its about 1 AND A HALF INCH
I very much doubt it is methane, it probably is related to respiratory i.e carbon dioxide and/or nitrogen gas from nitrate reduction. You could test it by stirring the sand to release the gas and hold a naked flame near the water surface where the bubbles emerge. If you lose your eye brows then it probably is methane however I think the more probable result will be extinguishing the flame.
Hope this helps
Joe