Author: Samer Naif

We retrieved the final ocean-bottom EM receiver just in time for lunch today, in dramatic style with 20 foot waves and 35 knot winds including occasional 60 knot gusts. We faced far more troublesome storms compared with our first voyage, making for some epic nail biting recoveries but also spectacular scenery.

Our final tally for this cruise is 42 out of 42 receivers successfully recovered. Each receiver was equipped with four sensors amounting to four channels of data, and we were fortunate to get 164 channels of high quality data with two channels of intermittent data and only two channels of unusable data. All in all, this amounts to a score of yasss*41.25! Another celebration worthy result after 128 successful recoveries during the first cruise.

Stay tuned for more details on preliminary data processing results in the coming days.

After 26 hours of transit from Auckland port, the day team successfully recovered three receivers during our first shift. All instruments came back with 45+ days of high quality data! Hoping for “yasss*42” when all is said and done.

The weather has been well behaved so far, but we are expecting a rough patch from a nearby cyclone in the coming days. It won’t be as bad as initially forecast, knocking on wood for manageable seas.

We are back on the R/V Revelle, and just left Auckland port to begin the second and final leg of our Hikurangi subduction zone survey! Bon voyage!

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.

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.

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).

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.

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!

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..

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.

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