Gabriel R. Venegas

Effects of marine infauna on the acoustic properties of sediment

Marine infauna alter the surrounding habitat in many ways. Compact mud burrows, tubes built from shell hash, and large subsurface galleries are a few examples of these alterations. Structural changes such as these can have varying effects on the geoacoustic properties of sediment. Here, we investigate how infauna may affect the sound speed and attenuation in sediments both in natural diverse communities of organisms, and in laboratory mesocosm experiments using controlled monocultures of species representing potentially important functional groups. Field studies were conducted at Petit Bois Pass off the coast of Dauphin Island, Alabama in May 2017 in which sediment cores were collected and brought back to the lab for acoustic measurements. These measurements can be directly compared with those taken in situ using a deployable field apparatus. For sediments with both natural and manipulated communities of infauna, sound speed and attenuation were measured at multiple depths and at high frequencies with wav...

Compressional and shear in situ measurements in the Lower Laguna Madre

This presentation describes the apparatus, procedure, and results from a field experiment conducted in July 2017 in the seagrass beds of the Lower Laguna Madre. The Laguna Madre is a shallow hypersaline lagoon on the western coast of the Gulf of Mexico. The relatively flat bathymetry is covered by a thick meadow of Thalassia testudinum. Acoustic piezoelectric transducer probes measured compressional wave speed and attenuation in the seagrass canopy as well as geoacoustic properties (compressional and shear wave speed and attenuation) of the underlying sediment. Geoacoustic parameters were measured at depths between 5 cm and 20 cm in 5 cm increments. Compressional wave measurements were made over a frequency range of 20 kHz to 100 kHz, and shear wave measurements were made at 500 Hz and 1 kHz. The compressional wave data from this experiment indicates dispersion within the seagrass canopy and sediment. This dispersion is likely due to the high presence of gas bubbles in both the seagrass and supporting sed...

Observations of the effects of benthic biology in measurements from the Acoustic Coring System collected in the New England Mud Patch

This paper presents in situ measurements of sediment sound speed and attenuation collected in the New England Mud Patch using the Acoustic Coring System (ACS) with comparisons to sound speed measurements from a multi-sensor core logger (MSCL). The ACS uses rod-mounted piezoelectric cylinders mounted below the penetrating tip of a gravity corer to obtain a continuous record of sound speed as a function of depth as the corer penetrates the seabed. The MSCL measurements of the recovered sediment cores were acquired in 2 cm increments. In both the ASC and MSCL measurements, an elevated sound speed is observed in the upper 25 cm of the seabed which is the portion of the seabed where most infauna live. The sediments collected in the cores were analyzed for density, porosity, bulk organic matter, carbonate content, mineral composition, and grain size. Additional surficial sediment samples recovered using a box core and a multi-core were sieved for infauna collection. The organisms were preserved, identified, and classified according to acoustically relevant traits. The acoustic measurements are interpreted in terms of the physical measurements of cores and the abundance of different types of organisms present in the seabed. [Work supported by ONR.]This paper presents in situ measurements of sediment sound speed and attenuation collected in the New England Mud Patch using the Acoustic Coring System (ACS) with comparisons to sound speed measurements from a multi-sensor core logger (MSCL). The ACS uses rod-mounted piezoelectric cylinders mounted below the penetrating tip of a gravity corer to obtain a continuous record of sound speed as a function of depth as the corer penetrates the seabed. The MSCL measurements of the recovered sediment cores were acquired in 2 cm increments. In both the ASC and MSCL measurements, an elevated sound speed is observed in the upper 25 cm of the seabed which is the portion of the seabed where most infauna live. The sediments collected in the cores were analyzed for density, porosity, bulk organic matter, carbonate content, mineral composition, and grain size. Additional surficial sediment samples recovered using a box core and a multi-core were sieved for infauna collection. The organisms were preserved, identified, and...

