OPERATION REPORT:
THE 8-14 OCTOBER, 2003, CRUISE OF THE R/V PELICAN,
TO DEPLOY THE CMRET VERTICAL LINE ARRAY IN
MISSISSIPPI CANYON 798 and ATWATER VALLEY 14,
NORTHERN GULF OF MEXICO
by
Tom McGee1, Erika Geresi1, Carol Lutken1, Bob Woolsey1,
Paul Higley2 and Scott Sharpe2
1
MMRI/CMRET
University of Mississippi
University, MS
2
Speciality Devices Inc.
Plano, TX
Introduction
During October 8-14, 2003, the Center for Marine Resources and Environmental
Technology (CMRET) of the University of Mississippi conducted a research cruise
onboard the R/V Pelican operated by the Louisiana University Marine Consortium
(LUMCON). The purpose of the cruise was to deploy the prototype vertical line array
(VLA) and record seismic data while making runs on the VLA with the survey vessel.
The VLA is a prototype hydrophone array developed as a component of the
proposed Gulf of Mexico sea-floor gas-hydrate monitoring station. The intention of
the cruise was to collect high-resolution seismic data for use during development of
software for analyzing monitoring station data. Two types of seismic source were
used. One was an 80in3 watergun towed behind the Pelican at the sea surface and the
other was the noise of the Pelican itself running at full speed.
On runs with the watergun, it was intended to tow a single-channel hydrophone
several hundred meters below the gun to collect zero-offset seismic reflection profiles
simultaneously with the VLA data. Use of this recording geometry, dubbed the
surface-source/deep-receiver (SS/DR) technique, allows the source signature to be
recorded for each shot so that it can be used to improve resolution during post-cruise
digital processing. Unfortunately, the deep-tow cable developed a major electrical
fault and it was not possible to record SS/DR profiles.
The prototype vertical line array
A schematic of the prototype VLA is shown in Fig.1. It has a total length of
slightly more than 200m. It is supported in the water column by glass-sphere flotation
and is fixed to the sea floor by an expendable concrete anchor.
The upper portion of the VLA consists of 16 channels evenly spaced at 12.5m
intervals. Each channel comprises a single hydrophone and preamplifier One
preamplifier was set to lower gain than the other 15 to insure a non-clipped recording
of the direct arrivals even at short offsets. This was done because data collected on
the initial VLA test cruise (August, 2002) exhibited some direct arrivals that had been
clipped, making them unusable as source signatures.
The lower portion of the VLA accommodates electronic devices in the pressure
housings. These include a data logger, a battery pack, an acoustic-doppler current
profiler (ADCP) and acoustic releases to disengage the anchor.
All 16 hydrophone channels are analog wired to the data logger which includes
a data acquisition and telemetry system (DATS). The signals are further amplified by
a programmable gain amplifier before each channel is digitized to 16 bits at 10,000
2
samples per second. A recording of 2-to-10-second length is stored in memory before
being transferred to a hard drive following each shot.
The DATS is connected to a two-way acoustic modem operating at about 38
kHz and capable of 1200-baud communications. This modem is used to receive a
command to start recording data. The modem is also used to monitor the DATS
house-keeping status, transmit compressed sample record information, provide surface
control of acquisition parameters including gains of the programmable amplifiers and,
if requested, transmit a recording for quality analysis in near-to-real time.
A pressure-compensated battery pack mounted below the data logger provides
all system power for up to a 10-day deployment. Energy conservation includes power
control of the hydrophone array, signal conditioning circuitry and hard drives.
An acoustic-doppler profiling current meter is located below the battery pack.
The current profiler is directed upward to aid in determining the geometric
configuration of the VLA in the water column.
Recovery of the system is initiated by activating a pair of acoustic releases that
connect the VLA to the expendable concrete anchor. Only one of two is required to
release the anchor successfully. The glass spheres then provide sufficient positive
buoyancy to bring the entire system to the sea surface.
Fig.1: Schematic of the prototype vertical line array
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The Work Area
The work area was about 120 kilometers south-southwest of the mouth of the
Mississippi River in the vicinity of the bathymetric feature known as Mississippi
Canyon (Fig.2).
Fig.2: Gulf of Mexico bathymetric and physiographic map (Bryant & Bryant).
The VLA was deployed twice, once in Mississippi Canyon Lease Block 798
(MC798) and once in Atwater Valley Lease Block 14 (AT14). Both lease blocks are
located on the continental slope of the northern Gulf of Mexico (Fig.3, Fig.4). The
water depth at the site in MC798 was 821m and that in AT14 was 1309m.
4
Fig.3: Bathymetry of the northern Gulf of Mexico showing VLA deployment
sites (adapted from USGS Hydrate Cruise Report G1-03-GM).
Fig.4: Detail of Fig.3 showing locations of lease blocks Mississippi Canyon 798
and Atwater Valley 14 (adapted from USGS Hydrate Cruise Report G1-03-GM).
5
Determining the speed of sound in the water column
Since post-cruise processing would require knowledge of the speed of sound in
the water column, CTD casts to measure conductivity, temperature and density as a
function of depth were made at each of the VLA deployment sites. Seabird software
was used to calculate the speed of sound profile in the water column from the
measurements. The results of the calculations are shown in Fig.5.
Fig.5: Speed of sound profiles calculated from CTD casts.
Current Measurements from the ADCP at the Base of the VLA
The ADCP was deployed at the base of the VLA only in MC798, the water depth in
AT14 being too great for its pressure housing. Current measurements recorded in
MC798 are shown in Fig.6. They indicate subsurface currents to be southerly at a
maximum of about 90 mm/sec (less than 0.2 knots). Thus the shape of the VLA at the
MC798 site probably did not deviate very much from being vertical.
