Drilling's isn't the first thing that comes to my mind when I think of geophysical methods and I guess it is more geology and geophysics, never the less that is something I work a lot with these days and it is a method that should never be ignored.
Drilling really seems quite straightforward and it basically is very simple. Making a hole in the earth with pretty much the same radius all the way down.
In general something can be said about drilling as such when compared to other methods:
- You only get information from one horizontal point (or maybe more accurately a small area with the radius of the drilling and perhaps with certain log-methods some information from the earth surrounding the drilling but I have yet to see a drilling to be used as more than a point information for all practical purposes) to the depth that you drill.
- You usually get extremely accurate information, when used with other geophysical methods drilling information is often used as very certain information which is used to check data and improve on the processing of data acquired through other geophysics methods.
There are many ways of doing drilling's, each with it's advantages and disadvantages.
One method is using a real drill head and turning/pushing that down the earth, the big advantage of this is that if you pull up the drill every time you have drilled the depth of the drill head you get a pretty undisturbed sample of the earth at that depth, that makes it possible to make very accurate description of the earth. One of the big disadvantages of this method is that it is far from always possible to do this, the harder the earth is the more force is needed to turn and push down the drill and the more strength is required from the drilling equipment.
This method is what is for example usually used for geotechnical investigations to check the underground before a building is build and to decide what kind of foundation is needed, for this purpose the drilling's need not be much deeper than to the layer which can carry the weight of the building which often here in Denmark means only a few meters for a normal house even though sometimes deeper drilling's are needed even for small houses depending very much on what is found in the drilling.