First cruise of HT-RESIST successfully completed

After 29 days of 24 hour operations, we have finally reached the conclusion of our grand voyage at the Hikurangi subduction zone. We successfully completed all of the planned data acquisition: 170 OBEM receiver deployments and 128 recoveries, as well as 500 line-km of SUESI tows (which is 100 line-km more than we set out to collect). Only three instruments came back with no data. A marvelous achievement given the inherent difficulty of collecting data at the bottom of the ocean. One that was only possible thanks to the dedication and hard work of the science party, the marine EM lab at Scripps Institution of Oceanography, and the captain and crew of the R/V Revelle. I am struggling to digest the overwhelming range of emotions now that our journey has reached its end.

Onshore celebrations in Wellington await.

One of the last of the OBEM deployments
The very last anchor to be deployed

Stay tuned for more blog posts during the second, shorter (and more relaxed) voyage beginning February 21st, when we will set sail to recover the 42 OBEM receivers left behind to collect five+ weeks worth of MT data.

Preliminary look at the MT & CSEM data

In the first figure below, I have plotted the processed MT response functions from three sites on the southern line of receiver deployments. Site s03 was located on the incoming plate, s14 was near the trench axis, and s33 was on the Hikurangi fore-arc. The data show the apparent resistivity and phase of the transverse-magnetic (TM) and transverse-electric (TE) modes. The TM and TE modes represent the two geomagnetic field polarizations that induce electric currents parallel and perpendicular to the line of receivers, respectively. They have sensitivity to different electrical conductivity structures, which will become important when the data are modeled using geophysical inversion methods. Details aside, what is most important here is that the data look smooth, have reasonable error bars, and therefore are fairly high quality. The data shown here are generally representative of the quality of the processed MT for the southern line of sites (have yet to process the MT data from the other profiles).

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Processed MT data from southern profile sites s03 (on the incoming Pacific plate far from the trench axis), s14 (on the incoming Pacific plate near the trench axis), and s33 (on the Hikurangi fore-arc slope).

In this next example, the top panel in the figure shows two repetitions of the 4-sec long waveform that is continuously transmitted by SUESI during deep-tow operations. The bottom panel shows a real snippet of the time-series from site s14, when the transmitter was within range of the receiver. There is a clear resemblance between the transmitted and recorded waveforms, but the recordings contain much more structure. This is because the transmitted waveform is being distorted by the electrical structure of the seafloor, which is what we hope to disentangle from the data with inversion modeling.

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Top panel: the raw SUESI transmitted waveform. Bottom panel: waveform recordings at site s14. Channels 1&2 are the magnetic fields recorded with induction coil magnetometers. Channels 3&4 are the electric fields recorded with silver-silver chloride electrodes.

EM Lab completes deep tow operations in extensive survey of the Hikurangi Trench off New Zealand

PI Samer Naif led a large team that included Lamont graduate students Christine Chesley, Bar Oryan, and Daniel Blatter onboard the R/V Roger Revelle in the waters east of North Island, New Zealand to carry out a marine electromagnetic (EM) survey of the Hikurangi subduction zone. The project includes over 168 seafloor EM receiver stations and over 400 line-km of transmitter deep-tow tows, making it the largest marine EM experiment at a subduction zone yet.  The data will allow the team to assess the role that fluids play in both generating subduction zone earthquakes, and in controlling the seismic behavior of faults more generally.

Click here to see photos and read the cruise blog.

 

Final SUESI tow complete!

I’m delighted to report that we have succesfully completed our final SUESI tow of HT-RESIST. We managed to collect nearly 20% more controlled-source EM data on top of the ~400 line-km originally planned. That may not seem like a big deal, but in the marine research world, it’s an achievement to celebrate (and celebrate we will, likely with oysters and karaoke). It’s all thanks to the 29 days of continuous and very hard work put in by the science party and the crew of the R/V Revelle.

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The entire science team keeping it cool and prepping for the final SUESI deployment of the cruise. Clockwise from back left: Samer Naif, Eric Attias, Christine Chesley, Christopher Armerding, Joanna Sherman, Gesa Franz, Andrea Adams, Bar Oryan, Jacob Perez, and Daniel Blatter.

