Is it necessary to do a survey or emperial research for law related research? Is case studies not enough.? Like secondary sources only

You will have to first decide whether you want your research to be a Doctrinal one ( Where you use case studies etc and primary & Secondary sources) or an Empirical one ( Surveys, Graphical Data, Data from Govt sites are used).

Where do theory type topics fall under ? Like jurisprudence or human rights theory or TWAIL etc ?

Research on HRs related topics can be done through both the methods. Jurisprudence falls under Doctrinal research category.

Thanks. Last question, Is it true that empirical research is preferred more than doctrinal. Or are both the same just depends on the output being produced?

Neither is preferred over the other, your ability to use them to produce good output is what matters.

Even TWAIL falls under the doctrinal research category..

Such ignorant views aired here! The research domain and methodology are two completely different things. One can conduct empirical research in jurisprudence and theories of TWAIL. Just like one can conduct doctrinal research in almost any field of law.

Oh great one...please do explain..how empirical research can be done in a theoretical field ?

I lack the bandwidth to fully explain the details here, but you may try looking up for example the work being done at KU Leven's Centre for Legal Theory and Empirical Jurisprudence. LI trolls obviously won't know real research if it comes and dances before them, but for those genuinely interested, the underlying principle is simple, in order to assess the impact of a development through the lens of a theory, empirical data may still need to be collected and analysed.