6
Fig.6: Data from the ADCP at the base of the VLA (per Vernon Asper).
7
Recording Conditions, Meeting with the Objectives and Data Quality
Seas were generally calm but occasionally rose. Survey speeds ranged from 3.5 to 4.5
knots. Unfortunately, the cruise could not meet with all planned objectives. At the
beginning of work in the first area, MC798, the SS/DR data was found to be very
noisy. The problem was caused by electrical shorts in the deep-tow cable. Attempts to
make repairs failed and no SSDR data were recorded during the cruise.
The VLA data collected using both the watergun source and ship noise were
quite good, however. The low-gain preamplifier was on channel 15 at the MC798 site
and on channel 16 at the AT14 site. A map of runs on the deployment in MC798 is
shown in Fig.7 and a map of those in AT14 is shown in Fig.8. Locations where the
runs start and end are given in Tables I and II.
Fig.7: Tracks of runs on the VLA site in Mississippi Canyon 798 (background
bathymetry provided by C&C Technologies, Inc.)
8
Fig.8: Tracks of runs on the VLA site in Atwater Valley 14 (background
bathymetry provided by C&C Technologies, Inc.)
Acknowledgements
The competent cooperation of the captain and crew of the R/V Pelican is gratefully
acknowledged. Graphics for the report were made by Paul Mitchell.
9
Table I: Locations for VLA Runs on Mississippi Canyon 798 Site
(VLA deployed at 28o 08.1180’N, 89o 39.6696’W)
Line
Number
FFID
Start
FFID
End
Start of Run
Lat/Lon
End of Run
Lat/Lon
Start of Run
UTM (meters)
End of Run
UTM (meters)
238604.074
3111589.613
235952.235
3115094.176
242743.733
3112448.390
241365.055
3117939.495
238589.103
3112293.191
236471.317
3115095.605
242171.274
3112099.318
241053.228
3117535.942
238647.742
3119206.960
233979.052
3115033.402
241684.273
3111963.528
241573.993
3118086.253
238574.366
3118004.479
235243.350
3115001.916
240804.950
3112900.691
240609.584
3117130.688
WATER GUN RUNS
Ln1
SN
Ln2
WE
Ln3
SENW
Ln4
NESW
3770
4248
6679
7195
5428
5998
7769
8387
2806.2433’N
8939.6348’W
2808.1078’N
8941.3002’W
2806.7566’N
8937.1202’W
2809.7115’N
8938.0342’W
2806.6238’N
8939.6533’W
2808.1148’N
8940.9834’W
2806.5611’N
8937.4649’W
2809.4895’N
8938.2192’W
NOISE RUNS
Ln1
NS
Ln2
WE
Ln3
SENW
Ln4
NESW
8483
8693
9179
9390
8840
9070
9525
9733
2810.3651’N
8939.7101’W
2808.0512’N
8942.5036’W
2806.4819’N
8937.7603’W
2809.7933’N
8937.9086’W
2809.7137’N
8939.7387’W
2808.0494’N
8941.7316’W
2806.9786’N
8938.3093’W
2809.2650’N
8938.4846’W
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Table II: Locations for VLA Runs on Atwater Valley 14 Site
(VLA deployed at 27o 56.5212’N, 89o 17.0856’W)
WATER GUN RUNS
Line
Number
Ln29
SENW
Ln30
SENW
Ln31
SENW
Ln32
SN
Ln33
SWNE
Ln34
NESW
Ln35
NESW
Ln36
NESW
Ln37
SWNE
Ln38
WE
FFID
Start
16303
FFID
End
16559
10691
11154
19696
19949
13040
13588
17021
17354
17490
17724
15232
15788
18366
18699
18907
19157
14430
14950
Start of Run
Lat/Lon
2755.2641’N
8917.2288’W
2755.0292’N
8915.6841’W
2756.5969’N
8915.5033’W
2753.9501’N
8917.1424’W
2756.4915’N
8918.9550’W
2757.7937’N
8916.5233’W
2758.0772’N
8915.3642’W
2757.1746’N
8915.6449’W
2755.1762’N
8917.1171’W
2756.3730’N
8919.6041’W
End of Run
Lat/Lon
2755.5171’N
8917.4465’W
2755.2603’N
8915.9064’W
2756.8310’N
8915.7062’W
2754.2285’N
8917.1487’W
2756.6767’N
8918.7501’W
2757.5872’N
8916.7712’W
2757.7785’N
8915.5825’W
2756.8142’N
8916.0337’W
2755.3788’N
8916.8867’W
2756.4139’N
8919.1621’W
Start of Run
UTM (meters)
274924.397
3090563.204
277450.675
3090082.028
277800.865
3092972.194
275020.884
3088133.404
272135.348
3092883.585
276168.911
3095213.923
278079.429
3095702.165
277588.227
3094043.631
275104.650
3090397.409
271066.457
3092684.900
End of Run
UTM (meters)
274576.070
3091037.054
277093.901
3090515.633
277476.053
3093410.780
275020.170
3088647.784
272477.836
3093219.209
275755.206
3094839.951
277711.238
3095157.171
276938.266
3093389.770
275489.626
3090764.531
271792.860
3092746.625
277290.474
3089998.611
275071.133
3088533.546
272931.532
3092707.909
276910.175
3090765.977
275147.700
3089814.838
273795.546
3092813.612
NOISE RUNS
Ln30
SN
Ln32
SN
Ln38
WE
21314
21450
20538
20787
21112
21244
2754.9824’N
8915.7808’W
2754.1672’N
8917.1163’W
2756.4046’N
8918.4677’W
2755.3939’N
8916.0212’W
2754.8614’N
8917.0842’W
2756.4706’N
8917.9424’W
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