Hi

Most measurement microphones extend well into the infrasound range. 

I believe there are several ways to calibrate microphones in the infrasound range.

The simplest is just to send the microphone to a laboratory for traceable calibration, e.g. this one which I find to be very reasonably priced and use myself (no commercial links between us)

http://www.cmrcalibrate.co.uk/ 

If you still wish to dig into the topic of calibration, I suggest taking a look at the Bruel and Kjaer Microphone Handbook. It is a bit old but very useful

http://www.bksv.com/doc/be1447.pdf

Sincerely

Claes

Alternatively, if you have any calibrated microphone on your side, then the simplest way to go is by coherence analysis. You may take a simple measurement with your microphone and the calibrated microphone. Put the test microphone on position x1 and the calibrated one on position x2. Switch on your any sound source and capture the signal with the two microphones. Do the FFT analysis and calculate the coherence of the captured signals. If you found the coherence value is 1 for the entire frequency, it means your microphone is fine. For further check purpose, you may also check the transfer function of the test microphone and set the calibrated microphone as the reference signal. Change your configuration and switch your test microphone position x2 and the calibrated microphone on position x1. Measure the transfer function with test microphone as a reference signal. Compare the transfer functions. If you found that the both transfer function is completely symmetrical, you may use your test microphone with no worries.

Norsonic delivers a low-frequency closed coupler calibrator with a range from below 0,1 Hz to 250 Hz. Below 50 Hz the response is quite flat. More information may be provided.