Geophysical Methods For The Investigation Of Subsurface

[Valerija]

Hi, I have a course work where I should suggest geophysical methods for the investigation for the construction of the rock tunnel. I know the location of the tectonic zone and I need to investigate soil conditions and bedrock quality.
Unfortunately I have no previous experience with geology. Hence, would like to get some suggestions, please.
Tunnel depth will be between 30 and 100 m below the ground. It is known that soil depth is 3 - 10 m and it mainly consists of till, sometimes covered by peat. Minor areas consist of sand and gravel. Gneiss is dominating rock type and it can be cut by amphibolite dykes.

[Davor]

Mafic minerals are usually a problem for detection by electromagnetic methods. In this case your biggest problem is depth and diverse deposits that will confuse identification. Commercial GPR penetrates to maximum of, say, 10m, and it is not enough for your application. There are GPR-s described in literature that operate at lower frequencies, but I have no idea if there are any of those available commercially. There are so-called "cable locators" that operate with distant transmitter and detect deep buried objects, but I'm not betting my sandwich for those to help you with your quest, as there is nothing specific to detect.
I'd say acoustical imaging is better suited for this task. It will indicate bedrock as homogeneous mass, while sediments will be indicated as rough material deposits, and you'll clearly see boundaries between those, and also slopes of tectonic zones.

[Dave J.]

I agree with Davor, this is a job for microseismology. But you need to know in advance that it's not a matter of just walking around with some kind of gadget that will tell you where things are. It involves complicated field procedures and complex data analysis to map the data and then to interpret what the data means in terms of engineering. If you don't have background in geophysics and math, it's not something you'll learn on your own in three weeks. You'll need to hire someone who already has that knowledge and experience.

Some information can also be gained by other methods. Proton magnetometry is routine geophysical investigation stuff, and vector magnetometry using fluxgates is also a method with a long history. Then there's various electromagnetic methods. And resistivity surveying. And core drilling. None of these methods gives a complete picture of underground structure, none are easy to use correctly, and all of them require extensive data analysis in order to interpret the measurements as geological structure. In most geophysical investigations, multiple methods are used since the different methods produce different results and combining the information presents a more accurate and detailed picture of the geological structure than any one method used by itself. `

I'm guessing that your biggest engineering concerns are the structural strength of the rock. You don't want to tunnel through glacial till if it can be avoided, meaning you want to know how deep the bedrock surface is. Since you've got mostly solid rock with glacial till and sediment overburden, resistivity surveying will probably give good results and is among the simplest and least expensive of the geophysical methods, you could possibly learn it on your own (or explain it or even demo it in coursework which ever comes first).

Investigating bedrock quality with that much overburden presents some challenges. If the amphibolite dikes are less than a meter thick, you're probably out of luck. If they're several meters thick you may be able to pick them up using magnetometry. However, from the perspective of constructing the tunnel, it has to go where it has to go, and the amphibolite dikes (which might be associated with lower strength and higher water permeability) will just have to be dealt with when you encounter them. Same with active shear zones if any. Active shear zones are usually aquifers and lack bearing strength; furthermore in tectonically active regions they represent places where structural failure may occur after the tunnel is already built. An active shear zone may show up as a topographic discontinuity on the map of the bedrock surface.