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1. REPORT DATE (DD-MM-YYYY) 12. REPORT TYPE 13. DATES COVERED (From - To)

4. TITLE AND SUBTITLE Sa. CONTRACT NUMBER

Real-Time Characterization of Mine Scour Burial at the Martha's VineyardCoastal Observatory Sb. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER0602782N

6. AUTHOR(S) 5d. PROJECT NUMBERMichael D. Richardson, Edward F. Braithwaite III, Sean Griffin, JohnBradley, Carl T. Friedrichs, Arthur C. Trembanis, Peter Traykovski

59. TASK NUMBER

5f. WORK UNIT NUMBER74-6639-05

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATIONNaval Research Laboratory REPORT NUMBERMarine Geosciences DivisionStennis Space Center, MS 39529-5004 NRL/PP/7430-04-1

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Office of Naval Research ONR800 North Quincy Street 11. SPONSOR/MONITOR'S REPORTArlington VA 22217-5660 NUMBER(S)

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Approved for public release, distribution is unlimited

13. SUPPLEMENTARY NOTES6th International Symposium on Technology and the Mine Problem

14. ABSTRACTMine burial by scour was measured in real-time using two cylindrical instrumented mines connected to a shore-based facility at theMartha's Vineyard Coastal Observatory (MVCO). Data on mine movement (heading, pitch, and roll), scour pit geometry, percentburial, and environmental processes responsible for scour burial including significant wave height, period, and tidal heightwere analyzed daily and presented on the NRL web site http://www7430.nrlssc.naw.mil/bblp/mine/realtimedata/.Scour pits developed in response to storm generated significant wave heights of up to 2.5-m that occurred within the first 5 days ofthe experiments. The two instrumented mines pitched (3-90), rolled (35-550), and reoriented to align axially (up to 400) withincoming swell as they rolled into scour pits. The mines were buried level with the seafloor after a second storm. Subsequent stormwith wave heights up to 3.5-m were unable to further bury the mines. A comparison of the observed mine burial and real-timepredictions http://www.vims.edu/phvsical/ proiects/CHSD/projects/MBP/using a modified Whitehouse -Soulsby wave-inducedscour model were nearly identical suggesting mine burial by scour is predictable from bathymetry, sediment type, andmeasured or predicted surface wave conditions.15. SUBJECT TERMSMine burial by scour, Martha's Vineyard, Minecountermeasures

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON

a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT OF Michael D. RichardsonPAGES

Unclassified Unclassified Unclassified PARES 19b. TELEPHONE NUMBER (Include area code)SAR 6 228-688-4657

Standard Form 298 (Rev. 8/98)Prescribed by ANSI Std. Z39.18

Real-Time Characterization of Mine Scour Burial at the Martha'sVineyard Coastal Observatory

Michael D. Richardson, Edward F. Braithwaite III, Sean Griffin, John Bradley, Carl T. Friedrichs,Arthur C. Trembanis, and Peter Traykovski

Abstract: Mine burial by scour was measured in real-time burial at impact usually in low strength muddy sediments;using two cylindrical instrumented mines connected to a scour and infilling; bedform migration or transverse bedformshore-based facility at the Martha's Vineyard Coastal movement; bedform morphological alterations, such asObservatory (MVCO). Data on mine movement (heading, changes to shorerise or bar-berm conditions; liquefaction orpitch, and roll), scour pit geometry, percent burial, and fluidization of the sediment; and biological processes thatenvironmental processes responsible for scour burial scour or promote scour by altering seafloor physicalincluding significant wave height, period, and tidal height properties.were analyzed daily and presented on the NRL web sitehttp://www7430.nrlssc.navv.mil/bblp/mine/realtimedata/. The mine burial experiment described in this paper wasScour pits developed in response to storm generated conducted at the Martha's Vineyard Coastal Observatorysignificant wave heights of up to 2.5-m that occurred (MVCO) during the winter of 2003-04. The Woods Holewithin the first 5 days of the experiments. The two Oceanographic Institution (WHOI) developed thisinstrumented mines pitched (3-90), rolled (35-551), and observatory, off the south shore of Martha's Vineyard Island,reoriented to align axially (up to 400) with incoming swell to study coastal processes in an area dominated by openas they rolled into scour pits. The mines were buried level ocean conditions [2].with the seafloor after a second storm. Subsequent stormswith wave heights up to 3.5-m were unable to furtherbury the mines. A comparison of the observed mine burialand real-time predictions http://www.vims.edu/phvsical/proiects/CHSD/proiects/MBP/ using a modifiedWhitehouse -Soulsby wave-induced scour model werenearly identical suggesting mine burial by scour ispredictable from bathymetry, sediment type, andmeasured or predicted surface wave conditions.