Our work schedule will mellow out for the remainder of the cruise, and I should be able to post a few more blurbs on how the data are turning out, and what is left for us to do before returning to Wellington port.

Hunting for earthquakes

Ever since Kerry Key was knighted as an ‘Earthquake Hunter’ on San Diego’s local Fox News station back in 2010 (during the SERPENT research cruise), I too have always wanted to claim this title. After perusing through some of the time-series data from this survey, I am happy to report that we recorded several earthquakes.

The OBEM receivers are equipped with induction coil magnetometers, which are magnetic field sensors that moonlight as seismometers since they are highly sensitive to tilt motions (check out the recent paper by Barak et al on the publications webpage for more insight). The screenshot below shows the signal generated by a magnitude (Mw) 5.0 earthquake on the magnetic field data of three OBEM receivers. This particular earthquake occured over 1100 km away!

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Yellow star shows epicenter of (Mw 5.0) earthquake that occured at 12:48pm on Christmas day (UTC). Red triangles show the location of the three OBEMs whose magnetic field recordings are plotted in the figure below.

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Crossing line SUESI tow underway

We deployed 24 OBEM receivers across the central Hikurangi subduction zone, oriented parallel to the trench axis. After email discussions with a GNS Science colleague, I decided to extend the profile length considerably.  The middle of the profile is now centered on the central tow line from about 10 days ago (the original plan was to tow only the southern half of the profile with 18 receivers). If all goes well and the weather doesn’t get in the way, it should take us about 48 hours to tow SUESI over the ~125 km long transect. Then back to navigating and recovering receivers. At this stage, we have now deployed the OBEM receivers nearly 150 times! Our crew are a hardworking bunch..

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Map of central Hikurangi subduction zone. The triangles show the OBEM site locations from the central profile, where we collected SUESI data about ten days ago. The squares show the OBEM sites we just deployed in the last day. We are currently towing SUESI up from the southern end.

(not quite) 100% data recovery: part 3

We completed OBEM recoveries for the northern transect early this morning. Although the SUESI operations proceeded without a hitch, this time around we hit a snag with logger setups. Three OBEM receivers were programmed to wake up and begin recording on the wrong date (Dec 2019 vs Dec 2018), and so were recovered with no data on them. Unfortunately, these were deployed consecutively and were centered on the trench axis. I was quite irritated by this all day yesterday, but such is life. Besides that, the remainder of the sites were recovered with plentiful high quality data, including the sites where we performed the additional crossline tows. Overall, very proud of our science team’s performance. It also helps that Poseiden must have sensed my frustration, because he gifted us with a wonderful display of dolphin magic that same afternoon, and I actually managed to snap a few pics of some bow surfing action.

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Currently deploying more OBEM receivers and getting ready to start ops for our central crossing transect, the final SUESI tow of our voyage!

Boring is good, & then some

We’ve ironed out all the wrinkles from our first two SUESI tow lines, with extra starch, leading to a nearly perfectly executed performance on our northern profile (except for a few nail biting run-ins with extremely steep topography that left me in a panic; SUESI dropped down to just 25 meters above the seafloor at one point, which may seem like a lot, but it most certainly is not and scared me half to death). An added bonus, we are ahead of schedule. It doesn’t hurt that the seas have been flat with almost no wind over the last two days.

To make use of some of the extra science time we have accrued, we are collecting a handful of crossing profiles along a relatively flat portion of the fore-arc seafloor. After the prior close encounter, I was in no mood for any more drama. Plus, one of the crossing lines is very near to site U1519 from IODP drilling expeditions 372/375, which should complement those suite of data nicely.

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Map of OBEM stations (green circles), main SUESI tow (black line), and newly added crossing tows (blue lines). 

We will be done towing in about eight hours, then begin OBEM recoveries for the second to last time. The most difficult aspects of the survey operations are now nearly completed, let’s hope things settle down a bit for the remainder of our voyage.

For your viewing pleasure, a photo of today’s sunset, and an even brighter night sky. In the words of one Kiwi fisherman, ‘cool bananas’!

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100% data recovery: part 2

We completed receiver recoveries for the central profile, and have maintained our 100% recovery rate. We are currently working on the northern profile, and expect to begin SUESI ops in less than 24 hours time. Our science team is now a well oiled machine, cranking out receiver deployments every half hour. The saga continues..