Back to Mine Burial site survey page
PDF version of this document
CH02-15 Cruise Report
Site Survey
of the
In Support of
the ONR Mine Burial Prediction Program
Chief Scientist:
John A. Goff (UTIG)
Co-PI’s: Robert Evans
(WHOI), Larry Mayer (UNH), Steve Schock (FAU), Peter Traykovski (WHOI), and Roy
Wilkins (UH)
Additional
participants: Chris Jenkins (U Co), Ilya Buynevich, Hilary
Gittings (WHOI), Barbara Kraft, Eric Jabs,
Peter Simpkin, Andy McLeod, Jarrod Millar (UNH), Jim
Wulf, Gwendoline Quentin, Pierre Beaujean, Csaba Vaczo, Hernando Nieto (FAU)
Cruise CH02-15 was conducted
aboard the R/V Cape Henlopen from August 1 to
Figure 1. Location of the
Our goal was to characterize the shallow sedimentology of the MVCO and surrounds: to define the variability in sedimentary properties at the seafloor, characterize the shallow (< 5m) structure, and to understand the near-shore sedimentary dynamics governing the region. Most of our efforts were focused in a small region encompassing the MVCO node – the permanent seafloor station to which the experimental mines will be connected – but also included broader coverage so that the mine burial experiment can be placed in a larger regional context. Data collection included sediment samples (vibracores and grab samples), in situ geotechnical measurements (velocity and resistivity), and ultra-high resolution, deep-towed chirp seismic reflection. The survey work was conducted in three individual 2-day legs, embarking and debarking at the Woods Hole Oceanographic Institution dock. Leg 1 collected 35 vibracores; Leg 2 collected grab samples and geotechnical data at 87 stations; Leg 3 collected chirp data along densely spaced (75 m and 150 m) shore-parallel lines (Figure 2).
Figure 2. Locations of grab samples (numbered yellow squares), cores (red diamonds; station numbers identified on Figure 4), and chirp track lines (dashed light blue lines) collected during CH02-15. Stations 14, 16, 19, 20 and 21 have multiple locations. Velocity/resistivity measurements were colocated with all grab samples. Background image is submetrix sidescan data (USGS, September 2001) with regional bathymetric contours in meters. Higher backscatter intensity is represented by lighter shades. Heavy dashed lines outline area covered by July, 2002 Reson 8125 multibeam/backscatter survey. Location of MVCO node is indicated.
Weather conditions were
generally excellent, the crew were very capable and accommodating to our
requests, the ship was well equipped and well suited to our needs, the
equipment worked almost flawlessly, and the science parties were enthusiastic
and tireless. All of our scientific
objectives were met and/or exceeded.
CH02-15 follows a number of
previous data collection efforts in the MVCO, including grab sampling in 2001
(Peter Traykovski, WHOI), boomer and hull-mounted chirp seismic data in 2001
(USGS Woods Hole), and three swath mapping programs: (1) a Feb. 2001 sidescan
survey (USGS Woods Hole), (2) a September 2001 Submetrix sidescan and
interferometric bathymetry survey (USGS Woods Hole), and (3) a July 2002 Reson
8125 focused multibeam/backscatter survey (Larry Mayer, UNH). Together these data sets establish a baseline
set of observations against which physical changes in the seafloor with time
can be measured.
Rippled scour depressions
(RSDs) are pervasive within the MVCO.
RSDs are ~shore-perpendicular bands of coarse sands separated by
overlying fine sands (references). The
term itself implies that the coarse sands are heavily rippled (~0.5-1 m
wavelength, ~0.1 m amplitude) and slightly depressed relative to the fine sands
which, in the MVCO, are generally just a few 10’s of cm thick. The RSDs are clearly evident on sidescan data
as bands of high backscatter (Figure 2).
For the most part, grain size measurements confirm a strong positive
correspondence between mean grain size and backscatter intensity. However, a critical exception is seen in
deeper water where, well within the area of fine sands, backscatter increases
noticeably as mean grain size decreases from ~150m to \~130m. Topographic expression related to the RSDs is
confined primarily to evident scour depressions at the edges. The RSDs are highly asymmetric: backscatter
is higher, the coarse/fine transition is more sharply defined, and the scour
depression is deeper on one side than the other. This pattern changes within the survey: the
higher backscatter edge is always to the west in the western part of the
survey, and vice versa to the east. The strike of the RSDs also changes, from
being slightly east of north in the western part of the survey to slightly west
of north to the east.
