This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). This award provides funds to build 20 new seafloor magneto telluric (MT) instruments using fluxgate magnetometer technology. The instruments will be designed and built for deployment offshore Cascadia as part of the proposed onshore-offshore seismic effort. Land-based MT data already exist through the Earthscope program, and PI Evans is funded to collect a more detailed MT profile in Washington State, coincident with a large seismic experiment. The instruments will reside at WHOI but their use will eventually be open to the community. Broader Impacts: This project will involve persons with disabilities by collaborating with CapeAbilities, a non-profit organization on Cape Cod, to have its clients assist in the manufacture of the sensor electrodes. Additionally, the research enabled by these instruments will have direct societal relevance in that the seismogenic zone of Cascadia represents a significant geologic hazard in a highly populated region of North America. The proponents also express a strong desire to entrain new students into the MT field, which has been criticized as lagging behind other countries.
This grant supported the design and development of a new fleet of 20 seafloor marine magnetotelluric instruments. Magnetotellurics (MT) is a geophysical technique used to image deep into the Earth. The method measures naturally occurring electric and magnetic fields at the seafloor, generated by the interaction of the Earth’s magnetic field and the charged particles of the solar wind. These charged particles create electric currents in the ionosphere which induce current flow within the Earth. The pattern of current flow beneath Earth’s surface is dependent on the electrical conductivity structure, a physical property sensitive to temperature, composition and particularly the presence of fluids, in essence properties that are important for understanding a variety of tectonic processes. MT has been used extensively on the seafloor to study melting at mid-ocean ridges and fluid release and melting at subduction zones, the nature of the oceanic mantle and is also used by the Petroleum industry as an exploration tool. Each MT instrument is self contained with battery powered electronics housed in a glass sphere that can withstand the pressures at the seafloor. Measurements are made by dropping instruments in a profile across the region of interest and leaving them in place for a minimum of ~3 months. Instruments are recovered by sending an acoustic signal which releases a weight, allowing the instrument to float to the surface. The instruments record time series of electric and magnetic fields which contain a broad spectrum of signals. The time series are processed into frequency domain and the ratio of the electric to the magnetic field is calculated as a function of frequency to produce an MT response function. Higher frequency data are sensitive to shallow structure while the lowest frequencies can probe to depths in excess of 200km beneath the seafloor. Through this grant we designed a new generation of instruments that used up to date low-power fluxgate magnetometers, temperature compensated clocks, and 24 bit A/D boards to improve the resolution of the fields. The mechanical packaging of the new instruments is simplified over previous instruments we have operated, with all electronics housed in a single 17" glass pressure sphere. New acoustic releases were implemented based on those used by our Ocean Bottom Seismology group, but adapted for our purposes. Funds from the project allowed us to hire and train a new technician who is now transitioning to other projects within WHOI. Electric field measurements are made with silver silver-chloride electrodes. As part of the project, we redesigned the materials and manufacture of these so that they could be manufactured through a partnership with Cape Abilities, a local non-profit that finds work for people with disabilities. More information on the program can be seen at www.whoi.edu/page.do?pid=106256. A video on the program is also available at www.youtube.com/watch?v=JUX8r_H3ka8 As a final test of the new units, we deployed 14 instruments in a profile south of Martha’s Vineyard. Two land stations were deployed for reference on the Vineyard and in Wareham, Massachusetts. At the time of writing, we have recovered 12 of the instruments. Although full data workup and analysis will require additional funds, we have calculated initial response functions for sites that were down for a short period of time and have compared them with those from the land stations. Data appear high quality in the bandwidth from 20s out to several thousand seconds which is satisfying especially given the noisy conditions typically encountered in shallow water.
