Noisy bedrock is always an issue when metal detecting, especially with a VLF. However, sometimes that noise you hear can be a target. You have to learn the characteristics of the bedrock you are working through trial and error and sometimes you just have to break some up to determine if the signal you are getting is just hot rock or a valid target. One useful way to check certain types of bedrock is with a magnet. Out here in Az. we have lots of bedrock in certain areas that is loaded with magnetite. By checking to see if it is magnetic this will often tell the story of the noise without any further effort. Sometimes what appears to be smooth bedrock can have hidden cracks and crevices that can be revealed by hitting the area with a hammer. Look for material that moves or jumps out of a hidden crack when impacted with the hammer or small sledge. Even the narrowest of cracks can carry surprisingly chunky pieces of gold. Bedrock can expand and contract with temperature change and cracks can close up and or be sealed over with material and be hidden from view.
First and foremost you need to know your detector. It will tell you a lot of info if you understand what it is telling you. Most of the time a valid target will be just a blip of a signal. Broad or elongated signals are often just mineralization. A valid target is also usually repeatable from different angles. Bedrock is rarely perfectly flat over the entire length of your coil, so any voids between your coil and the bedrock can often cause a false signal. This is often the case over dips or bumps and especially edges of bedrock.
If you have a detector with an iron readout, this will often show a high iron reading when you get a response from a particular spot that is questionable. A valid target will usually not have a high iron reading. Valid gold or silver targets will usually make the iron reading drop to a lower point on the scale, even without getting an actual target read out.
Even when there is no crack or crevice, targets in solid bedrock can be valid, even without a visible vein in the bedrock. Sometimes you just have to get a small sledge hammer and a chisel and break some rock to make the determination. You might just find a mineralized pocket in the rock, but you can find a gold or silver deposit in the right scenario. I myself have realized 2 small silver deposits this way. Breaking solid bedrock isn't easy and you will learn one way or the other what is making your detector sing. As you chisel out pieces of bedrock, keep checking the spot and pieces you break out with your detector. As you break into the bedrock, valid targets will give a stronger signal as you get closer to them.
The 1st silver deposit I found this way was in seemingly barren bedrock right in the gut of a dry wash. It took me about 2 hours of hard work to get roughly 8 inches into that solid rock before I started removing the mineralization that was causing the positive signal. I got most of it out that 1st night, but had to return the next week to get it all removed. After having the material tested by my local jeweler it was verified to be silver. I figure close to an ounce of silver is locked up in the ore I recovered. At the surface there was no indication of a vein or any cracks or crevices. This was smooth hard gray bedrock with a definite positive signal using a Minelab SD2100 V-2 PI unit.
The second deposit, which I found recently, was along an obvious vein system that I and others had been just walking past. I am sure I am not the 1st person to get a signal on this vein, but there is so much iron hot rock in the area that we just assumed it to be an iron deposit when using our Minelab PI units. One day I scanned the vein with my Gold Bug SE, non pro version, And to my surprise I got a valid target signal with a readout showing low to no iron and numbers in the range of what gold or silver would typically show. I actually got two valid targets close together and proceeded to remove the vein system where the signals originated. Once I had all the valid signals removed from the vein and in my bucket it was obviously similar to the silver deposit I had found previously. After a more thorough examination I realized that I got mostly silver from this deposit with some small specks of gold included along the rich seam of this ore.
So the moral of this story is to learn the capabilities of your detector to the best of your abilities. When all else fails, dig, dig, dig, and dig some more until you are satisfied with the solution. If nothing else you will better learn what your detector is telling you. Dennis