Don't worry about digressing. At this stage in the game, i.e. waiting for RPi, its good to think laterally about what can be done. The open source movement has always had the concept of re-usability so designing something that can be used for many different applications is better than a stand alone.
There are quite a few sites on the net where people have built proton magnetometers; have a look at:
This is one of the more sophisticated ones and uses quite a bit of electronics. I have not looked at it in detail but it may be possible to replace some by software.
Resistivity is relatively simple but I seem to remember that large power supplies/generators are needed? I seem to remember that there are some good archeology sites for it but would have to look.
I have not looked at gravitometers since I was a student but I seem to remember that they used quartz springs which are not easy things to make. Maybe there are better ways by now?
For seismic the problem would be the geophones. But I have just found this on youtube:
? 8:46? 8:46
and there seem to be quite a few more on googling 'diy geophone'. Maybe worth a look and a try.
As far as analysis goes, there are some very good open source mathematics apps; SAGE is a Mathematica/Maple clone, Octave is a Matlab clone. They have some very sophisticated graphics and things like DFTs are no problem. There may well be open source geophysics code for these platforms out there.
Tech_Monkey's site give a tablet for £46 which is even better. However, I did some reading of reviews on cheap Android tablets and its not too clear what processor they are using. Some of the more expensive tablets, ca. £150, have ARM processors and have had Debian mounted on them in dual boot with Android. From a practical point of view this seems desirable since it means that a much wider range of existing software/languages can be used. With Android you have to work with Java. I don't know Java but I think that there are APIs for using C, python etc. but I would rather not have to go to the trouble of learning another language just at the moment. Most negative comments I have seen about the cheap tablets concern not being able to play various games on them. If anyone knows about getting Linux on these cheap tablets please let me know. For the moment I will concentrate on the RPi/instrumentation side I think.