NEPTUNE Canada: Project Details

Project Details

Bullseye Vent Gas Hydrates

Geophysical Imaging of Gas Hydrate at the ODP889 Node-Bullseye Vent Site.

Principal investigator(s)

Nigel Edwards and Ele Willoughby (U. of Toronto)

Co-investigators

Carsten Scholl (now at Fugro Electro Magnetic GmbH); Reza Mir and Lisa Roach (University of Toronto); Katrin Schwalenberg (Bundesanstalt für Geowissenschaften und Rohstoffe (BGR)), Rob Evans (Woods Hole Oceanographic Institution); Engineering support: Peter Hurley, Shuqing Li, David Rogerson (University of Toronto)

Discussion & Collaboration

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Abstract

Gas hydrate increases both the resistivity and stiffness of marine sediment and can be located and assessed by geophysical techniques which generate data sensitive to changes in these properties. Using the NEPTUNE Canada cabled ocean-floor observatory we plan continuous, real-time monitoring of the gas hydrate-associated, “Bullseye” cold vent offshore Vancouver Island. We are preparing two stationary semi-permanent imaging experiments: Controlled Source Electromagnetic (CSEM) and Sea Floor Compliance (SFC).

For CSEM, a precise electromagnetic signal is sent out from a transmitter (TX) dipole on the seafloor and recorded at several seafloor receiver (RX) dipoles located at a range of distances from the transmitter. The data are sensitive to the subtle changes in resistivity in the sediment between the TX and RXs probably caused by the displacement of conductive pore water by electrically-insulating gas hydrate. Like all NEPTUNE Canada experiments, the apparatus are controlled remotely and the data are transmitted to local laboratories for analysis. Repeated soundings at Bullseye, over the lifetime of the Neptune project, will track changes in resistivity and reveal the evolution of the gas hydrate deposit. The CSEM method used is described theoretically by Edwards (1997) and experimentally by Schwalenberg et al (2005).

The SFC experiment serves a similar purpose. It is designed to monitor changes in the stiffness of the hydrate zone caused by the cementation of sediment grains by interstitial hydrate. The seafloor is displaced a few atomic radii by the action of kilometer long ocean surface gravity waves. Measurements on the dynamic pressure and displacement time series on the seafloor may be analysed to yield a stiffness profile as a function of depth. Changes in the profile are indicative of the evolution of the hydrate deposit.

Study Sites/Locations

The Bullseye Vent is on the mid-continental slope (1260m) off south-central Vancouver Island. Our experiment will be connected to the NEPTUNE Canada ODP 889 node (click the link for detailed maps of this location).

Equipment Summary

CSEM Experiment

CSEM Transmitter.

The core of the CSEM experiment is a high-tech transmitter. It was designed and constructed by IE Power to our specifications. It is essentially 5 KW DC to switched DC converter controlled by an onboard computer. The electronics are shown in picture at right. They fit in a pressure cylinder 36 cm diameter, 92 cm long and are powered by 400 V 10 A power supply provided by the NEPTUNE Canada Junction Box. The local operator running any given experiment selects the waveform to transmit, possibly a square wave, a sine wave, or a pseudo-random binary sequence.

Scematic diagrams for Controlled Source Electromagnetic experiment. (Click to enlarge.)

Components of the CSEM Array. The receivers have their own controller, in RC. The controller is connected to the receiver string using fibre optics while each receiver is connected to two grounded silver/silver chloride electrodes which form the RX dipole sensor. While only the nearest receiver dipole is illustrated, the receiver dipole assemblies (RX box with penetrators and two electrodes mounted coaxially with the inter-receiver cable) may be joined together like Lego.

Anomalous electrical resistivities recorded with the U of T towed CSEM array over the Bullseye Vent and three other cold vents in this region. (Click to enlarge.)

Compliance Experiment

The high-tech nature of SFC is best illustrated by some typical data of recorded time series at the seafloor (shown below). The gravity waves at the surface of the ocean have produced a pressure field on the seafloor of the order of 1 Pa, about one hundred thousandth of an atmosphere. It forces displacements of the seafloor of about 0.1 microns. We cannot measure these directly; they are too small. However in the frequency band of 10^-3 to 10^-1 Hz, displacements of this order correspond with accelerations of the order 10^-11 to 10^-7 m/s² (or .001 to 100 microgal), which are well within the sensitivity of high-precision gravimeters and long period seismometers.

Gravitymeter.

Differential Pressure Gauge.

At left is a photo of a modified gravitymeter arranged in a powered gimbal system so it may align itself vertically. To its right is a differential pressure gauge (DPG). The DPG consists of two chambers. The first is held at time-averaged pressure. The second is in contact with the ocean through a large rubber diaphragm. The sensor detects the difference in pressure between the two chambers.

How this project will foster collaboration

We acknowledge the assistance of the following industrial partners:

Dr. Shashi Dewan, IE Power (Toronto)
Dr. Chris Nind, Scintrex Ltd. ( Toronto)
Dr. Timothy Niebauer, micro-g Lacoste (Boulder)
Stuart Laswell, micro-g Lacoste (Boulder)
Shell International Exploration and Production Inc. (Houston)

Resources & References

Powerpoint presentations at NEPTUNE Canada workshop, 2/09

References

Edwards, R. N., 1997. On the resource evaluation of marine gas hydrate deposits using sea-floor transient electric dipole-dipole methods. Geophysics 62, 63 (1997); DOI:10.1190/1.1444146
Schwalenberg, K., Willoughby, E.C., Mir, R. & Edwards, R.N. 2005. Marine gas hydrate electromagnetic signatures in Cascadia and their correlation with seismic blank zones, First Break, 23, 57-63.
Willoughby, E.C. & Edwards, R.N., 1997. On the resource evaluation of marine gas hydrate deposits using seafloor compliance methods, Geophys. J. Int., 131, 751-766.
Willoughby, E.C., K. Latychev, R.N. Edwards, K. Schwalenberg and R.D. Hyndman,. (2008) Seafloor Compliance Imaging of Marine Gas Hydrate Deposits and Cold Vent Structures, J. Geophys. Res., 113, B07107, doi:10.1029/2005JB004136.
Yuan, J. & Edwards, R.N., 2000. The assessment of marine gas hydrate through electrical remote sounding: Hydrate with a BSR?, Geophys. Res. Lett., 27, 16, 2397-2400.