**NORTHERN GULF LITTORAL INITIATIVE (NGLI),**

**GEOLOGY AND PHYSICAL PROPERTIES OF MARINE SEDIMENTS**

**IN THE N.E. GULF OF MEXICO: DATA REPORT**

The Naval Research Laboratory (NRL), Seafloor Sciences Branch, was
tasked in calendar year 2000 to analyze sediment cores and grab samples
collected by the Naval Oceanographic Office (NAVO) (Code N33T) in
support of the Northern Gulf Littoral Initiative (NGLI), a multi-agency,
regional scale modeling program.

( see <http://www.navo.navy.mil/NGLI/main_frame.html> ).

One objective of the NGLI project was to establish a sedimentological
and physical property database for Northern Gulf of Mexico that can be
immediately utilized by the existing Navy R&D activities within the
area. Results from the core and grab sample analyses will be used to
populate this database.

NAVO's ambitious sampling efforts resulted in the recovery of over 50
sediment cores (averaging \~111 cm. in length), and 35 bottom grab
samples during the 1999 and 2000 field seasons consisting of 6 separate
sediment sampling cruises aboard the RV Pelican (LUMCON). NGLI bottom
samples were collected in water depths ranging from \~3 to over 300
meters with an average depth of 36.5 meters.

NRL conducted non-destructive testing of the cores utilizing x-ray
radiography and multi-sensor core logging to determine the structure and
physical properties of the marine sediment cores. A *Geotek*
Multi-Sensor core logger (Schultheiss and McPhail, 1989) was utilized to
determine, and digitally log p-wave velocity (using a pair of 500 kHz
piezo-electric ceramic transducers), wet bulk density by gamma-ray
attenuation (using a 137-Cs gamma source and scintillation counter),
core diameter deviation, and temperature at one centimeter intervals
down-core. The core logger is highly automated, runs in a windowed PC
environment, and stores raw data straight to computer disk where it can
be further processed. Derivative acoustic impedance (g/m^2^sec) and
fractional porosity are additionally calculated and reported for each
sample interval. Fractional porosity values are reported relative to an
average grain density value of 2.65 g/cc, and a pore water density of
1.026 g/cc. P-wave velocity values are reported at a standard laboratory
temperature of 23^o^ C, and 35 ^o^/oo salinity.

Upon completion of the non-destructive testing, the core liners were
split open along the length of the core with an electro-mechanical core
liner splitter. Once the liners were split, the sediment core was then
split apart utilizing an osmotic knife, which parts the cylinder of
sediment into two equal halves while maintaining the visible structural
detail within the sediment cores without the effect of smearing the
split sediment surface. One half of the core was then photographed in
\~20 cm/shot increments downcore, while the other half was used for
physical properties and shear strength testing. Most of the processed
core material is currently archived at NRL's core refrigerator facility
at Stennis Space Center, MS.

One surficial sample (\~0-5 cmbsf (centimeters below the seafloor) in
sediment cores) was analyzed from each NGLI core and grab sample
location for particle size distribution and mean grain size statistics.
Classical sieving techniques were utilized for the sand sized sediments,
and the fine particles (silt and clay) were measured via settling using
pipette analysis and a Micromeritics Sedigraph (Model 5000). Grain size
data are presented in phi (φ) values where φ = -log~2~(d (mm)), (d =
particle diameter). Equivalent particle size in millimeters is noted in
the individual grain size histograms, and presented in the NGLI sediment
map.

Undrained shear strength testing was conducted on cores containing
cohesive marine sediments by using either a torvane, or a miniature
laboratory vane shear apparatus. High resolution shear strength testing
(5 miniature vane samples in the upper 10 cm) was conducted in the
uppermost 10 cm of each core sample. Below this 10 cmbsf level, the
laboratory torvane was used to measure undrained shear strength at
intervals of \~20cm to the end of the core. Most cores were considered
sufficiently cohesive to run the shear tests. No strength measurements
were made on grab samples because the samples were sandy and
cohesionless.

Results from the core logging effort, the core and sediment laboratory
analyses, and the textural classification of the sediment samples are
presented on this compact disk (CD) data report. The CD contains 2
directories, one contains data, plots and photos, and the other contains
maps. Core logger data, (p-wave velocity and relative amplitude, gamma
density, fractional porosity, and acoustic impedance), and laboratory
physical property data (natural water content and undrained shear
strength values) are provided in a standard spreadsheet or workbook
format that can be read by most software (see Appendix I data format
details). Also presented are textural or grain size data and statistics
for surficial samples from each NGLI sample location. Down-core
photographic images, of the split core halves are also provided for each
core (one lower resolution and one higher resolution jpeg formatted
files in increments of \~20 cm. down-core). Photographs from grab sample
sub-samples are also provided. Budgetary and time constraints prevented
completion of creating scanned files of the core x-radiographs on this
CD, however the x-ray films are available to be viewed at NRL.

