How far from basalt bedrock can magnetometic survey find archaeological features? Magnetometry is used to find archaeological features such as stone walls or ancient hearth. But it usually cannot be used for archaeological prospection in areas where the bedrock is strongly magnetic. Let's assume basalt - it is quite common near the town I come from. With an average cesium vapour magnetometer, how thick must be the soil so that I could find remains of highly magnetic features like remains of a burnt wattle-and-daub house or an iron depot? And if operating near a fault between magnetic and non-magnetic rock, how far from the magnetic rock can I find weakly magnetic situations like disturbed soil?

EDIT: I expect approximate answer. An answer I would expect could start: "Iron is more magnetic than basalt, so several kilograms of iron (or iron rust) could be found even less than a meter above the bedrock under condidtions X, several meters otherwise. Very small iron items and piles of burnt soil would usually need several tens of meters..." I just guess all the facts, I don't know, I'm just an archaeologist with no background i geophysics. The aim of this edit is to emphasize that I don't need exact numbers, but I need an idea which numbers might be exact and which are likely to be completely wrong.
You can get a good idea of the magnitude and wavelengths of typical archaeological magnetic anomalies with a literature search. For instance this page shows a magnetic map of fire hearths http://en.wikipedia.org/wiki/Magnetic_survey_(archaeology)

Geology is a lot more complicated though and there are many variables that might affect the background magnetic signature. It doesn't matter how much you study up on basalt rock properties you still won't know enough about the rocks in your project area to predict the outcome of a survey. 

Your best hope is to use prior experience from other surveys close to your project area. If a survey worked in one farmer's field then it will probably work in the adjacent field as well. 

If no one has collected any magnetic data close to your project area then go ask the geophysicists if they will do a test line or two. If it works great, if not then try something else like magnetic gradiometry, resistivity or ground probing radar. 

Sorry I can't give you a definitive answer to your question. I understand what you are asking but geology is just too complex to give you a useful rule of thumb.