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3.5kHz Profiling with Vertically Separated Source and
Receiver
"making a mini reflection
survey around a deep-water drill hole"
Woods Hole Oceanographic Institution,
360 Woods Hole Road, Woods Hole, MA 02543-1542 USA
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ABSTRACT
The spatial resolution of hull mounted 3.5kHz echo
sounding systems is often limited in deep water by the large footptint (Fresnel
zone) of the insonifying energy on the seafloor. On
ODP Leg 200 at Site 1224, we tested a system that could be used from the drillship
while on station to improve resolution. In 4970 m water depth, a 3.5 kHz pinger
was mounted on the video camera frame and lowered down the drill string to a few
meters above the seafloor. The ship's 3.5 kHz receiver recorded the returns. Having
the source and receiver at differing distances above the seafloor provides two
advantages: 1) the area returning reflections is greatly reduced, 2) the amplitudes
of the sub-seafloor reflections are less affected by the spreading effect. Reflections
were observed to 40 ms beneath the seafloor on lowerings at three closely spaced
holes. The heave of the ship and camera frame shifted the travel times of the
reflection sequences. A level-discriminator and correlation MatLab routine was
used to align the traces and to stack them to enhance signal. The reflections
showed good correlation with the limited geotechnical data obtained from the sediment
cores. Although tested from the drill ship this system could be used to provide
a low cost, shallow penetration profiling system from ROV's and AUV's. This work
was supported by a grant from JOI-USSAC.
Figure 1. Hole 1224 is near the H2O observatory
between Hawaii and California.
Figure 2. This 3.5-kHz echo sounder recording
shows that the seafloor dips smoothly ~6 m from the H2O junction box to the
drill site, 1224. One subbottom horizon at ~9 m is fairly uniform throughout
the area. Based on drilling results, this is a mid-sediment reflector. A second
reflector at ~30 m below the junction box can be associated with basaltic basement
although it appears only occasionally in the record. One objective of lowering
the 3.5 kHz source to the seafloor was to improve the resolution of the basement
reflection. PDR = precision depth recorder.
Figure 3. This shows a schematic of the
3.5 kHz Experiment. The 3.5kHz source was on the VIT frame. The ship's hull
mounted 3.5kHz Transducer was used to receive the signals. (This figure was
adapted from the Leg 203 Intial Reports volume.)
EXPERIMENTAL SETUP
The area of the seafloor insonified (within the
first Fresnel zone) can be greatly reduced by lowering a broadbeam source to a
few meters above the seafloor. By comparing the echo sequences observed at three
holes at Site 1224, and at four elevations above the seafloor, inferences as to
the lateral character of drilled interfaces can be made. In these data, there
was little difference in spreading loss as the direct and reflected returns were
recorded on the ship’s 3.5 kHz receiver nearly 5 km above the source and
seafloor.
The 0.25 sec data window, digitized at 24 kHz, was
triggered by the direct water arrival from the back lobe of the transducer. A
MatLab travel time alignment program was written to align the returns from the
free-running source. Ten groups of aligned traces were stacked to reduce noise,
and are shown in figure 7.
Figure 4. On the VIT frame, the 4.252-kHz
transducer (a nominal 3.5 kHz source) is the squat cylinder in the middle under
the frame's horizontal member. The pinger batteries and electronics are in the
yellow cylinder to the upper left marked "Pinger."
Figure 5. (TOP) Installation of the reentry
cone and steel casing in Hole 1224D as observed on deep-source records. This
figure is excerpted from the EPC graphic recording on 2 January. The traveltime
interval shown is ~410 ms; the light horizontal traces are spaced at 100 ms.
The heavier vertical traces are 5-min marks. The 14-ms fluctuation with an amplitude
of up to 4 ms in the traveltime of the direct water wave is primarily due to
the heave of the ship pulling and slackening the vibration isolated television
(VIT) cable. The 1- to 2-ms fluctuations in the traveltime for the reentry cone
is a measure of the VIT heave plus the uncompensated heave of the drill string
because the cone is rigidly linked to the ship’s heave compensator by the
drill string. (BOTTOM) This shows the same data plotted using MatLab to show
the match between the analog PDR recording and the digital data acquirred.
Figure 6. This figure contains part of
the data shown in Figure 5. (TOP) A quarter (1/4) second window of data is shown
with time 0 starting at the direct water wave arrival. The data at about .035
sconds is the seafloor arrival. Note the roughness due to ship's heave. (BOTTOM)
The data here have been lined up at the first motion of the waveform representing
the seafloor. Only 60 milleseconds (0.060 seconds) of data are shown below the
seafloor. Note how the subbottom reflectors line up very coherantly in this
figure. Both of the figures are a result of the MatLab code used to process
the data.
RESULTS shown in figure 7
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Reflector
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Comment
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Approximate
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Travel time
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Depth (Meters)
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11 ms
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Consistent between lowerings and differing source elevation;
appears to be an extensive interface
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9.4
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14 ms
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Varies between lowerings and with differing source elevation;
probably a less planar interface
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11.9
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27 ms
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Weak return which varies between lowering and source
elevation; perhaps a prominence in basement
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24.5
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32 ms
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Strength varies with elevation; perhaps indicating a
concave
surface which focusses and defocusses
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29.5
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36 ms
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Strong basement reflector; change of echo envelope between
lowerings suggests irregular topography
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33.5
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39 ms
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Big changes between lowerings and with source elevation;
perhaps represents a small hollow in basement surface
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36.5
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Taken together, the 32, 36 and 39 ms reflections suggest
basement relief of up to 7 m in the vicinity of Site 1224.
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Figure 7. This shows the stacks from 10
different windows at this site. There is good agreement of the data irregardless
of the height of the source off of the seafloor. The different horizontal lines
are referred to in the box above and to the left discusing results.
Conclusions
Two subseafloor reflecting interfaces were identified
that were drilled. Based on the changes in acoustic impedance indicated by the
two reflections, the following stratigraphic spacing is suggested:
A. The 11-ms interval between the seafloor and the first subsurface
reflection corresponds to the 9 m of unconsolidated yellow to brown clay cored,
assuming that the sound velocity of this material is near that of water.
B. The 25-ms interval between the first and second subseafloor
reflectors corresponds to a somewhat more consolidated sedimentary sequence
~19 m thick. There was little core retrieved from this interval.
C. Basaltic basement is an irregular surface at a depth of ~28
m bsf. Using the 3.5-kHz hull-mounted source, the basement reflector could be
identified at the H2O junction box but not at the drill site. At the drill site,
the basement reflector was only identified with the deep source.
Contact us for further logistic details. These
measurements do not take rig time. We would like to make further measurements
and would welcome the opportunity to join future drilling legs.
hhoskins@whoi.edu
rstephen@whoi.edu
tbolmer@whoi.edu
This was created on December 12, 2003 by Tom
Bolmer
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