Introduction: Buried mine detection is one of the greatestthreats facing shallow water Mine CounterMeasures (MCM)operations [1]. The possible presence of buried mines canchange MCM tactics from one of mine hunting to one ofminesweeping or area avoidance. The ability to predict mineburial both for planning and during operations (strategic and , .... • , ,° N U.

tactical scenarios) is therefore of great importance to Naval Fig. I. Cartoon of the relative location of the instrument and non-

forces. Processes known to contribute to mine burial include instrumented mines, rotary sidescan and pencil beam sonars, and far-fieldbedform measurements instruments to the 12-m communication node atMartha's Vineyard Coastal Observatory.

Mike. Richardson and Edward Braithwaite are with the MarineGeosciences Division, Naval Research Laboratory, Stennis Space Center, Instrumented Mines: The cylindrical mines (2.033 m long,MS 39529-5004 USA; E-mail mikerichardon(dnrlsscnavtmil 0.533 m in diameter, 1,984 kg m3 density) are constructed of

Sean Griffin and John Bradley are with OMNI Technologies Inc., 7412 non-magnetic bronze and covered with 112 flush mountedLakeshore Drive, New Orleans LA 70124 USA; E-mail acoustic sensors (1.5-MHz and 3.0-MHz) which are used tosgriffingotiengineering.com determine percent burial (% surface area) and measure scour

Carl Friedrichs and Art Trembanis are with Virginia Institute of Marine pit geometry [3]. Acoustic backscatter from sedimentScience (VIMS), College of Marine Science, Department of Physical suspended in the water column can be used to measure flowSciences, Gloucester Point, VA 23062 USA; E-mail cfried@vints.edu and estimate sediment concentrations near the mine. The

Peter Traykovski is with the Applied Ocean Physics & Engineering acoustic sensors have the potential to measure suspended

Department, Woods Hole Oceanographic Institution. Woods Hole, grain size distribution using the two-frequency inversionMA 02543 USA, E-mail ptraykovski@whoi.edu. techniques developed by Thorne [4]. Each mine has 6

pressure sensors which are used to determine tidal heights,

significant wave heights and periods, and when compared to Oceanographic Data: Tidal and wave data were measureda spatially-fixed reference pressure sensor calculate mine using the six pressure sensors located at 600 intervals aroundburial relative to the sediment surface. Changes in mine each mine (Fig 3). Only data from AIM3 is presented as noorientation (heading, roll and pitch) are measured with a significant difference between oceanographic data seriesthree-axis flux gate compass and three-axis accelerometer. measured with the two AIMs mines was detected. Tidal,

temperature and wave statistics were the nearly identical totime series reported by sensors located on at node of MVCOover the same time period (http://mvcodata.whoi.edu/cgi-bin/mvco/mvco.cgi). Bottom temperature decreased 100 overthe 62 day sampling interval (1 October through 5December). Seven storm events with significant wave heightsof 2-m or greater were recorded on day dates 279, 289, 295,303, 318, 324 and 333.

