MPR’s and BPR’s

Dr. Nooner, what have you accomplished so far during the pressure dive?
“We have visited the majority of the benchmarks and made pressure measurements at them. In 2011 an eruption at Axial buried two out of six benchmarks that we had in place at the time (along with one BPR, one hydrophone, and a fluid sampler). We have put in new benchmarks for the two that were lost and an additional four to expand the network. For the first visit at each of these new benchmarks we have to move them to a stable, flat location where they will spend the rest of their lives growing old. Then we release the floats which pop up to the sea surface and are recovered by the deck crew onto the ship. This takes a fair amount of time, so this first circuit is a long one.” –Dr. Nooner

Dr. Nooner plugging in the MPR's just before the launch.

Dr. Nooner plugging in the MPR’s just before the launch.

So far our Jason dive is going very smoothly, we have successfully made pressure measurements at 9 benchmarks over the last 24 hours. We are making measurements at each benchmark beginning in the center of the caldera and moving north then east and south in somewhat of a question mark pattern covering all ten benchmarks as efficiently as possible. We are hoping to traverse the course (down and back up) two full times totaling four measurements at almost every benchmark within these 5 days.
For these ROV based measurements, we are using what we call Mobile Pressure Recorders (MPR’s) which are carefully placed in the black rectangular indentions on the benchmarks for about 20 minutes allowing the pressure sensors to stabilize. Essentially, these instruments measure the pressure at that particular location on the seafloor. The benchmarks make sure that we are measuring the pressure at precisely the same location each time we come here. Two of the problems faced by using MPR’s are accounting for tides and drift. Making multiple measurements at all the benchmarks over a short period of time (hours to days) allows us to calculate the drift. The tides are dealt with using a small tide gauge that we leave on the seafloor for the duration of the survey. Once the data is brought back to the lab it is corrected for tides and the pressures are converted to a corresponding depth in meters. We then use the depths to monitor the inflation or deflation of the volcano. Since Axial erupted in 2011 the volcano is currently re-inflating as magma makes its way from deep within the Earth and into the magma chamber, priming it for its’ next eruption!

Jason being lowered into the water for the pressure dives.

Jason being lowered into the water for the pressure dives.

Data from the MPR’s is useful for long-term inflation and deflation cycles but since measurements can only be made every year or two (when there is funding for a cruise), Bottom Pressure Recorders (BPR’s) are left on the seafloor for an extended period of time. BPR’s also record pressure in a specific location on the seafloor, but unlike MPR’s they are battery powered and record a measurement every 15 seconds. The main reason that BPR’s cannot solely be used is because the drift of the instruments cannot be distinguished from motion of the seafloor. We constraint this drift using our MPR measurements. Back in the lab, the BPR data is corrected for tides and converted to a corresponding depth. Since these instruments are left on the seafloor for 1-2 years, they are critical for collecting rapid inflation or deflation events, such as an eruption (BPR data confirmed the 2011 eruption).

Combining the data received from both MPR’s and BPR’s proves to be the most efficient and effective way to monitor the overall inflation-deflation cycle of Axial.

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