updated 5/20/97
Prepared by: Marcia McNutt
Massachusetts Institute of Technology
Richard Von Herzen
Woods Hole Oceanographic Institution
Dates: August 22, 1997-September 18, 1997
Ports: Honolulu, Hawaii - Honolulu, Hawaii
R/V Roger Revelle
Summary of Scientific Objectives
It has been suspected for some time that marine hydrothermal circulation is responsible for the fact that conductive heat flow in young oceanic lithosphere is significantly less than predicted by theoretical models of thermal cooling of oceanic plates. Heat flow values collected in the past decade from the midplate swells surrounding young hot spot chains have similarly been less than predicted by geophysical models that fit other sorts of data, such as geoid height and subsidence rates. The most dramatic departure from theoretical expectation is a profile across the Hawaiian chain, where heat flow near the peak in the swell is actually less than the off-swell mean. The most anomalous points from the Hawaiian swell were recorded in the flexural moat, a submarine sedimentary basin that was rapidly filled with volcaniclastic debris eroded from the Hawaiian volcanoes. The purpose of this expedition is to investigate to what extent fluid circulation in this large sedimentary basin is responsible for perturbing conductive thermal gradients. We plan to collect 2 transects of finely-sampled heat flow observations across the northern limb of the Hawaiian moat, one near Oahu and the other near French Frigate Shoals. We will use these data to constrain finite-element models of fluid circulation in archipelagic aprons and its temporal evolution as the moat fills and subsides. Ancillary data to be collected in support of the data reduction and modeling effort include single-channel seismic data, underway gravity data, Seabeam bathymetry, and piston cores.
Cruise Plan
August 22 Depart Oahu
August 25 Arrive French Frigate Shoals, deploy seismic gear
Total distance ~ 550 nm
August 26 Complete survey Line 1 with seismic, gravity, Seabeam
Length of line ~167 nm
September 2 Complete heat flow survey of Line 1 (~100 measurements)
September 4 Complete 4 cores along Line 1
September 7 Arrive at start of Line 2 near Oahu
Total distance ~ 550 nm
September 8 Complete survey Line 2 with seismic, gravity, Seabeam
Length of line ~167 nm
September 15 Complete heat flow survey of Line 2 (~100 measurements)
September 18 Complete 4 cores along Line 2
September 18 Return to port at Oahu
TOTAL of 28 days at sea.
We have chosen to survey the French Frigate Shoals line first. This will give the scientific personnel 3 days of transit time to familiarize the watch standers with their duties and 4 days to prepare the heat flow instrumentation for the first deployment. The alternative of surveying the Oahu line first would result in only a few hours to prepare the watchstanders and only a day to get heat flow equiment ready.
To optimize ship control during heat flow work, we want to start the survey of each line at the upwind end of the line. The heat flow work will then begin at the downwind end of the line.
It may save time to intersperse the cores with the heat flow measurements. We will discuss with the Res Tech whether this is feasible since the core winch will be needed for both operations.
Figure 1. White lines show locations of proposed heat flow profiles across the Hawaiian moat. Based both on bathymetry and altimetric gravity (not shown here), we expect the physical structure of the moat to be similar along the two profiles, except that the age of the sedimentary basin is 10 m.y. older along profile 2.
Seismic Data Collection
The sound source for the seismic reflection profiling will be a 6-airgun, tuned array with the maximum volume available on the R/V Revelle, which is approximately 2000 cu inches. The shots will be received on an AMG seismic streamer with 2 active sections. We will digitally record all seismic data. During the moat profiling, ship speed will be kept to 8 knots or less for better data quality. At this speed, each line requires about 20 hours to complete. We only anticipate about 2 days of seismic data acquisition.
Gravity Data Collection
We will be installing a NAVO Bell BGM-3 gravimeter on the R/V Revelle for this expedition. The gravimeter will be shipped directly to Honolulu. Mr. Randall Herr from the Naval Oceanographic Office at Stennis Space Center in Mississippi will perform the installation. The gravimeter will be installed on mounts as close to the center point of the ship as possible. We will provide the mounts, based on a design used on the R/V Knorr last year.
Other equipment that we will be providing for using the gravimeter include a PC and a buffer to interface the BGM-3 with the PC. The buffer will either be fabricated by Dan Fornari at WHOI or borrowed from Robin Bell at Lamont. The PC will be DOS-based (no windows) and have a serial port. It needs a hard drive with about 50 Mbytes of available space to log the data. If possible, we would like to link this dedicated PC to the real-time computer which is logging the science data.
The gravimeter will be continuously operated during the expedition.
3.5 kHz Data Collection
We will be running the 3.5 kHz profiler during all heat flow and coring operations to look for areas blanketed with sufficient sediment for sinking the probe and core. It would also be a benefit to run the 3.5 while conducting the single-channel seismic surveys of the two lines, such that heat flow and core sites can be selected in advance.
SEA BEAM Data Collection
We have budgeted for 4 days of Seabeam operation in "basic mode" and 24 days in "dataless mode". The 4 days in "basic mode" will be collected while surveying the two lines with the seismic equipment. Seabeam will be continuously running in "dataless mode" during all other times for positioning the ship with respect to our survey data.
Heat Flow Data Collection
The heat flow data will be collected along the two profiles using the pogo method [Von Herzen et al., 1989]. Each deployment of the heat flow instrument lasts 36 to 48 hours. After taking a measurement at one location, the instrument is lifted above the seafloor while the ship slowly steams 1 to 2 km to the next site. Each measurement takes about 20 minutes, and it takes about 1 hour to move the ship. Therefore 20 to 30 heat flow measurements may be obtained without having to recover the heat flow instrument.
In order to achieve full and vertical probe penetration, as required for high quality geothermal measurements, we will be using 100 m/min lowering speeds during the last 50-100 m before penetration. During other times, a winch speed of 75-80 m/min is required to complete the 200 heat flow stations.
Our deepest sites are at a depth of approximately 5000 m. It is highly desireable to have 10,000 m of winch cable aboard the ship. In that case, if the wire parts at the seafloor, we will still be able to complete all of our scientific objectives without having to return to port for more winch cable.
Expedition Personnel
Marcia McNutt MIT Chief Scientist
Dick Von Herzen WHOI Senior Scientist
Grant Garven Johns Hopkins Senior Scientist
Alain Bonneville Universite Francaise du Pacifique Senior Scientist
John Sclater SIO Senior Scientist
Seiichi Nagihara U-Houston Senior Scientist
Jim Kirklin WHOI Heat flow technician
John Hallinan WHOI Heat flow technician
George Pelletier WHOI Heat flow technician
Rob Harris U-Miami Post Doc
Kelsey Jordahl MIT-WHOI Graduate Student
Jennifer Georgen MIT-WHOI Graduate Student
Robyn Kelly MIT-WHOI Graduate Student
Linda Rasmussen MIT-WHOI Graduate Student
Rebecca Lattier U. Southampton (U.K.) Undergraduate Student
We will be standing 3 shifts of 4-hour watches coordinated with those of the ship's crew. At least one senior scientist will be on call at all times to answer questions from the bridge and assist with over-the-side operations.
Logistical Details
Agent in Honolulu
Stan Winslow
Marine Center
#1 Sand Island Road
Honolulu, Hawaii 96819
Berthing and Meals in Port
We will have priority for bunks and meals beginning with the dinner meal on August 19th. We will sail at 0800 on August 22nd, and return to port at 0800 on September 18th. We must move off the ship by noon on September 19th. Any expedition personnel who will require lodging and meals in Honolulu before the evening of August 19th or after noon on September 19th should make other arrangements.