# 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.

• If you are in a magnetic environment and you feel that that your target may or may not be detectable. There are two paths you can take. First use magnetic anomaly forward modelling code to simulate the setting and the target. A geophysicist should be consulted if this is the route you want to go. Secondly, Try another geophysical method in addition to magnetometry. Since You can't see beneath your feet, two images are recommended to reduce uncertainty in both location and depth. Its like using a flashlight in a dark room first you shine a flashlight into the middle of the room and make out gen Mar 14, 2016 at 20:31
• (Michael Wallace comment continued) general shapes and size of objects. Then you step into the room and shine your flashlight to pinpoint the specific item you are looking for. With Iron its possible there could be an electrical conductivity contrast that is detectable and may be easily located with EM conductivity as well as magnetometry. (Michael Wallace)
– gerrit
Mar 15, 2016 at 17:47
• (Michael Wallace comment continued) There are some great youtube videos on using geophysics for archaeology that should help to explain the methodologies and when to use them. youtube.com/watch?v=lU9aRZK4j84 My apologies about not answering the question as specifically as you want I just don't think anybody will run the forward model for free. (Michael Wallace)
– gerrit
Mar 15, 2016 at 17:48
• @MichaelWallace Hi Michael. I converted your answer to a comment on the question because it didn't really answer the question. Hope that's okay!
– gerrit
Mar 15, 2016 at 17:48
• @gerrit Finally got enough rep to comment :) Thank you for preserving my insight. That said Still a little miffed about the down vote. Pavel V. if you tell me your location maybe I can suggest a geophysicist to help you since I have a lot of contacts and have worked all over the world. Otherwise please award a winner as the answer given is the best your going to get for free. Apr 3, 2016 at 18:16

## 1 Answer

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.

• Slightly better than before, but still not what I need. Your first paragraph shows that you understood what I mean. But as an archaeologist, all I need to know is which situations I should and shouldn't discuss with geophysics experts or with fellow archaeologists with some background in geophysics. I already knew what can be found (from school, reminded by the wiki article) and that I could spend months doing literature search so that I really understood the problematics. But I simply can't afford this. Apr 29, 2014 at 17:50
• @PavelV. This maybe the best answer you get for free. See my comments under your question. Its possible that with some legacy data a university may be able to apply some magnetic forward modelling to give relative strengths of anomalies like you are asking for. Also Consulting an experienced geophysicist is always recommended because I think you would rather have an accurate and well informed answer than more questions. Apr 3, 2016 at 18:09