Geoacoustic properties of seagrass-bearing sediments

Seagrasses are commonly referred to as “ecosystem engineers” due to their ability to modify their environments to create unique ecosystems believed to be the third-most valuable in the world. While engineering nursery habitats for small invertebrates, fish, and microalgae such as diatoms, they also aid in water filtration and are capable of storing 11% of the ocean’s organic carbon each year. Seagrasses mainly grow in shallow salty waters within sandy sediments, but over time can drastically alter the sediment by entrapping silt particles, organic matter and decaying organisms that collect on the seafloor to form an organic-rich mud that incorporates itself with the underlying sand. Anaerobic microbes break down the buried organic matter to produce biogas and more stable organic compounds, which can drastically affect sediment geoacoustic properties. Sediment cores were collected within a Thalassia testudinum meadow in the Lower Laguna Madre, TX, and analyzed in 2-cm depth increments for grain size, density, porosity, sound speed, and attenuation from 100 kHz to 300 kHz, and organic carbon content. Results are compared with those from a seagrass-free sediment core to investigate how these valuable “ecosystem engineers” can alter geoacoustical properties of the seabed. [Work supported by ONR and ARL:UT IR&D.]Seagrasses are commonly referred to as “ecosystem engineers” due to their ability to modify their environments to create unique ecosystems believed to be the third-most valuable in the world. While engineering nursery habitats for small invertebrates, fish, and microalgae such as diatoms, they also aid in water filtration and are capable of storing 11% of the ocean’s organic carbon each year. Seagrasses mainly grow in shallow salty waters within sandy sediments, but over time can drastically alter the sediment by entrapping silt particles, organic matter and decaying organisms that collect on the seafloor to form an organic-rich mud that incorporates itself with the underlying sand. Anaerobic microbes break down the buried organic matter to produce biogas and more stable organic compounds, which can drastically affect sediment geoacoustic properties. Sediment cores were collected within a Thalassia testudinum meadow in the Lower Laguna Madre, TX, and analyzed in 2-cm depth increments for grain size, densi...

Effects of benthic biology on geoacoustic properties of marine sediments

Infauna dwell in the benthic zone and have the capacity to modify the physical and acoustic properties of the seabed through bioturbation. To investigate such effects, in situ measurements of compressional and shear wave speed and attenuation were conducted in Petit Bois Pass, near the mouth of Mobile Bay, Alabama, USA. The measurement system was deployed at multiple locations within the pass, and acoustic measurements were conducted at depths up to 20 cm into the sediment to scan the portion of the seabed where most infauna live. Additionally, diver cores were collected and analyzed for infauna abundance and geotechnical properties, such as porosity and grain size distribution, for comparison with the acoustic data. While sediment geoacoustic models do not explicitly account for effects of biology, they do allow for parameterization of various physical properties like porosity, grain size, or pore fluid viscosity, all of which can be modified by the presence of biological organisms and bioturbation. Results from the in situ acoustic measurements and core analysis will be compared with such models to determine if any additional insight into the acoustic effects of infauna can be gained. [Work supported by ONR and ARL:UT IR&D.]Infauna dwell in the benthic zone and have the capacity to modify the physical and acoustic properties of the seabed through bioturbation. To investigate such effects, in situ measurements of compressional and shear wave speed and attenuation were conducted in Petit Bois Pass, near the mouth of Mobile Bay, Alabama, USA. The measurement system was deployed at multiple locations within the pass, and acoustic measurements were conducted at depths up to 20 cm into the sediment to scan the portion of the seabed where most infauna live. Additionally, diver cores were collected and analyzed for infauna abundance and geotechnical properties, such as porosity and grain size distribution, for comparison with the acoustic data. While sediment geoacoustic models do not explicitly account for effects of biology, they do allow for parameterization of various physical properties like porosity, grain size, or pore fluid viscosity, all of which can be modified by the presence of biological organisms and bioturbation. Resu...

Effect of carbon content on sound speed and attenuation of sediments in seagrass meadows

Globally, seagrass-bearing sediments contain 19.9 billion metric tons of carbon (C), and account for 10% of all organic C buried in the ocean each year. Protection of these C stores is vital in mitigating climate change [Fourqurean, J. W., et al., Nature Geoscience 5, 505–509 (2012)]. Some sediment acoustic properties are sensitive to the presence of gas bubbles entrained in such C stores due to inherent anaerobic decomposition. Measurement of these properties could therefore provide a means to indirectly monitor C stores and overall seagrass meadow productivity. As a preliminary effort to investigate the relationship between C content and acoustic properties of seagrass-bearing sediments, cores were collected in the seagrass meadows of Lower Laguna Madre, Texas. Sound speed and attenuation from 100 kHz to 300 kHz were measured radially in 2-cm-depth increments. The cores were subsequently frozen, sliced along the same depth increments, and their C content estimated using an elemental analyzer. Acoustic p...