Sub-directories (Gif, Jpg, and Png) in the map directory contain several
very generalized regional maps depicting surficial sediment grain size
characteristics (mean phi, % sand, and % clay) and spatial distribution,
both seaward and inshore of the LA-MS-AL barrier islands. Additional
data sources were combined with the NGLI grain size data to populate the
2D grid (using a natural neighbor gridding algorithym). Measured
undrained shear strength values were used to construct a surficial
sediment cohesion map (average undrained shear strength value in the
interval 0-10 cm) in the open Gulf of Mexico region of the NGLI domain
(seaward of the barrier islands). ASCII grid files are also provided in
the map directory on the CD with a readme file explaining the file
format.

One important note here is that sample data used to construct the
sediment maps vary greatly both in spatial data density, and temporally
(significant historical data covering more than 20 years of varied
sedimentological research was combined with the NGLI data set for
purposes of constructing these maps). Because of these concerns, natural
variability within marine sediments in the region is obviously not
addressed by map content.

**Data Sources and References**

The authors are greatly appreciative to the following U.S. Government
and State Agencies, universities, and individual researchers and authors
who have assisted NRL in attempting to better define and understand the
geological nature of bottom sediments and seafloor characteristics
within the NGLI geographic domain.

1.  NAVO Data Warehouse (Mark Geiger).

2.  National Geophysical Data Center-NGDC DOC-NOAA, MS. Carla Moore

3.  Gulf Coast Research Laboratory

> a.) USM Institute of Marine Sciences Dr. Ervin Otvos and Dr. D. Jay
> Grimes, marine sediment sample data.
>
> b.) Gulf Coast Research Laboratory, Sept. 1988, A Location Map of the
> Oyster Resource in Western Mississippi Sound, Project Leader W.J.
> Demoran.

4.  MS Dept of Environmental Quality, Office of Geology Mr. Keil Schmid

> Provided 2 unpublished MS thesis with Mississippi waters sediment
> information:
>
> a.) Wilson, Andrew Moore, "Response Modeling of Holocene Sediments
> within the Shallow Substrate of Central Mississippi Sound",
> unpublished MS Thesis University of Mississippi, Department of Geology
> and Geological Engineering, 1984, 89 pp.
>
> b.) Chastain, David Lawrence, "Western Mississippi Sound: Analysis of
> Sedimentary Responses and Recent Deposition in a Semi-Enclosed
> Brackish Water Basin, Northeast Gulf of Mexico", unpublished MS Thesis
> University of Mississippi, Department of Geology and Geological
> Engineering, September 1981, 90 pp.

5.  Environmental Monitoring and Assessment Program (EMAP), U.S.
    EPA/NHEERL/Gulf Ecology Division, Gulf Breeze, FL , MS Linda Harwell
    and MS Virginia Engle.

6.  NGLI Sampling Cruises 1999-2000, Naval Oceanographic Office, Dr John
    Blaha and Carl Szczechowski, LUMCON RV Pelican.

7.  Schultheiss, P.J., and S.D. McPhail, 1989. An automated P-wave
    logger for recording fine scale compressional wave velocity
    structures in sediments. In: W. Ruddiman, M. Sarnthein, et. al.
    (Eds.), Proc. ODP, Sci. Results, 108: pp. 407-413.

8.  Thompson, M.J., W.W. Schroeder, N.W. Phillips, and B.D.
    Graham, 1999. Ecology of Live Bottom Habitats of the Northeastern
    Gulf of Mexico: A Community Profile. USGS/BRD/CR-1999-0001, OCS
    Study MMS 99-0004, 104 pp.

Appendix I

Data Format and Definitions

Sediment geoacoustic and physical property data from the multi-sensor
core logger are reported down core in both decimal meter and centimeter
intervals. Core section is indicated beginning with section one to
section 'n' depending on core length and number of sections that were
cut to length on the coring vessel (typically \~1 meter lengths). Core
diameter deviation (mm) data are logged from output from linear voltage
displacement transducers (LVDT) co-located with the acoustic transducer
pair. This measure (core diameter deviation) takes into account small
differences in acoustic travel path distance, and is used in the
calculation of p-wave velocity down core. A relative amplitude value
provides the logger operator and the data interpreter with a measure of
how well the acoustic transducers are coupled with the core liner, and
also how well the sediment fills the plastic core liner. P-wave velocity
is measured using a pair of spring-mounted, oil-filled rolling acoustic
transducers (500kHz) and is reported in m/s down-core. Gamma-ray density
(analogous to wet bulk density) determined using gamma-ray attenuation
is calculated from raw counts/second data and calibration values
determined from a calibration water-filled core containing an aluminum
(density \~2.7g/cc) slug of varying thickness. Acoustic impedance
(g/m^2^s) is calculated from the product of density and velocity.
Fractional porosity is calculated from standard relationships of the
soil skeleton and the water filling the voids in the sediment core.
These relationships assume a selectable but constant sediment density
(2.65 g/cc used here) and selectable water density (1.026 g/cc), and
that the voids within the sediment are 100% saturated.

