First measurements of the methane concentration in the North Sea with a new in-situ device

Abstract

A device is briefly described for the continuous in-situ measurement of methane gas dissolved in sea water. The device extracts dissolved gas from a continuous flow of water and quantitatively determines the methane partial pressure in the gas phase by laser absorption spectrometry. A lower detection limit of 30 ppt (parts per trillion, 30 pg of methane per g of water) is achieved, and the dynamic range exceeds six orders of magnitude. Thus, the natural background concentration of the open ocean (35 ppt) can be measured. First measurements with the device have been taken in the North Sea. Depth profiles at Statpipe, in the Bømla Fjord and at the location of a seepage in the Gullfaks area are shown. Horizontal profiles across a natural seepage show the distribution of the methane around the source. Measurements at a gas injection wellhead may give a hint of a microleakage of the structure.