Acoustics of biologically active marine sediments

Biological organisms are prevalent on and near the seabed; however, most predictive theoretical models do not explicitly address the acoustic behavior of biologically active marine sediments. Sediment-acoustics models, such as those based on Biot theory or grain-shearing theory, allow for parameterization of various physical properties including bulk density, porosity, shear strength, or pore fluid viscosity, all of which can be modified by the presence of biological organisms or bioturbation. Other models can include the effects gas bubble distributions, which can also be associated with the presence of benthic biology, on compressional and shear wave propagation. In this paper, we examine sediment-acoustics models in comparison with data from a field experiment focused on a muddy sediment containing infauna. The model parameters are partially constrained by physical data from core samples collected at the field experiment sites. Empirical relations between the core data parameters and acoustic data are also examined.Biological organisms are prevalent on and near the seabed; however, most predictive theoretical models do not explicitly address the acoustic behavior of biologically active marine sediments. Sediment-acoustics models, such as those based on Biot theory or grain-shearing theory, allow for parameterization of various physical properties including bulk density, porosity, shear strength, or pore fluid viscosity, all of which can be modified by the presence of biological organisms or bioturbation. Other models can include the effects gas bubble distributions, which can also be associated with the presence of benthic biology, on compressional and shear wave propagation. In this paper, we examine sediment-acoustics models in comparison with data from a field experiment focused on a muddy sediment containing infauna. The model parameters are partially constrained by physical data from core samples collected at the field experiment sites. Empirical relations between the core data parameters and acoustic data are ...

Acoustical characterization of a seagrass meadow in the Lower Laguna Madre

This talk presents preliminary results from an experiment conducted in the Lower Laguna Madre, Texas to characterize the physical and acoustical properties in a meadow of Thalassia testudinum. Concurrent measurements were collected using (1) acoustic probes, (2) side-scan and parametric sonar, (3) broadband propagation, and (4) sediment cores. The acoustic probes provided localized, direct measurements of sound propagation in the seagrass canopy as well as geoacoustic properties (compressional and shear wave speed and attenuation) of the seagrass-bearing sediment. The side-scan and parametric sonars were used to survey for seagrass abundance and sub-bottom layering. Broadband signals produced by a combustive sound source were recorded at several ranges by hydrophones and geophones and were used to infer geoacoustic properties of the seagrass and underlying sediment for rapid environmental assessment. The sediment cores were analyzed in the laboratory using both low-frequency resonator measurements and hig...

Low frequency acoustic properties of Posidonia oceanica seagrass leaf blades

The acoustics of seagrass meadows impacts naval and oceanographic sonar applications. To study this environment, a one-dimensional resonator was used to assess the low-frequency (1-5 kHz) acoustic response of the leaf blades of the Mediterranean seagrass Posidonia oceanica in water. Three separate collections of plants from Crete, Greece, and Sicily, Italy were investigated. A high consistency in effective sound speed was observed within each collection while a strong variability was observed between different collections. Average size, mass, and epiphytic coverage within each collection were quantified, and discoloration and stiffness are discussed qualitatively with respect to the observed acoustic variability.

Physical and acoustical properties of marine sediments containing a wide particle size distribution

Naturally occurring marine sediments can possess poorly sorted or multimodal particle size distributions. Furthermore, while well-sorted coarse grained sediments are typically characterized according to their grain size, for mixed or fine grained sediments the shape of the particles can have significant influence on the acoustical properties. In this work, particle size and morphology are investigated using scanning electron microscopy (SEM). Images of dry particles are analyzed to determine the sphericity and angularity of sand and silt grains and to estimate the shape (e.g. platelets or needles) and aspect ratio of clay particles. SEM images of wet samples are analyzed to understand the sediment microstructure, including the interaction between sand, silt, and clay particles. Additionally, the effect of the proportionally varying composition of sand, silt, and clay particles on the bulk properties of the sediment, including the porosity and compressional wave speed, will be examined using a multiphase effective medium model. These techniques are applied to sediments collected in the New England Mud Patch as part of the Seabed Characterization Experiment.Naturally occurring marine sediments can possess poorly sorted or multimodal particle size distributions. Furthermore, while well-sorted coarse grained sediments are typically characterized according to their grain size, for mixed or fine grained sediments the shape of the particles can have significant influence on the acoustical properties. In this work, particle size and morphology are investigated using scanning electron microscopy (SEM). Images of dry particles are analyzed to determine the sphericity and angularity of sand and silt grains and to estimate the shape (e.g. platelets or needles) and aspect ratio of clay particles. SEM images of wet samples are analyzed to understand the sediment microstructure, including the interaction between sand, silt, and clay particles. Additionally, the effect of the proportionally varying composition of sand, silt, and clay particles on the bulk properties of the sediment, including the porosity and compressional wave speed, will be examined using a multiphase e...