Download Seafloor Compliance Apparatus Guide

Seafloor Compliance Apparatus Guide
Dr. Eleanor Willoughby
April 17, 2012
The seafloor compliance (SFC) is the transfer function between pressure and displacement at the
seafloor. The source of the pressure changes are wind- and coastline-excited ocean surface gravity
waves, which, though evanescent, propagate down to the seafloor and measurably palpitate the ocean
bottom. The ratio (more precisely, transfer function) between the pressure and the deformation is
extremely sensitive to the shear modulus of the underlying material. This in turn, can be related to the
materials which make up the seafloor and their time evolution. Hence, SFC data have been used to
characterize gas hydrate deposits, which stiffen sediments when they displace seawater, and to monitor
partial melt at mid-ocean ridges, which present an anomalously low shear modulus.
This guide summarizes the hardware, software, use and maintenance of the NEPTUNE Canada SFC
apparatus.
The Apparatus
To measure minute changes in pressure a Scripps Institute of Oceanography (SIO) Cox-Webb differential
Pressure Gauge (DPG) can be used. Measuring minute displacements of the seafloor poses a serious
technical problem as displacements can be as small as microns! The solution is to measure analogues of
displacement. For instance, typical precision gravimeters have resolutions of 0.1 μgal or 10-9 m/s2 which
are appropriate for compliance measurements. The NC SFC apparatus contains a custom Micro gLacoste gPhone gravimeter. To produce seafloor compliance spectra, time series data of pressure and
acceleration must be gathered at 1 second sampling rates.
1. Gravimeter
The gPhone gravimeter has ± 50 μgal dynamic range. It is based on a zero-length spring
suspension system; acceleration data are the voltages applied to return the beam to its null
position. It provides precise digital measurements of acceleration with 0.1 μgal resolution. The
provided gMonitor software allows for the recording of location, time, date, raw gravity, sensor
temperature, outer oven temperature, cross and long level values, and level corrections. The
instrument itself has been customized for this deployment. Rather than recording atmospheric
pressure (a redundant feature for an ocean-bottom deployment!) it has been interfaced to the
DPG so that its ADC records the pressure data. The gMonitor program also handles two-way
communication with the gravimeter, including the clamping and unclamping of the sensor.
NOTE: during any transportation the sensor must be CLAMPED. Prior to recording meaningful
data in situ it is necessary to communicate with the gPhone via gMonitor to set the sensor to
UNCLAMPED.
gPhone Specifications
System Performance
Resolution: 0.1 μGal
Precision: 1 μGal
System Noise: 6 μGal/√Hz or better
Drift: 1.5 milliGal (or better) per month when new. With aging, drift values are usually less
than 500 μGal/month
Range: 7,000 milliGal uncalibrated (worldwide)
Feedback range (during measurement): ±100 milliGals
System Performance
Size: 33 x 27 x 46 cm
Mass of sensor: 14 kg
Power: 24 V
Further details on the gPhone can be found in the gPhone Brochure and gPhone/P.E.T. Hardware
Manual (both available on the NEPTUNE Canada wiki). Note however that ‘TideDaq’ software has been
replaced with gMonitor, the instrument has been customized (so there is no atmospheric pressure data,
but it does record the DPG data) and the power specifications and the Timing Module are not pertinent
to this instrument. For information on software, see the gMonitor help files.
NOTE: The instrument manufacturer’s warranty ran for 1 year from the date of purchase (2007) and has
now expired. The manufacturers however have shown great enthusiasm for NEPTUNE Canada and have
always been extremely helpful with any questions or difficulties.
2. Differential Pressure Gauge
The DPG is a SIO DPG. It measures minute changes in pressure by comparing ambient ocean
pressure introduced via a diaphragm on the bottom with the pressure in a reference chamber
by means of resistor bridge (specifically a piezoresistive strain gauges implanted into a
Wheatstone bridge configuration formed on a miniature silicon diaphragm, a Lucas NovaSensor
NPH Solid State Pressure Sensor). The sensor requires a constant current excitation to produce a
voltage output. A ±7.2 V power input is rectified to 5V (on the analogue circuit board, following
the SIO circuit design for the DPG power board) and fed to the DPG across two pins on its
bulkhead connector (Impulse VSG-4-FS); the sensed voltage returns across the other two pins.
The signal is input to a special analogue input on the custom designed gPhone.
Figure 1 Typical analog board for a SIO DPG. The U of T version is based on the one designed by SIO, like the one above found
on the NEPTUNE wiki.
Further information can be found in the General Differential Pressure Gauge (DPG) Documentation
(available on the NEPTUNE Canada wiki) and in Cox, C.,T., Deaton and S.C. Webb, ‘A Deep-Sea
Differential Pressure Gauge’, Journal of Atmospheric and Oceanic Technology, (1984), volume 1, pp.
237-246.
The DPG calibration is 0.186 ± 0.008 mV/Pa (see E.C. Willoughby, ‘Resource Evaluation of Marine Gas
Hydrate Deposits Using the Seafloor Compliance Method: Experimental Methods and Results, Ph.D.
thesis, (2003), University of Toronto).
Note: The DPG has been replaced since the initial SFC deployment at Bullseye Vent. The original DPG did
not perform to spec. The manufacturer’s warranty only covered its initial shipment to the University of
Toronto and there is no further warranty.
