I thought I would write a bit about the things I work with, hopefully this will only be the first of a row of posts about different geophysical methods.
The geophysical method I work with the most is called MEP, Multiple Electrode Profiling, I think in some areas this is better known as static DC-profiling.
The idea behind this method is to send an electric current between two points on the surface of the earth and measure the potential difference between two other points which then will give you some kind of resistance measurement.
Now this resistance says something about what it is you are sending the current through. Of course you measure higher resistance the further apart the electrodes are, but when you do some calculations on this you find out that this can be taken out of the equation by dividing by something called the geometrical factor, you then have something that is measured in ohms/m, called relative resistance which if you stand on a "homogeneous halfspace" (a flat earth consisting of the same material) will give the same output no matter how you arrange the electrodes. Now the earth is not a "homogeneous halfspace" but this relative resistance that you measure says something about what it is you send the current through.
Another thing that is being used is that the further apart the electrodes are, the deeper the current will run into the earth. So this way you can get a lot of information about the resistance of the earth. By doing measurements with different configurations of the electrodes you get ever more information on the resistance structure of the earth.
There are almost always more than one resistivity model that will fit data, but if you also have some other geological information and put on some boundaries on what kind of model you accept (like for example the 1-d model which says that the earth is composed of horizontal layers, which often is close to true) you can get a pretty good idea of what is beneath the surface.
The way this is done practically is most often (at least in my experience) with some special equipment from a company in Sweden, Abem, the company is closely connected to the university in Lund.
Usually you lie out 400 meters of multi conductor cable, connect that cable to the ground using spears that you connect to the cables outlets and then let the machine in the middle send out current and measure potential differences on a lot of preprogrammed configurations. When it is done you move 100 meters of cable to the other end of the line, move the machine back to the middle and let it measure again, then move another 100 meters and so on and on.
One of the good things of this method is that you can measure in almost any terrain as long as the fieldworkers are able to lie out the configuration in a straight line (can be a bigger challenge than what you might imagine, but it is can almost always be done). All the equipment and work can be done by two men carrying the equipment.
With the 400 meters configuration you can usually get reliable data down to at least 50 meters depth and sometimes even 200 meters depending a lot on what structure the underground has. As with almost all of these method the deeper you get the less resolution you will have on the structures in the underground.