A4M3 Wow TvmuUý

278 29 298 3WO 310 3M 0

AIM3 Trid

Fig.2. Underwater photograph of Acoustic Instrumented Mine (AIM) 14j IExperimental Description: Four acoustic instrumented • j I --

mines together with six optical mines and six inert shapes 0 1were deployed near the Martha's Vineyard CoastalObservatory (MVCO) 1-2 October 2003 and most were 21 2a 9 2W83 310 3W 3o

recovered 16-18 April 2004. The observatory (MVCO) 14 AIMMw8,,, P*8

consists of a shore station with power and communication i,2facilities connected via buried electro-optical cable to a 0 't<1 t j "power and electronics node located 1.0 kin offshore in 12-m : ! •' •-' I ,/ 1 4, . ,,

water depth. Two acoustic mines were connected to the 14 0 ý11 t 1power and communications at the MVCO node to provide i4 i 2 2 378 310 -3 330

real-time burial and environmental data. Only data from the -, . . I M3 9V490V8 ,W H&

first experiment (2 Oct to 5 Dec 2003) using the two node- I

attached acoustic instrumented mines is reported in this -paper. The first experiment was terminated when the minesiwere moved on 5 December 2003. Data from the two H j• .... ',"• :acoustic mines connected to the communication node were 2 2 0 310 2 2 34

stored on shore-based computers at MVCO, downloaded to G" 000

the Naval Research Laboratory daily over the Internet, andanalyzed to determine hourly values of percent burial, roll,

Fig. 3. Summary of temperature, tide and wave data collected by AIM3 duringpitch, heading, tidal state, significant wave height and period, deployment at MVCO between 1 October 2003 and 5 December 2003.

and bottom water temperature. The data were then displayedin graphical and tabular form on the NRL web site Mine Movement and Burial: Changes in mine orientation(http://www7430.nrlssc.navv.nilibblin/ine/realtimedat-l). (heading, pitch and roll) generally occurred during stormLater analyzes provided data on percent burial based on events when significant wave heights were greater than 2acoustic sensors covered, a geometric description of the scour meters (Figs 4 and 5). During the first two storms (day datespit, and sediment concentration in the water column. 278 and 289) both mines pitched, rolled and changed headingSediments in the vicinity of the experiment were moderately suggesting mines rolling into scour pits. No movement waswell-sorted, fine quartz sand with a mean grain size of 0.18 detected during storms 3 and 4 (day dates 296 and 303)mm (phi 2.5), average grain density of 2661 kg m3 (SD = 11 indicating scour pit development was insufficient to reorientkg mi ), porosity of 38.5% (SD = 0.7 %) and bulk density of the mines. Wave conditions during the fifth storm (day date2042 kg m3 (SD = 11 kg m 3) [5]. An average of 95% of the 317) correlated with movement of AIM3 only. The 6th stormsediments were sand-sized (by weight) with most grains (day date 325) generated lower significant wave heights thanbetween 0.1 and 0.35 mm. previous storms and resulted in only a slight change in pitch

for AIM4. The last storm (day date 332) resulted in a changein pitch and heading of AIM3 but little movement of AIM4. 3

Over the 64 day period, the mines rolled roughly 40 (AIM3clockwise, AIM4 counter clockwise), pitched 3-4'; andreoriented parallel to dominate storm wave conditions (wavesfrom the southwest). _ ____ _

Mine burial was calculated relative to the sediment surface 2 W no 30 310 320 33 30

(Figs. 6 and 7) by comparing mean pressure measured with 3

the six pressure sensors around each mine relative to pressure ,1measured at a fixed point on the MVCO node. The •- --- __calculations assume negligible changes in sediment surfaceelevation which may accompany larger-scale bedform 170 n0

migrations. Mine burial was also measured as a function of _0

surface area covered (Fig 8 and 9) using the 120 acoustic 4.

sensors that covered the surface of the mine. Bothinstrumented mines buried nearly 40-cm relative to the -sediment water interface (pressure measurements) after thefirst storm (Day Date 278) and an additional 10-cm after the 6 o W Wo 210 3iO , 3,0

second storm (Day Date 289). Changes mine burial wasconcordant with changes in mine orientation during the same ,period and can be interpreted as mines rolling into scour pits. • !, ,• '• i•

Little change in burial was measured after the second storm I , ,

suggesting that scour during subsequent storms was 200 200 300 31o 330 33. 3,0

insufficient to further bury the mines but sufficient to allow __

some mine movement.Fig. 5. Changes in orientation (heading, pitch and roll) related to significant

wave height of AIM4 over the period of October 2 (day date 274) to December

AM3 mod", P,• 14 (day date 340) 2004.

