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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 2. REPORT TYPE 1. REPORT DATE (DD-MM-YYYY)
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5a. CONTRACT NUMBER
4. TITLE AND SUBTITLE
5b. GRANT NUMBER SPATIAL AND TEMPORAL VARIATIONS IN SEDIMENT COMPRESSIONAL WAVE SPEED AND ATTENUATION MEASURED AT
400 KHZ FOR SAX04
5c. PROGRAM ELEMENT NUMBER N602782N 6. AUTHOR(S)
5d. PROJECT NUMBER
KEVIN B. BRIGGS, MICHAEL ZIMMER, AND MICHAEL D. RICHARDSON
Se. TASK NUMBER
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Naval Research Laboratory
NRL/PP/7430-05-7
Seafloor Sciences Branch Stennis Space Center, MS 39529
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Office of Naval Research
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Arlington, VA 22217-5660
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Approved for public release; distribution is unlimited 13. SUPPLEMENTARY NOTES Boundary Influences In High Frequency, Shallow Water Acoustics, Pages 29-37, University of Bath, UK 5th - 9th September 2005. 14. ABSTRACT Diver cores were collected from various locations within the SAX04 site from 21 September to 3 November 2004 and compressional wave speed and attenuation were measured every centimeter at 400 kHz. Although measured values of sound speed and attenuation fall within established ranges for archived data from similar medium quartz sands, fluctuations in measured values were observed in the data that can be explained by the pattern of storm events during the experiment. Three significant storm events occurred during the period in which cores were collected: a category-4 hurricane, a tropical storm, and an early-winter cold front. Following these events values of sound speed initially increased, but then later decreased; values of sound attenuation did not show this pattern, but were generally lower (mean = 92.1 dB/m) than values measured 15. SUBJECT TERMS
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SPATIAL AND TEMPORAL VARIATIONS IN SEDIMENT
COMPRESSIONAL WAVE SPEED AND ATTENUATION MEASURED AT 400 KHZ FOR SAX04 KEVIN B. BRIGGS, MICHAEL ZIMMER, AND MICHAEL D. RICHARDSON Naval Research Laboratory,Seafloor Sciences Branch, Stennis Space Center,MS 39529-5004 USA E-mail:
[email protected]
Diver cores were collected from various locations within the SAX04 site from 21 September to 3 November 2004 and compressional wave speed and attenuation were measured every centimeter at 400 kHz. Although measured values of sound speed and attenuation fall within established ranges for archived data from similar medium quartz sands, fluctuations in measured values were observed in the data that can be explained by the pattern of storm events during the experiment. Three significant storm events occurred during the period in which cores were collected: a category-4 hurricane, a tropical storm, and an early-winter cold front. Following these events values of sound speed initially increased, but then later decreased; values of sound attenuation did not show this pattern, but were generally lower (mean = 92.1 dB/m) than values measured five years ago at the SAX99 site nearby (mean = 177.5 dB/m). Values of sediment sound speed measured at the SAX04 (mean velocity ratio = 1.162) were generally greater than those measured at the SAX99 site (mean velocity ratio = 1.155). Values of coefficient of variation for sediment sound speed were lower for SAX04 measurements (0.55%) than SAX99 measurements (0.70%). Lower values of sound attenuation measured at the SAX04 site was probably due to a lack or absence of shell fragments that may have been segregated by the sediment resuspension and settling during and after storms. The roles of sediment transport, grain size, grain sorting, porosity, and density in controlling sediment compressional wave speed and attenuation at the SAX04 site are discussed.
1 Introduction Sediment compressional wave speed and attenuation are important parameters required
in assessing acoustic scattering from the sea floor [1,2]. The contrast between the sound speed within the sediment and the sound speed in the overlying water is the chief characteristic determining the fate of acoustic energy reaching the sea floor. The magnitude of the attenuation of the acoustic energy within the sediment determines to what depth the sound penetrates the sea floor (and how much scattered energy is reradiated back out of the sea floor). Moreover, fluctuations in sediment sound speed may be responsible for the scattering of acoustic energy penetrating into the sediment. As part of the geoacoustic measurements collected by the Naval Research Laboratory during Sediment Acoustics eXperiment 2004 (SAX04), sediment compressional wave speed and attenuation were determined at 400 kHz on 58 diver cores. The cores were collected throughout the experiment, from 21 September to 3
29 N.G.Pace and P Blondel, (Eds), Boundary Influences In High Frequency,Shallow Water Acoustics, Pages 29-37,, University of Bath, UK 5 th- September 2005
20060721010
30
K. B. BRIGGS, M. ZIMMER, AND M. D. RICHARDSON
November and measurements were made aboard ship within 24-36 hours after collection. Consequently, the measurements exhibit both spatial and temporal variations in these parameters for the SAX04 site. The important factors controlling sediment compressional wave transmission in sands are porosity, grain size, grain sorting, grain shape, grain packing and mineralogy; although, porosity and grain properties are correlated factors [3]. Previous studies of the area off Fort Walton Beach, Florida show the sediment to be a relatively homogeneous, moderately well sorted, medium quartz sand [4]. Differences in sound speed transmission within the sediment are thought to be essentially a matter of grain packing, which can be changed by bioturbation and hydrodynamic events [5]. Because there were three major storms during the execution of SAX04, the effects of reconfiguring the grain-to-grain structure by storm resuspension and sedimentation may be examined.
