Here is a link to the gas phase IR spectrum of methane ― the peaks should be around 3020 and 1300 cm⁻¹. I suspect that the absorptions you observe at 3300 and 1600 cm⁻¹ are due to water (see second link). 

If you cannot find the molar absorptivity coefficient as reference data somewhere, you can start to design an experiment to determine the concentration by constructing a calibration curve.

You'll have to have a way to prepare samples that have a range of concentrations of dissolved methane ― you could consider using an inert and IR silent gas like dinitrogen to displace the methane. The solubility of a gas in a liquid is dependant upon the partial pressure of that gas and its solubility constant, k, from Henry's Law (C = kP).

Henry's Law solubility constants can be found for methane at the first NIST link (I'd use 0.0014mol/kg*bar). Prepare several samples with known partial pressures of methane (mixed with nitrogen) over water and take the spectra. From that construct a calibration curve, fit your unknown, and calculate the concentration.

http://webbook.nist.gov/cgi/cbook.cgi?ID=C74828&Type=IR-SPEC&Index=1#IR-SPEC

http://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Type=IR-SPEC&Index=1#IR-SPEC

@Michael Smithback. thanks for your reply.  I have these transmittances as attached.

Molar absoptivity is better determined experimentally by constructing a calibration curve and calculating the molar absorptivity from the slope, because molar absorptivity is a function of slitwidth as well as wavelength.

I agree with Dr. Gharaibeh. Construct the calibration curve as described.

ID2 is water with a little carbon dioxide around 2350cm⁻¹.

BB2 looks to be a difference (subtracted) spectra with noise where the water manifolds should appear and again, the absorption at 2350cm⁻¹ is CO₂.

I do not see evidence of methane in these spectra.

thankyou @michael Smithback and Ahmad Gharaibeh for guiding. this interpretation was very helpful