Hey Hanyuyue, sorry for the delayed response. I see that your deadline was 1st of Dec... I agree with rricenator's suggestion regarding SEG. You never know who you'll meet. People always remember your face or the event if you run into each other in 2-3 years' time. It worked for me, so far. The oilfield is a small world and I keep bumping into people I haven't seen for ages. There is the annual meeting in Denver in the last quarter of 2014, for instance. SEG is not the only group/society out there. There is AAPG, AGU, BGS, SPE. Most of these will take you as a student member with no costs or a minimal entrance fee.
There are many graduate schemes, offered by operator and service companies. I will suggest you to try your luck with those after you get your MSc. I'm sad to admit, if you are American or British, your chances are greater than other nationalities. However, it doesn't mean they will just accept you with open arms. I would recommend doing internships with operators or slightly smaller geophsysical companies during the summers to increase your chances. Attend seminars, show your interest in your CV that you didn't spend the past x number of years doing whatever.
Go to career days. I swear by them. It's how I got my internships, my current job. Those recruiters are there to hire folk! Even if you don't get hired in the end of the process, it will be a great opportunity to see how the interviews go. I recommend using glassdoor.com, to learn what to expect. See what Baker Hughes, Schlumberger do when they hire graduates, for instace.
To answer your question about Leeds/Imperial... To be honest, I don't think much of Leeds in respect to geophsyics. Imperial has a mad petroleum geoscience department though. Some lectures are given by guests from Shell, BP, Schlumberger. You get to make connections and learn about the sector directly from the people who work in it. I can't stress enough how much this matters, when you compare it to a lecturer who has no connection to the industry. Logging tools change every other month. There are new technologies.
These being said, I don't know how a Masters degree form Leeds or Imperial would put you ahead when you return to the States. Based on what would you think Imperial is any better than other universities in USA? Going international to UK may not be the solution.
Regarding the petrophysicist question: By itself, yes, there will be little advancement within the job. You will get amazing training though. Putting that aside, normally there is a fixed-step training programme that companies love to assign you. Normally people won't be doing the same job for over 5 years. Unless you insist on staying in your current department, there is always a grading system. For example, one can start as a field engineer (data acquisition), move to a support center (helping the field population, quality check data etc.) after a few years, then move to consulting (interpretation, where the petrophysicist would be), then maybe assistant domain champion etc.. I have not met anyone who got hired as a petrophysicist directly, normally, like I said, you would go through other steps to get to an interpretation position.
It's all a big race.
I am currently working for one of the 3 major service companies, if you want further information, I can gladly help you persoanally. By no means, I am an expert in anything, I've been doing this for about 2.5 years.