I2| ,oApparent fluctuations in mine burial relative to the sedimentsurface are probably artifacts of the filtering of pressure

- series measurements. Burial of the mines measured as surface300 M 3 1'0 .M area covered yielded a much different result with a maximum

of 40% (AIM3) to 45% (AIM4) burial compared to near total

"21 1burial relative to the sediment water interface. It is apparent[ that only relatively minor infilling of the scour pits occurred•4 4 after storms and the mines although fully buried rested in

".. 1 larger scour pits (Figs 8 and 9). Sonar images (sector scan

t.0 20 30 3 , M 30 and pencil beam sonar) and multibeam surveys of the minesso collected by Traykovski et al [6] during the mine burial

experiments support the observations made using theinstrumented mines. Most of the surface area burial occurred

S...during the first storm which is concordant with the0 oobservations of burial relative to the sediment-water

.6 200 20 30 10 320 n 30 interface. Increases in surface area exposed for AIM3 were

evident during the second (day date 289) and fifth (day date

3I 318) storms probably as a result of scour. The second storm

12 (day date 289) resulted in increased burial of the surface of1, AIM4 and a decrease in surface burial occurred during the 5ih

. storm (day date 318). In summary, sediments were scoured

67 200 200 30 310 32U2034

0" ,away from the mine as significant wave heights exceeded 2meters and significant scour pits developed. At the height ofthe storms the mines began to pitch, and then rolled into the

Fig. 4. Changes in orientation (heading, pitch and roll) related to significant scour pits changing heading to axially align with thewave height of AIM3 over the period of October 2 (day date 274) to December i14 (day date 340) 2004. incoming waves. Scour around the mines occurred during

four subsequent storms with significant wave heights greater

than 2.5 meters without additional burial with respect to the transducers looking into the water column. Increased scatterseafloor. During two of these storms scour significant enough due to particles in the water column are evident during allto allow mine movement (pitch, roll and changes in heading) seven storm events (Fig. 10). These data will be used tooccur and high concentrations of sand-sized particles were determine temporal changes in the concentration and sizedetected in the water column (see next section). distribution of particles in the water column.

Acoustic reflections derived from transducers directed at,MfOs , ENA t Mrý othe sediment surface can be used to measure the distances

between the mine and sediment surfaces (Fig. 11). The scouri ,, pit dimensions can then be derived from an analysis of

501 • ?", /reflection distances calculated from all sensors surrounding

0, ,the mine and a careful consideration of mine orientation.Changes in mine orientation, especially roll, must be

02! accounted for in developing images of the temporal evolutionS/ J of the scour pits. Figure 12 shows scour pit development

during the first storm (day date 278). AIM4 is sitting in a

-iscour pit at least 20 cm below the sediment surface.

300l~tl 310 20 30i34 02 ___ _ _

Fig. 6. Mine burial (in meters) relative to the sediment water interface N-1oacabg •sO Data

determined as the difference between filtered pressure series measured withsix pressure sensors (AIM3) on the surface of the mine and filtered pressure _ _ _ _ _ _

series measured at a fixed point on the MVCO node (WHOI). L

Sg.4n1ran W-, 30.501

05 4000,0004 E alh 00W & * ~fW~oM to WOW-Sa40wl 8.05

m8 2o 31

v 1 -

041- /1

Fig. 8. Temporal relationships among burial measured as depth below thesediment water interface, burial measured by surface area covered and

0 significant wave height measured using AIM3.4ig4 7 b ( t e s n r f BS 00 TAW Dato

280 200 300 310 320 330 340 2.0 2.0 M 30 510 W0 3 3

Fig. 7. Mine burial (in meters) relative to the sediment water interface00 1 .~- 4- '~-i -

determined as the difference between filtered pressure series measured with j _ __ "six pressure sensors (AIM4) on the surface of the mine and filtered pressureseries measured at a fixed point on the MVCO node (WHOI).

