Seafloor
Instruments for monitoring submarine volcanoes
One of the
goals of the NeMO Program
is to have monitoring instruments in place at Axial Volcano. The instruments
described below are used to measure vertical and horizontal movements
of the seafloor. Why would we want to do that? Well, we're studying how
submarine volcanoes work, and we know from volcanoes on land that the
surfaces of volcanoes actually deform in response to movements of magma
(molten rock) underground. For example, if magma is accumulating beneath
a volcano, the ground may gradually swell or inflate. Or if a large volume
of magma is removed, the ground may suddenly subside or deflate. So by
making precise measurements of ground movements on the surface, we can
interpret what is going on underground inside a volcano. Measuring ground
deformation is one of the main methods scientists use to monitor volcanoes
on land, but the techniques that work best on land (GPS, lasers, optical
leveling) are impossible to use in the deep ocean. So we had to come up
with new instruments to measure ground movements on the seafloor. As explained
below, Bottom Pressure Recorders can measure vertical ground
movements and Acoustic Extensometers can measure horizontal
ground movements of the seafloor.
Bottom Pressure
Recorders
Bottom Pressure
Recorders (BPRs) precisely measure the pressure from the overlying ocean
at a specific site. Since the pressure is a function of the height of
the water column above them these instruments can measure vertical movements
of the seafloor, after the predictable variations from ocean tides are
removed. Thus, if the seafloor moves up or down, then the instrument measures
that there is less or more pressure, respectively. BPRs are usually deployed
for a year or two at a time and make a measurement every 15 seconds. As
part of the early stages of the NeMO Project, there were BPRs in each
of the "rumbleometer" instruments (developed
by Chris Fox) that were in place at Axial volcano when the 1998 eruption
occured. The 1998 eruption caused a large volume of magma that had been
stored in a reservoir beneath Axial's summit caldera to be removed and
intruded into the south rift zone. The removal of this large volume of
magma caused the whole summit of Axial seamount to subside, like a balloon
that had some of the air taken out of it. The BPR that was at the center
of Axial caldera at the time of the 1998 eruption measured 3.2 meters
(10.5 feet!) of subsidence. Another BPR was in the rumbleometer that was
stuck in the 1998 lava flow until it was extracted in the summer of 1999.
This second BPR showed a subsidence of 1.4 meters (4.6 feet) at the eruption
site, which is 3.5 km (~2 miles) from the caldera center. These two measurements
can be combined with the Acoustic Extensometer measurements (described
below) to model the depth of the magma reservoir beneath Axial caldera
that caused these ground movements and how much volume was removed during
the 1998 eruption.
For more
information:
Go to the
NeMO Explorer
web site for Virtual views of the rumbleometer
stuck in the lava flow and an animation
of what happened to the rumbleometer during the eruption. Also see the
NeMO Net
web site to see BPR data transmitted back to shore in near real time from
the seafloor at Axial volcano
Photo
of the rumbleometer stuck in the 1998 lava flow
Photo of ROPOS rigging
the stuck rumbleometer to be pulled out of the lava flow
Photo
of the previously stuck rumbleometer now released from the seafloor
Photo
of the rumbleometer back on deck after it was rescued
Photo
of one of the "hard hats" on the rumbleometer slightly melted
by the lava
Plot of the BPR data from the two rumbleometers
showing subsidence of Axial caldera at the onset of the 1998 eruption
Plot showing a model for vertical and
horizontal movements for Axial
Acoustic
Extensometers
[pronounced
"x-ten-som-i-ters"]
"Extensometers"
are instruments that measure extension - the increase (or decrease) in
horizontal distance between two fixed points. To measure distance underwater
we use sound. The speed that sound travels in seawater can be precisely
calculated if you know the temperature, pressure, and salinity. The extensometers
measure distance by carefully recording the travel time of acoustic pulses
between pairs of instruments (and the water temperature). If we know the
time and the speed, then we can calculate the distance. The travel times
are clocked to the nearest 500 nanoseconds (that's 500 billionths of a
second!) and the distance of each range measurement can be calculated
to less than half a millimeter (0.015 inches)! Of course there are other
errors involved so ultimately the distance measurements are good to about
1 cm. The instruments are designed to be deployed in a segmented array,
somewhat like a picket fence, with about 100 m between adjacent instruments.
The total distance spanned is dependent on the total number of instruments,
but is intended to be between 0.5 and 1.5 kilometers (about 1/2 mile).
The instruments make one set of distance measurements each day. The extensometers
are specifically designed to detect seafloor spreading events across the
axis of a mid-ocean ridge, and establish relationships with other oceanographic
or geophysical phenomena, such as earthquake swarms, hydrothermal venting,
and biological responses. Seafloor extension across the ridge may occur
due to faulting or magma intrusion or both.
There are
now two versions of the Extensometer instruments, 1) Prototype Extensometers,
which were built first, and 2) Benchmark Extensometers, which are new
upgraded versions with more capabilities. The Prototype Extensometers
were designed and built with funding from NOAA/NURP
and can only record for 1-2 years and then have to be physically recovered
to get the data back. The Benchmark instruments, funded by NSF/RIDGE,
are redesigned to be able to stay down for 5 years or more, and are deployed
in separate bases that will remain permanently on the seafloor to facilitate
long-term measurements. Data can be retrieved by an ROV (via an infrared
data port) without disturbing the instruments. After 5 years the instruments
could be taken out of the bases, have their batteries replaced, and then
put back in the same bases to continue the measurements.
1) Prototype
Extensometers
The Prototype
Extensometers were in place at Axial volcano at the time of the January
1998 eruption. They were located along the north rift zone (7 km from
the center of Axial caldera) and they measured a 4 cm distance decrease,
related to the deflation of the volcano summit when the eruption occured
and magma was intruded into the south rift zone. When combined with the
subsidence measured by 2 BPR instruments deployed in the caldera (see
above), the extensometer data can be used to model the depth to the magma
reservoir beneath Axial's caldera (3.8 km).
Photo
of a Prototype Extensometer instrument on deck ready for deployment
Photo of a prototype instrument deployed
on the seafloor at Axial volcano
Map
of Axial summit showing the location of the BPRs ("rumbleometers")
and Extensometers at Axial
Data showing the distance change measured
at the time of the 1998 eruption
Plot showing a model for vertical and
horizontal movements for Axial
2) Benchmark
Extensometers
The Benchmark
Extensometers look different, but operate in basically same way as the
Prototype instruments. An array of Benchmark Extensometer instruments
was deployed in July 2000 at the NSF/RIDGE sponsored seafloor observatory
on the Cleft segment, on the Juan de Fuca Ridge. Data was recovered from
them in July 2001 and showed that no seafloor spreading events had occurred
during the first year. The instruments are still at Cleft recording data.
Photo
of a Benchmark Extensometer instrument ready for deployment
Photo of a close-up of the instrument
installed in the benchmark base
Schematic diagram of a Benchmark Extensometer
Diagram of how the instruments are
deployed across the Cleft segment
Bathymetric map of the extensometer
deployment site at the Cleft segment
last updated:
08/29/02 by Bill Chadwick |