IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS016 Oral Presentation 2342
Development of a pop-up ocean bottom station for investigations of
groundwater seepage and underwater earthquakes
Dr. Aleksey Kontar
Geophysics P.P.Shirshov Institute of Oceanology IASPEI
Submarine groundwater seepage could be of importance for variousunderwater operations (acoustic
interference, etc.). Radon (Rn-222) can be a valuable tracer of direct groundwater discharge on the
ocean floor. Also changes in the radon emission of groundwater have come to be known as one of the
precursory phenomena of a submarine earthquake. Although Rn-222 in water may be measured reliably
by classical methods such as radon emanation techniques, this approach can only provide information
about water bodies over limited time periods. Ideally, studies of groundwater discharge would include
measurements on the bottom of the ocean of dissolved radon concentrations integrated over various
periods including time scales of days to weeks using a pop-up bottom station. Unfortunately, fine-scale
temporal analysis is invariably limited by sampling logistics and time constraints. Therefore, it is
desirable to develop a detection system which could be deployed on the bottom of the ocean and
provide monitoring either in real time, for a rapid site assessment, or moored for a more extended
period to provide a continuous record. We have designed a few varieties of an underwater radon
detection system, suitable for deployment on the bottom of the ocean. One design is based on a new
plastic scintillator with unique features that make it appropriate for in situ measurement of the alphaactivity
of natural waters. A negative charge on the detector is used to attract positively-charged Rn-222
daughters to the detector surface. Monitoring of the 6.0 MeV alpha particles associated with the decay
of Po-218 allowed concentrations of 4 pCi/L to be determined in 15 minutes with an uncertainty of 11%.
The detector is made of a silicon semi-conducting PIN photodiode, an amplifier module, a water-proof
connector and a bias battery. All of these are protected from water by a hydrophobic microporous
membrane which allows diffusion of radon gas, but excludes water. The 25 m thick polypropylene
hydrophobic membrane has a pore size of 0.04x0.12 m and a 35% porosity, and is attached as a flat
sheet to the end of the cylindrical container. The microporous membrane is pinched between flanges of
the container and reinforced with a chlorovinyl board with 56 openings (1.0 cm ID each). These
characteristics allow only gas dissolved in the water to pass into the detector chamber, except at very
high pressures when water will move through the pores. A hydrophobic and insoluble polymer is used
as a polymer base for the plastic scintillator. This material is superior to such polymers as polycarbonate
and polyethylene-terephthalate with respect to size stability when humidity increases. The thickness of
the plastic scintillator is set equal to an average path length of the alpha-particles. The scintillator is
placed on the outer surface of a transparent (visual spectrum band of the electrical wave range)
waterproof case so direct contact with the environment is achieved. A photo-detector with a large
diameter (370-382 mm) hemispheric photoelectric cathode is located inside the waterproof case. This
scheme permits monitoring large water volumes with a relatively small sized counter (a 450-500 mm
diameter sphere) with low background (less than one background pulse per minute). The electronic
equipment consists of a threshold anode pulse discriminator; a transformation unit for converting
battery voltage into rated voltage values to supply the photo-detector and electronic equipment; a
counter, timer, and buffer memory for brief and permanent data storage; a signal forming unit with a
hydroacoustic package unit; and a hydroacoustic release mechanism. Data may be transmitted over the
hydroacoustic channel and written on the permanent data carrier. A lithium battery supply unit could
supply a minimum of 4,500 hours of continuous operation. Various deployment scenarios of a new popup
ocean bottom station also were proposed.