Jurassic Magnetism

Expedition to the Pacific
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Conclusions

Posted by Aric Velbel 
· Saturday, January 17th, 2015 

[Cruise Journal: the rest of the cruise]

A busy time in which: Sentry successfully completes the remainder of its science dives; watchstanding and data processing continue apace; we begin sharing our already limited satellite internet with several other research vessels in the region; we wrap up our science mission and begin transiting to Guam; and all of the various books, computers, notes, devices, containers, coffee, and other accoutrements of marine geophysics in the field begin to be packed away for shipping back to their home labs.

As the surface towed maggie is retrieved for the last time, we can take stock of the status of the science mission and reflect on our time onboard during the few brief hours that remain.

 

With this last surface maggie recovery, science operations of JQZ cruise 3.2 are complete

With this last surface maggie recovery, science operations of JQZ cruise 3.2 are complete

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Down time

Posted by Aric Velbel 
· Thursday, January 15th, 2015 

How do you fill the eight hours between watches, besides sleeping and catching whichever meals are served in the mess during your off time? No matter what you choose, you’ll not be able to get too far away from where you started, but that’s just another way to say that there’s plenty to do on board practically within arm’s reach. After stopping by the computer lab to keep the current watch entertained, putting some time into your own work in the main lab, or resting up a bit, you can indulge in the ship’s library or gym, catch up with your fellow watchstanders over a game or snack, learn a bit about the routine and responsibilities of the professional sailors on duty, or just take in the view and air on deck.

It's like this every day

It’s like this every day (A. Stote)

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Gravimetry

Posted by Aric Velbel 
· Thursday, January 15th, 2015 

Gravity is the force of attraction between any two masses; the study of this force is gravimetry; the instruments used to perform this study are gravimeters (or sometimes gravitometers). In the context of the Earth, when we measure the acceleration due to gravity, the Earth is so massive compared to everything else in the picture that there is little room for variance: we must be able to detect small variations on the scale of 1/1000th of a “gal” (the unit of gravitational acceleration named for Galileo, and equaling 1cm/s^2). Variations in density — the amount of mass per unit of volume — in the Earth lead to subtle variations in the gravitational acceleration at different points around the globe (higher density -> more mass -> more gravity and vice versa). These density variations arise from the composition and structure of the Earth, which are of interest to geologists and geophysicists. Therefore, data on the “gravity anomaly” (the difference between the mathematically expected and actually observed gravitational acceleration) at each location we visit is one of the staples of any marine geophysics cruise.

The portable gravimeter. Taking a reading involves manually moving a needle to the correct spot using the silver dial

The portable gravimeter. Taking a reading involves manually moving a needle to the correct spot using the silver dial

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Magnetometry

Posted by Aric Velbel 
· Thursday, January 15th, 2015 

Any time you use a conventional compass to check which way is north from where you’re standing, you are taking advantage of magnetism, specifically the tendency of a magnetic material to align itself, if its motion is appropriately unconstrained, to the direction of the surrounding magnetic field. Imagine a line extending outward from the north and south points of your compass needle; this line will coincide with one of the field lines which designate the direction of the magnetic field of the Earth. These lines all extend outward from the magnetic north pole and loop out and around (one of them going right through your compass needle, another going through every other magnetic compass needle anywhere around the globe) to dive back in at the magnetic south pole.

Because of what we know of the changing nature of the Earth’s magnetic field, it is possible to imagine that if you had very, very acute senses, you would be able to observe, over the course of several years, the direction to magnetic north from your current spot shifting slightly as the Earth’s field gradually oscillates and reorients itself. You could also observe, if you walked along the surface of the Earth, tiny fluctuations in the direction of the needle as local influences — magnetic fields from magnetic material within the rocks constituting the Earth’s crust — momentarily jostled with the global field for control of your compass’ direction.

This observable variation in the magnetic field as we move over different parts of the Earth is exactly the reason why we are in the Pacific Jurassic Quiet Zone; only instead of a compass and impossibly sensitive eyes, we rely on instruments developed for this specific task: magnetometers.

The DeepTow sled, carrying two magnetometers, is deployed overboard off the a-frame

The DeepTow sled, carrying two magnetometers, is deployed overboard off the a-frame

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R/V Sikuliaq

Posted by Aric Velbel 
· Wednesday, January 14th, 2015 

Conceived of to fill the need for an ice-capable research vessel in the Alaskan region, designed to provide a state-of-the-art platform for all manner of oceanographic research including the effect of climate change in that area, and already embarked on her second science mission en route from construction in the Great Lakes to her home port in Seward, AK, the R/V Sikuliaq is our lodging, transport, base of operations, and laboratory for the duration of our mission to the Pacific Jurassic Quiet Zone.

R/V Sikuliaq at Pier 30 in Honolulu Harbor (A. Stote)

R/V Sikuliaq at Pier 30 in Honolulu Harbor (A. Stote)

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About this expedition

From December 16, 2014 to January 15, 2015, our team including 10 scientists, 5 engineers, and more than 20 technicians and crew will sail to the Western Pacific Ocean near 20°N, 160°E to measure the magnetism, bathymetry, gravity, and structure of the Jurassic seafloor to investigate the history of the Earth’s geomagnetic field.

Funding

nsfThe Jurassic Magnetism expedition is a collaborative effort funded by the National Science Foundation.

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