Note that the data have been edited and blank values, or no values
usually indicate where the core logger system encountered acoustic
coupling problems which usually occurred near the core tops and bottoms
and/or near core section breaks. The deleted data were usually erroneous
p-wave velocity values and derivative acoustic impedance. Some of the
cores had desiccated due to the length of time between core recovery and
core logging. This resulted in poor acoustic coupling and no or
unreliable p-wave velocity values being logged. The following
spreadsheet sample is provided for clarification.

  ------------- --------- -------------- ----------- ----------- ---------- --------- ----------- ------------ ------ ------
  ALL           NGLI      SPREADSHEETS                                                                                

  Geotek MSCL                                                                                                         
  Version 4.2 -                                                                                                       
  299C_1.OUT                                                                                                          
  created at                                                                                                          
  11:25:56 on                                                                                                         
  07-12-2000.                                                                                                         

  299c1                                                                                                               

                                                                                                                      

  Section Depth Section   Section Depth  Core        relative    p-wave     gamma ray acoustic    Fractional          
  (m)           \#        (cm)           Diameter    p-wave      velocity   density   impedance   Porosity            
                                         Deviation   amplitude   (m/s)      (g/cc)    (g/m2s)                         
                                         (mm)                                                                         

                                                                                                                      

  0.01          1         1              6.666       23                                                               

  0.02          1         2              6.667       46                     1.263                 0.8541              

  0.03          1         3              6.667       66          1617.811   1.4063    2275.142    0.7658              

  0.04          1         4              6.668       97          1641.96    1.4344    2355.198    0.7485              
  ------------- --------- -------------- ----------- ----------- ---------- --------- ----------- ------------ ------ ------

Undrained shear strength was measured on all cores except those where
cohesionless sediments were encountered. For this study, a cohesionless
sediment is considered one which contained less than \~25% mud.
Intervals in the upper 10 cm of cores were measured for undrained shear
strength using a miniature vane shear device with a vane rotation rate
of 70 degrees/minute. A hand operated torvane was used for intervals
further down-core. Results are reported in the following example in
kilopascal units.

  ---------- ------------ ------------- ------------------ ----------------
                                                           

                                                           

             **NGLI Shear                                  
             Strength**                                    

                                                           

                                        Undrained          

  Cruise     Sample ID    Interval      Shear Strength     

  (MO-YR)                 \(cm\)        (kPa)              

  2-99       G1           .75-2.0       2.74               

                          2.75-4        1.45               

                          4.5-5.75      2.62               

                          6.50-7.75     4.24               

                          8.5-9.75      4.64               

                          11-16         5.75               
  ---------- ------------ ------------- ------------------ ----------------

Natural water content (% dry weight) was determined on freshly split
core halves, and are reported down core as shown in the following
example spreadsheet.

  --------- ------ ----------- --------- ----- ----- ----- ------------- ----- -----
                                                                               

                   **NGLI                                                      
                   Water                                                       
                   Content**                                                   

                                                                               

  Cruise    Core   Interval    Natural                                         
            ID                 Water                                           
                               Content                                         

  (MO-YR)                      (%)                                             

  2-99      C1     0-1cm       33.70                                           

                   1-2cm       35.97                                           

                   2-3cm       43.30                                           

                   3-4cm       34.92                                           

                   4-5cm       28.19                                           

                   5-6cm       37.03                                           

                   6-7cm       47.20                                           

  2-99      C2     0-1cm       122.40                                          

                   1-2cm       136.99                                          

                   2-3cm       140.60                                          

                   3-4cm       141.13                                          
  --------- ------ ----------- --------- ----- ----- ----- ------------- ----- -----

For additional information please contact:

Bill Sawyer

NRL Code 7431

bill.sawyer@nrlssc.navy.mil

Voice 228-688-5233-Core Laboratory

FAX 228-688-5752

Or

Chad Vaughan

NRL Code 7431

chad.vaughan@nrlssc.navy.mil

Voice 228-688-5369

FAX 228-688-5752