Swcnleao W_11.

Scour Pit Development and Re-suspension Events: The 4f-

112 acoustic sensors located over the surface of the mine aredesigned to characterize the dimensions of the scour pit anddetermine particle size distribution of sediment suspended in 0. 0

the water column as a result of scour and sediment transport.Although the characterization of particle size distribution incolumng ise sharacterilaiong diloed siev ont w r Fig. 9. Temporal relationships among burial measured as depth below thethe water column is still being developed, events where sediment water interface, burial measured by surface area covered andsediment particles are suspended in the water column are significant wave height measured using AIM4.

easily seen in images of acoustic scatter from unburied

2D,¶ r Tn, wý. 022 both percent of the surface area covered with sediment andburial relative to the sediment-water interface matched burialmeasured with the instrumented mines (Fig. 13). These

&V 330 results suggest mine burial by scour is predictable and can be2 U forecast based on local bathymetry, sediment type, and

measured or predicted surface wave conditions.

310f 310 310-

200- - I 0AJM4 3-D c. 5 ,o M" Day 278.0888 Rog Aorje 171616

291 i 20

26500 2 4 20 10 60 0o -1 -0 .......8.8Wo lnt 8.m8 R 4600 R081.40)

Fig. 10 Temporal evolution of acoustic backscatter strength from a single 10

upward looking 3 MHz acoustic transducer on AIM3. Note the increased Ascattering (darker intensity of red) at distances of 15-80 cm from the mine 2 Z

surface. Blanking as a result of saturation from the transmitted signal -precludes measurements of particle size distribution closer than 15cm from J00the transducer face. Also included are mine roll (degrees) and significant 1 0wave height (meters) measured by three-axis flux gate compass and three- -100 /.

axis accelerometer and pressure sensors in AIM3.o i 10 - - - -- -- , .10s 0 -40 -100 Yw,)

20 Rob" Wy for Tinukaver A 7

2 2 Fig 12. Estimated dimensions of the scour pit surrounding AIM4 based onacoustic reflections (see Fig. 11) from sensors located on the surface of the

22 292 Mmine. This image derived from data collected on day date 278.08 (0100S. n9.October 5 2003) which was near the height of the I" storm. The mine's roll

was 17.8' from vertical, heading 287.6', pitch 1.0° and the significant waveS'- IN 211111 height was 2.44 meters. The colors on the intensity bar represent the height

2_ 2 of the reflection in cm above the blue reference plane (referenced to thebottom of the mine).284 20428

282 Buri: by Depth Below Wab.-Sed.mw Bwrner

M 2V

W, 200278 0

270 260 290 w00 T15 20 330 3407" 2414 Perem• 8-1a by Alma

ftwl W Rwim)Rol ear;40

0 2 1 0 2O 0 B0' 2000

Fig. 11. Temporal evolution of backscatter strength from a 750 kHz a single j •transducer looking at the seafloor. The distance between the mine surface 20

and sediment surface (scour pit) is easily determined as is the burial state at 10_... . . .that surface location. Note the change in distance of the scour pit as the mine 270 280 290 2W M 320 330 240

rolls into the scour pit. 4 l,.tar W..HIa

3-

Mine Burial Prediction: Mine burial was predicted in real-time using a modified Whitehouse-Soulsby [7] wave-inducedscour model, water depth, sediment mean grain size, and 200 , 3 31 2o 33 540

significant wave heights predicted from the NOAA Wave 20

Watch III wave forecast model. Both 5-day forecasts andburial hindcasts of wave height, bottom orbital velocity, wind Fig. 13. Temporal relationships among burial measured as depth below the

speed and mine burial were updated every 12 hours on the sediment water interface, burial measured by surface area covered andsignificant wave height. Thick lines (red) are predictions based on