3. Power supply: 48 V to 24 V converter
The 48VDC power supply from the NEPTUNE Canada port is connected to the instrument and wired
directly to a 48V to 24V converter, mounted within the pressure sphere, on the bottom of the
gravimeter gimbals. Thus, the power supply does double-duty as ballast, ensuring the gravimeter
gimbals will swing like a pendulum, and reach equilibrium with the gravimeter upright, very close to
perfectly level.
The power supply is a VHK-100W-Q48-S24. It features regulated output with continuous short circuit
protection and means that the entire SFC apparatus is electrically isolated from the NEPTUNE Canada
power supply. It is 87% efficient and contains its own heat sink. It has its own over-temperature shutdown feature. A full data sheet can be found at:
http://products.cui.com/Product/Resource/3318/VHK100W.pdf
A schematic diagram for the wiring is below.
Figure 2 Schematic wiring diagram for SFC. Note the device is completely electrically isolated from NEPTUNE.
4. Power supply: 18 to 36 VDC to ± 7.2 VDC converter
To power the analog DPG circuit board, ±7V are required. A custom-designed 24V to ±7.2 V converter
was produced by TRUMPower. It is fully isolated, with continuous short circuit protection. A datasheet
was provided to NEPTUNE.
5. Communications: Moxa NPort 5210 RS-232 to Ethernet converter
The output from the DPG is interfaced to the modified gPhone and logged on its internal analog-todigital (Delta-Sigma type) converter. Thus, the gPhone handles all data and two-way communication. It
outputs its datastream in RS-232, which is converted to Ethernet by an NPort 5210 (see
http://www.moxa.com/product/nport_5210.htm ). This device is similar to those employed by
NEPTUNE for communication in all ports. The NPort 5210 requires 24V to operate.
6. Gimbals
The gPhone needs to be upright to record vertical accelerations of the seafloor. Ideally, it would be
placed within remotely-controlled, self-levelling gimbals. However, these can be extremely expensive.
We have employed our custom gimbals and allowed them to be freely moving, instead, as a means of
remaining within budget. The gravimeter in its gimbals moves like a pendulum until it settles at its
equilibrium, very nearly perfectly levelled. Laborious, precision fine-tuning of weights on the gimbals
and levelling screws are needed so that gimbals natural fall to this near perfect position. It is important
that the position selected for instrument deployment be a flat a possible (on a scale comparable to
deployment package, roughly 2 m in diameter). When communicating with the gPhone, via gMonitor
software, one can monitor the Long and Cross measurements. These record the divergence of the
sensor from being perfectly plum, in two orthogonal directions. They are in arbitrary units. If the data
are of the order of 30,000, then the readings are off-scale and cannot be trusted. Ideally, both Long and
Cross would read 0 units (though this sort of perfection is virtually impossible). Readings of less than
6,000 units are best for good quality data.
7. Pressure vessel and deployment package
The 60 cm diameter pressure sphere is rated for full ocean depths, and can be used down to 6 km. It is
made of anodized aluminum. It has two half-spheres, and a mid-ring (complete with o-ring groves and orings, and three ports to the exterior). The ports allow for 1) connection to NEPTUNE via MINK
connector with Delrin isolator; 2) connection to DPG and 3) connection to Prevco Pressure Release Valve
via Delrin isolator. The sphere is bolted shut with bolts on the threaded rods on the triangular, Delrin
bottom assembly box, and the thread rod on the tripod (each or which are supplied with a sacrificial
anode). There are lead weights below the triangular box to ensure the device lands upright if dropped
free-fall (not recommended – but the deployment package should be rugged enough to protect the
instrument should this happen). The DPG is mounted in one of three corner, grey, cylinders, on the
triangular bottom assembly box. Bolted to the side of the bottom assembly box is a holder for the
under-water-mateable connection.
Prior to deployment: the stainless steel screws on the MINK connector, holding it onto the isolator,
should be replaced with titanium screws, to ensure no dissimilar metals are in direct contact. The
MINK to under-water-mateable whip must also be added.
8. Pressure release valve
A standard 01104-001 Titanium Pressure Relief Valve from PREVCO Subsea LLC (see
http://www.prevco.com/Products/PressureRV.htm) was included on an otherwise unused mid-ring
port, as per NEPTUNE’s instructions. Since the sphere is anodized aluminum, a special Delrin isolator was
designed to prevent it from touching the sphere directly, to avoid corrosion.
9. Marine cables and bulkhead connectors
The power to and data from the DPG comes from the pressure sphere via Teledyne Impulse 4-pin VSG
marine cable and bulkhead connector (datasheet: http://www.teledyneimpulse.com/pdfs/1050/25InvGlassReinfEpoxyRubMolded/25.VSG_VMG.pdf)
10. Delrin isolators
Two custom designed Delrin isolators were engineered and manufactured in-house at the Department
of Physics of the University of Toronto. These allow the titanium Pressure Release Valve and MINK
connector to be safely mated to the anodized aluminum pressure sphere. The concept is similar to
isolators employed on the CSEM deployment. An o-ring seal (both barrel and face seals) is made
between the isolator and the mid-ring, and a second seal is made between the device to be connected
and the isolator.
11. Bottom assembly
The entire apparatus is housed in a large Delrin, triangular box. The steel frame is protected with
sacrificial anodes. The bottom is weighted with lead to ensure it lands upright. The assembly is rugged
and has been used to deploy similar equipment with great success for two decades.
Figure 3 The photo (by NEPTUNE Canada) shows the bottom assembly deployed at the Bullseye Vent JB, in its white Delrin
triangular box, with the under-water-mateable connection in front.