Stewart, T.J.; de Ronde, C.E.J. 2015 Mapping mid-water bubble (gas) plumes from Lake Rotoiti : a new tool for deciphering active lacustrine hydrothermal systems. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/53  p.
Abstract: A mid-water column study of bubble plumes within Lake Rotoiti in the Bay of Plenty has been undertaken for the first time. Lake Rotoiti sits astride the northwestern margin of the geothermally active Haroharo Caldera of the Okatanina Volcanic Centre. Fifteen line-kilometers of water column data and swath multibeam data were collected by a Kongsberg EM2040C multibeam echo sounder system over an area of known thermal activity on the lake floor. Water column data were processed and analysed by FMMidwater, a plug-in to the QPS Fledermaus software suite. Parameters and statistics were recorded for each individual plume before being exported as three different views. This water column data provides an insight into the physical nature of bubble venting and, when combined with high-resolution bathymetric data, spatial constraints of a lake floor hydrothermal system in Rotoiti. In total, 277 plumes were analysed within an area of 635 x 920 m. Two venting styles were identified: continuous (near) vertical plumes, and bubble clusters. The latter were focussed areas of high acoustic backscatter that could be either bubbles or fish. Plumes range in height, with some reaching the lake surface; the majority of plumes originate from the centre of one of the many craters. A method comparison analysis was also undertaken to determine the best way to select data to create a 3D volume object for individual plumes. (auth)