the machine in what the ground balance is, tending to sort of make a "brick wall" when your signal is encountering the changing types. Contrast to if the "soil" type is uniform all the way down, then it's not differing mineral types.
doesn't hurt to scan it though. Might have to turn down the sens, as I say. Or if you have a beach pulse type machine, bi and tri-level minerals should pose no problem, and it'll go deep, etc... However, every frickin staple, tack, nail, etc... that's down there, will give hits, d/t no disc.
We have recently purchased a new old house (mid 1880's) that has not been lived in for at least 15 years. It is located near downtown Lexington Ky on land that historically was a staging area for Civil War troops. The entire lot around the house has been under pavement since the 1950's and we plan on removing most of the pavement over the next couple of months. My question is, I want to make sure no relics or (dare I hope?) caches are not removed along with the pavement-will a metal detector be useful to detect anything that may be under the asphalt? I love treasure hunting and have always wanted a detector-this is a great excuse to get one-any ideas? Thank you!...Lisa