Leg 1 – Vibracoring
The vibracoring system (Figure 3), borrowed from Chuck Nittrouer (UW), was deployed with 3 m of pipe. We anticipated, with the coarse sands prevalent in the survey area, only minimal recovery (and, in fact, never penetrated more than 2 m). Also, this pipe length greatly simplified deployment from the Henlopen’s A-frame. Once up to speed, coring proceeded at a pace of one every half hour, which was considerably faster than expected. We collected 35 cores total (two more than best-case-scenario plans) in approximately 24 hour’s time, essentially depleting our supply of core liner.
Figure 3.
Deployment of the vibracore.
Core locations were chosen along 5 E-W transects (Figure 4), sited with respect to previous seismic reflection interpretations produced by the USGS-Woods Hole. Our coring priorities were as follows: (1) several samples to be taken in the vicinity of the likely locations of mine burial experiments; (2) samples to characterize the thickness of the fine sand unit and the rippled scour depressions; and (3) samples within the fill of the buried channels. Our first priority was satisfied with multiple samples at near node sites identified by Peter Traykovski as likely candidates for mine burial experiments – one in fine sands (stations C3.1-3.5) and the other in coarse sands (stations C2.1-2.4). Our more distributed sampling provided excellent regional sampling of the coarse and fine sands. Although the top of the cores were often disturbed, we able to define the thickness of the surficial fine sands (generally a few 10’s of cm). In a number of cores into the coarse sands of the rippled scour depressions, color changes likely identified the thickness of the surficial mobile layer. A number of cores sited over the buried channels sampled mud or stiff clay at their base; one core at the edge of a channel sampled peat (Figure 4).
Figure 4.
Vibracore locations (numbers give station identification) overlain on
USGS structural interpretation based on February, 2001 seismic reflection data. Highlighted station numbers indicate samples
with mud at core bottom. Station C1
contained peat. Dashed blue lines are
track lines. Regional bathymetric
contours are given in meters. Heavy dashed line is approximate location of chirp
seismic data presented in Figure 7.
The cores are presently
being stored at the WHOI repository. In
the coming months they will be split, subsampled (at ~10 cm intervals) and
analyzed for grain size distribution. If
possible, we will conduct C14 age dating on select subsamples.
Leg 2 – Grab Sampling and Velocity/Resistivity Probe (+ sector scanning
sonar)
Grab sampling and
velocity/resistivity measurements were conducted together so that grain size
analysis of seafloor sediments could be closely matched in time and space to
the geotechnical measurements. Our
primary objectives for the grab sampling and geotechnical probe work were to
(1) sample intensively in the vicinity of the node, where the mine burial
experiment is to take place, (2) intensively sample regions of transition
between fine and coarse sands at RSD boundaries, (3) reacquire samples at
locations previously sampled by Peter Traykovski in early 2001 to investigate
temporal changes in grain size distribution, and (4) sample across regional
variations in acoustic backscatter. Except
for a few near-shore stations, where water depth was precariously shallow for
station keeping, all planned grab/probe stations were sampled successfully.
On-deck
observations of the grab samples showed a number of interesting features: (a)
thin (5mm) crust in some samples of the fine sand facies, crumbly, possibly
organic-bonded but sufficient to impede current erosion; (b) dominance in the
fine sand facies of a 'soft worm burrow - annelid -hydroid' community with
vagrant gastropods, hermit crabs; high apparent biomass and black reduced
sediment below surface; (c) much sparser benthic colonization in the coarse
sand facies; occasional vagrant crabs, worm tubes, and large (10cm) infaunal
bivalves; (d) sand dollars (up to 7cm diameter) were commonly encountered
across both facies. Post-cruise interpretations of the video imagery associated
with the velocity probe will provide additional information on the character of
the seabed surface.
Intensive sampling across
RSD boundaries successfully pinned down the location of the transition along
those transects to within ~20 m at three sites, and to within ~50 m at a
fourth. Additional constraints were
provided by Peter Traykovski’s sector scanning sonar, which was deployed from a boom at the conclusion
of the sampling work while the ship either drifted or steamed slowly over the
same RSD boundaries targeted in the grab sampling. These data are still being processed for
navigation. A key preliminary result
(which we will elaborate upon at planned presentations to AGU and the January
mine burial workshop) is that the RSD boundaries have moved substantially – by
as much as 80 m - since the initial sidescan survey in early 2001.
Velocity/Resistivity (and attenuation) measurements were completed using the ISSAP (In-situ Sound Speed and Attenuation Probe) instrument (Figure 5). ISSAP used four transducer probes arranged in a square pattern, providing acoustic path lengths of ~30 cm and ~20 cm, and a maximum insertion depth of 15 cm. The transducers operated at a frequency of 65 kHz. Five acoustic paths were used; two long paths and three short paths. A ~15.4 ms pulse was generated at a repetition rate of 30 Hz. The acoustic signal detected by the receive transducer was amplified and combined with the transmitter gate pulse to generate a composite signal that was sampled at a frequency of 5 MHz with a National Instruments PCI-6110E A/D data acquisition board. Two resistivity probes mounted on the ISSAP platform were positioned in locations selected to limit interference with the acoustic signals. Also mounted on the platform were a color video camera and light, and a Jasco Research UWINSTRU, which measured platform pitch and roll angles, heading, depth, and temperature.