VIMS mine burial web site http://www.vims.eduiphysicali WaveWatch III and a wave-induced scour model. Thin lines are observationsprojects/CHSD/projects/MBP/index/. Wave forecast from AIMs; the green is AIM3 and the blue AIM4 (see Figs 8 and 9).

predictions from NOAA's WaveWatch III model matched Preliminary versions of both sets of data were presented in real-time on web

both measurements at nearby NOAA buoys and sites at the Naval Research Laboratory and the Virginia Institute of Marine

measurements at the MVCO site. Mine burial predictions of Science.

ACKNOWLEDGMENTS References:

Subsequent burial instrumented mines were built under [1] M.D. Richardson and S.G. Tooma, "Environmentalfunding from the Office of Naval Research (Roy Wilkins, requirements to support mine countermeasure operation inDawn Lavoie and Thomas Drake, program officers), the shallow water," U.S. Navy Journal of Underwater Acoustics,Naval Research Laboratory (program element N062435N), vol. 43(3), pp.977-991, 1983.and under an SBIR contract administered by NAVSEA, [2] J. Edson, T. Austin, W. McGillis, M. Purcell, J. Ware, M.(James Bloodsworth and William Kastner project managers). McElory, S. Hurst and C. Grant, "The Martha's VineyardThe Captain, Dan Nelson, and crew of the R/V Connecticut Coastal Observatory,". 6 pps.provided excellent seamanship during the deployment of the [3] S. Griffin, J. Bradley and M.D. Richardson, "Improvedequipment. MVCO is ably managed by Janet Fredricks and subsequent burial instrumented mines". Sea Technology 44(11),Marguerite McElroy whose organizational talents were pp 40-44 2003.invaluable. Divers from NRL (Ricky Ray, Robert Fisher, [4] P.D. Thorne and J. Taylor, "Acoustic Measurements ofChad Vaughan, and Kevin Briggs) and WHOI (Glenn boundary Layer Flow and Sediment Flux", J. Acoustic Soc.McDonald, Jay Sisson, Andy Girard, Peter Schultz, Fred Am. 108 (4), 2000.Thwaites, and Erich Horgan) installed equipment on the [5] M.D. Richardson and P Traykovski, "Real-timeseafloor and connected them to the communications and observations of mine burial at the Martha's Vineyard Coastalpower ports at the MVCO node. Grant Bower, Conrad Observatory" Proceedings of the 5"' International SymposiumKennedy, and Rick Mang helped prepare the instrumentation on Technology and the Mine Problem. Naval Postgraduatefor deployment and provided logistical support during the School, Monterey, California, 22-25 May 2002, 11 pps. 2002.experiments. This study was supported by the Office of Naval [6] P. Traykovski, M.D. Richardson, J.A. Goff, L Mayer,Research (Tom Drake and Brian Almquist, program officers). R.H. Wilkens, and B. Gotowoka. Mine burial experiments atPeter Traykovski was also supported by an award from the the Martha's Vineyard coastal Observatory. Proceedings ofYoung Investigator Program of the Office of Naval Research. the 6 h International Symposium on Technology and the MineMichael Richardson was supported by the Office of Naval Problem. Naval Postgraduate School, Monterey, California,Research through Naval Research Laboratory Core funding 9-13 May 2004.under PE62435N. This is NRL contribution number NRL-PP- [7] A. Trembanis, C.T. Friedrichs, M.D. Richardson, P.7430-04-0001. []A rmaiCT rercs .. RcadoP

-Howd and P. Traykovski. Real-time forcasts of scour burialat Indian Rocks Beach, Florida and Martha's Vineyard,Massachusetts. Proceedings of the 6 h InternationalSymposium on Technology and the Mine Problem. NavalPostgraduate School, Monterey, California, 9-13 May 2004.