STRATAFORM High-resolution Multichannel Seismic Reflection Survey, Eel
River Basin California
Principle Investigators:
Craig Fulthorpe (craig@utig.ig.utexas.edu) and Jamie Austin (jamie@utig.ig.utexas.edu) at the
University of Texas Institute for Geophysics (UTIG), Austin, TX
Greg Mountain (mountain@ldeo.columbia.edu),
John Diebold (diebold@ldeo.columbia.edu)
and Peter Buhl (buhl@ldeo.columbia.edu) at
Lamont Doherty Earth Observatory (LDEO),
Palisades, NY
Funded by: Office of Naval
Research
This is a joint effort between The University of Texas Institute for Geophysics (UTIG)
and Lamont-Doherty Earth Observatory (LDEO), designed according to the philosophy and task
guidelines developed by ONR's STRATAFORM program (STRATA FORmation on Margins).
STRATAFORM consists of three distinct but interrelated projects whose goals are to
study: (1) shelf sediment dynamics and the development of lithostratigraphy, (2) slope
geological processes and resultant geomorphology, and (3) stratigraphic sequences
resulting from shelf and slope sedimentation. The multichannel seismic (MCS) survey forms
the core of the third project (ref. task C9, multiple-frequency 2-D seismic profiling
efforts), but the data form part of a multi-faceted approach that ties all three projects
together. The overall goal is to link short-term (i.e., acting over hours to weeks)
biological and physical processes affecting sedimentation (so-called "event"
stratigraphy") to the sequence stratigraphy and facies architecture of the preserved
record (i.e., ~upper 100 m and occurring over the past ~106 years).
The goal of the third "stratigraphic sequences" project outlined above is
"to understand the creation of the stratigraphic record on continental shelves and
slopes as the product of geologic processes acting with spatial and temporal
heterogeneities." Ultimately, modern processes will be linked to the seismically
imaged and sampled (preserved) record through an evolution of increasingly sophisticated
models.
The key to this entire effort is the collection of "nested" geophysical and
geological data, through use of a variety of tools whose individual temporal and spatial
scales overlap to form a wide-ranging continuum of measurements. A primary reason for
choosing northern California as one of two STRATAFORM corridors is that a great deal of
such data already exists (see below). However, despite this data base, task C9 emphasized
the need for a regional, high-resolution set of 2-D MCS profiles in the frequency band of
~100-500 Hz. Such a comprehensive survey, using a new MCS system formulated with
STRATAFORM objectives in mind, was conducted in July - August 1996 using R/V Wecoma.
R/V Wecoma Cruise 9605B
15 July - 1 August 1996
STRATAFORM researchers from the University of Texas Institute for Geophysics (UTIG) and
Lamont-Doherty Earth Observatory (LDEO) collected ~2200 line-km of high-resolution
multichannel seismic (MCS) reflection data in the Eel River Basin during July 1996. System
resolution for the 1996 Wecoma data is intermediate between the very-high
resolution Huntec deep-towed seismic profiles and the commercial MCS (JEBCO) profiles
already available in this area. The new and existing data thus provide "nested"
geophysical coverage of the Eel River shelf and slope.Project principle investigators
designed the survey grid to 1) overlap and, in some cases, duplicate the existing seismic
reflection profiles in the basin; 2) provide complete coverage of the STRATAFORM EM1000
swath backscatter and bathymetry survey area and geologic features under investigation by
other STRATAFORM researchers; 3) provide closely spaced (~800m-1km) dip and strike
profiles that we will be able to tie into a three-dimensional interpretation of shelf and
slope stratal architecture.
 Jamie Austin and
co-chief scientist Craig Fulthorpe aboard R/V Wecoma
W9605B Science Party
The professionalism of the captain, officers and crew of the Wecoma contributed
greatly to the success of the cruise.
SEISMIC ACQUISITION, SURVEY COVERAGE AND PROCESSING
Source: Single 45/45 cu. in. GI air gun at 2000 psi. Firing time calibrated to
equal streamer group length at a nominal ship speed of 4.86 kt.
