A TECHNIQUE FOR OBTAINING A GEOPHYSICAL MODEL OF THE ...



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

1MMRI/CMRET

University of Mississippi

University, MS

2Speciality 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 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.

[pic]

Fig.1: Schematic of the prototype vertical line array

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).

[pic]

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.

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).

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.

Fig.6: Data from the ADCP at the base of the VLA (per Vernon Asper).

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.)

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.

| |

|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 |FFID |Start of Run |End of Run Lat/Lon |Start of Run |End of Run |

| |Start |End |Lat/Lon | |UTM (meters) |UTM (meters) |

| |FFID |FFID |Start of Line |End of Line Lat/Lon |Start of Line |End of Line |

|WATER GUN RUNS |Start |End |Lat/Lon | |UTM (meters) |UTM (meters) |

|Ln1 |3770 |4248 |28(06.2433’N |28(06.6238’N |238604.074 3111589.613 |238589.103 3112293.191 |

|S(N | | |89(39.6348’W |89(39.6533’W | | |

|Ln2 |6679 |7195 |28(08.1078’N |28(08.1148’N |235952.235 3115094.176 |236471.317 3115095.605 |

|W(E | | |89(41.3002’W |89(40.9834’W | | |

|Ln3 |5428 |5998 |28(06.7566’N |28(06.5611’N |242743.733 3112448.390 |242171.274 3112099.318 |

|SE(NW | | |89(37.1202’W |89(37.4649’W | | |

|Ln4 |7769 |8387 |28(09.7115’N |28(09.4895’N |241365.055 3117939.495 |241053.228 3117535.942 |

|NE(SW | | |89(38.0342’W |89(38.2192’W | | |

| |

|NOISE RUNS |

|Ln1 |8483 |8693 |28(10.3651’N |28(09.7137’N |238647.742 3119206.960 |238574.366 3118004.479 |

|N(S | | |89(39.7101’W |89(39.7387’W | | |

|Ln2 |9179 |9390 |28(08.0512’N |28(08.0494’N |233979.052 3115033.402 |235243.350 3115001.916 |

|W(E | | |89(42.5036’W |89(41.7316’W | | |

|Ln3 |8840 |9070 |28(06.4819’N |28(06.9786’N |241684.273 3111963.528 |240804.950 3112900.691 |

|SE(NW | | |89(37.7603’W |89(38.3093’W | | |

|Ln4 |9525 |9733 |28(09.7933’N |28(09.2650’N |241573.993 3118086.253 |240609.584 3117130.688 |

|NE(SW | | |89(37.9086’W |89(38.4846’W | | |

| |

|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 |FFID |FFID |Start of Run |End of Run Lat/Lon |Start of Run |End of Run |

| |Start |End |Lat/Lon | |UTM (meters) |UTM (meters) |

|Ln29 |16303 |16559 |27(55.2641’N |27(55.5171’N |274924.397 3090563.204 |274576.070 3091037.054 |

|SE(NW | | |89(17.2288’W |89(17.4465’W | | |

|Ln30 |10691 |11154 |27(55.0292’N |27(55.2603’N 89(15.9064’W|277450.675 3090082.028 |277093.901 3090515.633 |

|SE(NW | | |89(15.6841’W | | | |

|Ln31 |19696 |19949 |27(56.5969’N |27(56.8310’N |277800.865 3092972.194 |277476.053 3093410.780 |

|SE(NW | | |89(15.5033’W |89(15.7062’W | | |

|Ln32 |13040 |13588 |27(53.9501’N 89(17.1424’W |27(54.2285’N 89(17.1487’W|275020.884 3088133.404 |275020.170 3088647.784 |

|S(N | | | | | | |

|Ln33 |17021 |17354 |27(56.4915’N |27(56.6767’N 89(18.7501’W|272135.348 3092883.585 |272477.836 3093219.209 |

|SW(NE | | |89(18.9550’W | | | |

|Ln34 |17490 |17724 |27(57.7937’N |27(57.5872’N |276168.911 3095213.923 |275755.206 3094839.951 |

|NE(SW | | |89(16.5233’W |89(16.7712’W | | |

|Ln35 |15232 |15788 |27(58.0772’N |27(57.7785’N |278079.429 3095702.165 |277711.238 3095157.171 |

|NE(SW | | |89(15.3642’W |89(15.5825’W | | |

|Ln36 |18366 |18699 |27(57.1746’N |27(56.8142’N |277588.227 3094043.631 |276938.266 3093389.770 |

|NE(SW | | |89(15.6449’W |89(16.0337’W | | |

|Ln37 |18907 |19157 |27(55.1762’N |27(55.3788’N |275104.650 3090397.409 |275489.626 3090764.531 |

|SW(NE | | |89(17.1171’W |89(16.8867’W | | |

|Ln38 |14430 |14950 |27(56.3730’N |27(56.4139’N |271066.457 3092684.900 |271792.860 3092746.625 |

|W(E | | |89(19.6041’W |89(19.1621’W | | |

| |

|NOISE RUNS |

|Ln30 |21314 |21450 |27(54.9824’N |27(55.3939’N |277290.474 3089998.611 |276910.175 3090765.977 |

|S(N | | |89(15.7808’W |89(16.0212’W | | |

|Ln32 |20538 |20787 |27(54.1672’N |27(54.8614’N |275071.133 3088533.546 |275147.700 3089814.838 |

|S(N | | |89(17.1163’W |89(17.0842’W | | |

|Ln38 |21112 |21244 |27(56.4046’N |27(56.4706’N |272931.532 3092707.909 |273795.546 3092813.612 |

|W(E | | |89(18.4677’W |89(17.9424’W | | |

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