Figure 5.
Recovery of the velocity and resistivity probe – a lovely day for a trip
to the beach!
At each of the 87 stations, the ISSAP probe was lowered into seawater
to a location ~ 6m above the seafloor. A
measurement cycle was completed by transmitting 10 pulses on each of the five
paths and repeating three times for a total of 150 measurements. Resistivity measurements (10 per probe) were
obtained following completion of the acoustic measurements. The ISSAP platform was then lowered into the
seafloor where two acoustic and resistivity measurement cycles were completed
in the sediment. Probe insertion was
aided by the video signal which also provided imagery of the seafloor. The instrument was removed from the sediment
and a second seawater measurement cycle completed. Typically, a sequence of measurements (300
acoustic and 40 resistivity measurements in seawater and similarly in sediment)
was completed in ~ 4 minutes. A problem
with the resistivity measurements was discovered to have occurred at 15
stations (1-7, 68-75). These were
reacquired at the conclusion of the grab sampling, which provided duplicate
velocity measurements at these stations.
At many stations, acoustic measurements were obtained continuously
while the probes traveled through the water column and reached full insertion
in the sediment. It is hoped these
measurements will provide some indication of the sound speed profile in the
upper 15 cm of sediment. Due to
additional time, two of the 65 kHz probes were replaced with 100 kHz probes and
additional velocity measurements were obtained at 9 stations (36-38, 66-71).
Leg 3 – CHIRP seismic reflection
The chirp sonar (Figure 6) generated two simultaneous reflection profiles of the sandy seabed by alternately transmitting a 10 msec, 1-15 kHz FM pulse and a 40 msec, 1-4 kHz FM pulse. The wideband image of the sediment profile has a vertical resolution of 5 cm and a penetration of about 5 meters in sand while the 1-4 kHz band produces images with a vertical resolution of 20 cm and subsurface penetration of about 20 meters. The chirp seismic reflection profiles were intended to provide structural and physical property information in support of the mine burial prediction experiment. These seismic lines were coincident with coring, grab-sampling and resistivity probe work carried out during Legs 1 and 2, as well as the February 2001 boomer seismic lines carried out by the USGS. We completed a dense grid of chirp profiles with more than 200 km of coverage in the vicinity of the main MVCO node (Figure 2). An additional line along the entire south shore of the Vineyard was also run. Finally, the system was run carefully over locations of core sampling, so that physical properties inferred from acoustic impedances can be ground-truthed against sampled sediments. The shallow sub-bottom stratigraphic information provide by the chirp seismic reflection will help characterize the local sediment distribution, and, combined with coring results, can be used to understand the most recent episodes of postglacial reworking.
Figure 6.
Recovery of the FAU Chirp seismic reflection system – nice view of the
transducers on the bottom.
Two primary features are seen in the chirp seismic reflection profiles around the MVCO: (1) a distinct, shallow seismic horizon that separates coarse sands and the overlying fine sands which cover the seafloor between RSDs (Figure 7); and (2) the offshore extension of a Late Pleistocene fluvial channel complex presently occupied by the Edgartown Great Pond (Figures 4, 7). The main buried channel is 200-500 m wide and 1.5-3.0 m deep, becoming shallower in a seaward direction. Seismic profiles reveal sharp valley outlines and conformable to complex asymmetric channel fills that contrast with the surrounding deposits. Several tributaries 100-200 m wide and ~1.0 m deep join the main channel at wide angles. More buried channels were seen in the chirp line run along the south shore of the Vineyard.
Figure 7. Shore
parallel chirp seismic line demonstrating channel morphology and the contact
between surficial fine sands and basal coarse sands. A small rippled scour depression (RSD) is
present at a gap in the fine sand layer.
See Figure 4 for location.
Analysis of seafloor and sub-bottom reflection coefficients from the chirp system will be used to infer physical properties throughout the survey region. These estimates will be compared to cores, where applicable. A transducer extension which is rigidly attached to the chirp system and which provides signal offsets of approximately 2m from the main array, was used towards the end of the cruise. This extension, along with analysis of reflection coefficients from the chirp system should allow estimation of sub-bottom physical properties to depths of about 4m. Electrical resistivity data are to be collected through the region in a cruise later this fall, and will also provide porosity measurements, in this case to depths of 20m below the seafloor.