Recording: LDEO provided the acquisition system including a primary solid-state,
48-channel ITI streamer with a 12.5 m group length. We also leased an oil-filled
48-channel Geco streamer from Mitchum industries as a backup precaution. We recorded data
using the ITI streamer for the first 5.5 days of the survey and then switched to the Geco
streamer (15 m group length) because 10 of the 48 channels in the solid-state streamer
were unusable. An electrical failure in the leased power supply occurred after another 5.1
days of recording and we switched back to the ITI streamer for the final 2 days of the
cruise. Data were sampled at 0.5 ms and stored on 3480 tape cartridges (180 shots/tape).
Navigation: Differential GPS
3.5 kHz profiles: Continuous collection of good quality 3.5 kHz profiles
(including turns and other seismic down time.)
Seismic resolution and survey coverage: Preliminary processed lines show ~5 m
vertical resolution and ~900 m (one second two-way travel time) penetration. Gas
"wipeouts" along the outer shelf and upper slope as well as strong multiple
reflections in shallow water reduce data quality in places. The survey grid consists of 84
lines covering an area ~45 km from north to south along the coast from roughly Trinidad
Head to Eel River, and from the 40 m isobath to the 950 m isobath. Data collection
landward of the 50 m isobath was severely limitted by the need to avoid moored crab pots.
We opted to collect more data in deep water and to shoot two lines in the Eel River Canyon
instead of collecting closely-spaced strike lines in the gas "wipeout" area at
the shelf edge.
Processing: UTIG and LDEO divided the 3480 tapes at the end of the Wecoma cruise
and have each pledged to complete processing tasks for half of the data set. Processing is
underway at UTIG and we plan to complete this task during1998.
W9605B study area and track map (Seismic tracklines in blue and 1995
Simrad EM1000 bathymetry and sidescan coverage in green)
PRELIMINARY RESULTS
- Strong, continuous stratal reflections characterize the shallow subsurface on the upper
and middle shelf. Unconformable surfaces bounding prograding units, sedimentary structures
and subtle tectonism are all visible above the multiple. Investigation of deeper stratal
patterns on the shelf will require efforts to diminish the multiple. Units on the shelf
thin to the shelf edge, are thin or truncated along the upper slope and then thicken again
on the lower slope.
- Gas wipeouts and "bright spots" (associated with both stratigraphic and
structural traps) imaged in the MCS profiles provide ample evidence for pervasive
hydrocarbon accumulation in a ~5km wide north-south band along the outer shelf and upper
slope. Janet Yun, from UC Santa Cruz, has mapped these gas-related features using
preexisting commercial MCS data. Gas plumes extending 10's of meters into the water column
are also visible on the 3.5 kHz echosounder record.
- The new MCS data support the hypothesis that strike-parallel ridges within the Humboldt
Slide zone are structural features related to slope failure rather than bedforms. The data
reveal what may be earlier, buried failure surfaces, with high-amplitude seismic
reflections, possibly indicative of trapped gas-charged fluids, immediately landward of
the buried headwall scarps. These data should constrain the style and relative timing of
slope failure.
- Tectonic features, including the large plunging anticline at the center of the survey
area, are well imaged. Stratal onlap and growth faulting within the basin document the
history of progressive tectonic displacement within the basin.
- During sea-level lowstands, part of the shelf was exposed, as indicated by a pair of
large, buried channels, up to ~120 m deep and 1 km wide, occurring near the landward end
of line 25. These features may represent crossings of a single, meandering channel,
possibly related to Eel River drainage.
- Slope channels are of at least two distinctive types (Line 66, shown below, displays
them both). 1) Shallowly buried (~40 m) aggrading gullies, ~20 m deep, are spaced at 300 -
600 m intervals. Cycles of erosion alternate with periods of draping during which gully
morphology is retained. Sequence stratigraphic interpretation may confirm whether the
erosional episodes occur during periods of low sea-level. 2) Northward of the aggrading
gullies are larger buried channels, or canyons up to ~120 m deep and 1 km wide. These
channels are erosional and exhibit lateral migration and erosion of fill by subsequent
channeling.
High-resolution MCS image of aggradational gullies and erosional channel on the Eel
River continental slope. W9605B line66.
Correspondence to Craig Fulthorpe (craig@utig.ig.utexas.edu) in Texas or Greg
Mountain (mountain@ldeo.columbia.edu) at
Lamont. Questions about this page to Laurie
Schuur (laurie@utig.ig.utexas.edu)
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