Sounds like me self learning.....(our teacher just showed us how to maximise marks and stuff in HSC and was good at giving us exam preparation but the content was up to us to learn).
Just be careful.....if you have a topic assessment and its on a random option, then it might be a problem. Make sure your teacher explicitly knows that you are doing a completely different option. Also, it would be best if you had a few more people (preferably friends who would discuss stuff with you) so that you can learn from each others mistakes and stuff.
Also, I would recommend you do it only if there is NO assessment for the option modules.
I have no idea about Geophysics but I'm guessing it has a lot to do with seismic activity and tectonic plates and how the Earth behaves and its history and there is like 2% chance possibly of calculation questions in HSC if I remember looking to the section properly briefly.
Depending on your circumstances as I have iterated here, and here:
http://community.boredofstudies.org/18/physics/321171/physics-options.html
your decision might be constricted.....
You have 1 year left to decide it Axio, but I suggest you look into your performance and preference in 7 months time too, then keep all the options in your mind, then choose one close to the point you need to choose.
About 3-6 marks of HSC is allocated to analysing circuits, 3-5 marks are allocated to logic circuit design, 3-10 marks are allocated for LDR, Thermistors, LEDs, Cameras and transducers/interfacing, 3-5 marks on amplifiers and a big 6-8 marker at the end of the paper is allocated on a random discussion topic involving generally the future of semiconductor technology/sustainability of it continually improving or something related to how it has developed or the real impacts on society.
Its a neat intro into electronics and has a little bit of intro into circuits (it could have been better here but the electronics stuff is just as great and it is called Age of Silicon not "Age of Electrics and Electronics").