2
Methods
Diver cores that were made from 5.9-cm-inside-diameter, clear polycarbonate tubing cut into 48-cm lengths were used to collect the sediment samples from various locations within the SAX04 experiment site. Each core was carefully handled and kept upright during the collection, the sealing of the top and bottom, the recovery through the airwater interface, the storage in the temperature-controlled laboratory, and the acoustic measurements. Upon recovery, each core was sealed with tape to prevent draining of the overlying water and the level of the sediment-water interface was marked on the core to monitor settling. After allowing the cores to equilibrate to laboratory temperature for at least 24 hours, sediment compressional wave speed and attenuation were measured directly through the core liner with "earmuff" transmit and receive transducers, function generator, pulse generator, dual band-pass filter, and digital storage oscilloscope as depicted in Fig. 1 [6]. Measurements were made at 400 kHz at 1-cm increments downcore.
31
SEDIMENT COMPRESSIONAL WAVE SPEED AND ATTENUATION
Figure 1. Equipment used to measure compressional wave speed and attenuation (left) and a close-up view of the "earmuff" transducers with a core inserted between the transducers (right). Sound speed ratio is calculated as the ratio of sound speed in sediment to sound speed in the overlying water. Sediment compressional wave attenuation values are expressed as dB/m and normalized to the acoustic frequency as dB/m.kHz (k, after Hamilton) [7]. Sediment porosity was measured at 2-cm intervals after transport of the cores to a shore-based laboratory from one to 33 days after collection by divers. Bulk density was calculated from porosity values and measurement of grain density. Grain size distribution was determined by dry-sieving for sand- and gravel-sized particles and pipettes for the silt/clay fraction. 3
Results
Compressional wave speed (Vp) ratio and attenuation as a function of sediment depth is illustrated in Fig. 2. Low values of Vp ratio correspond to mud layers and flasers [8] and these features as well as the attenuation values associated with them will be disregarded in all of the subsequent figures, tables, and results for the purpose of this analysis of compressional wave energy in sand. Sediment sound speed increases slightly downcore to about 15 cm depth, and is generally constant below that depth. Sediment sound attenuation exhibits a slight increase throughout the measured depth. Values of attenuation show great variability in the top 14 cm, and these high values that depart from the overall trend are probably indicative of scattering of the 400-kHz sound by the mud-sand interfaces of mud layers and flasers. Note the relative absence of high attenuation values below the sediment depth where the mud is found in Fig. 2. Attenuation (dB/m.cHz)
Vp Ratio 1.00
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K. B. BRIGGS, M. ZIMMER, AND M. D. RICHARDSON
32
Figure 2. Compressional wave speed (Vp) ratio (left) and compressional wave attenuation (right) as a function of depth in the sediment for the SAX04 site. During SAX04 three significant storm events occurred: Hurricane Ivan, a category 4 storm, made landfall 100 km west of the experiment site on 16 September; tropical storm Matthew approaching from the west produced strong southwest to west winds during 8-10 October; and a cold front passed through the area on 2 November. These storm events effectively divided the experiment into four periods, which were determined from patterns of sediment acoustic data collected from cores (Table 1). The average sediment sound speed excluding obvious mud layers is 1775.6 m/s calculated for 23°C, 35 ppt, and atmospheric pressure and the average sound speed ratio is 1.162 with a coefficient of variation (CV) of 0.55%. Table 1. Values in sand of compressional wave speed (Vp), compressional wave speed ratio, and compressional wave attenuation (dB/m, dB/m.kHz) for four periods during SAX04; values of coefficient of variation (%) for the data; values of the data for all periods of SAX04. Period
Dates
V,
Vp Ratio
Att. (dB/m)
k (dB/m.kHz)
1.164 1.158 1.162 1.166 1.162
87.8 99.9 90.2 89.8 92.1
0.22 0.25 0.23 0.23 0.23
(mis)
post Ivan pre Matt post Matt post cold front All post Ivan pre Matt post Matt post cold front All
21-30 Sep 1-5 Oct 12-31 Oct 3 Nov 21-30 Sep 1-5 Oct 12-31 Oct 3 Nov
1779.9 1770.6 1776.0 1782.7 1775.6 0.60 0.55 0.50 0.40 0.55
21.07 31.02 31.26 21.56 30.26
Immediately following Hurricane Ivan, suspended sediments settled and fine-grained mud brought from the lagoon north of the barrier island was deposited on top of the freshly settled sand. For 14 days following Ivan (post Ivan), benthic fauna appear not to have begun to recover from the catastrophic resuspension of the sediment in which they were living. Indeed, with the turbulent fluidization of their habitat sustained for several hours, whatever fauna that were not destroyed were irretrievably buried. In the next 5 days before the effects of Matthew began to be felt, values of sediment sound speed and attenuation appeared to change, perhaps due to a delayed recovery response of the benthic fauna. There is a significant difference (a
