Acoustic Sub-bottom Profiling Research Articles

2 kHz high power smart transducer for acoustic sub-bottom profiling applications

In this study, a 2 kHz Tonpilz projector was designed using a Terfenol-D and modeled in ATILA. For the purpose of modeling studies, it has been determined that a radiating head mass exhibits better transmitting current response (TCR) at 136 mm diameter, where the resonance occurs at 2.4 kHz and the peak value of 118 dB re 1 μPa/A at 1 m occurs at 12 kHz. Also bolt at a 46 mm distance from the center of the head mass offers resonance at 2.4 kHz, and the peak value of 115.3 dB re 1 μPa/A at 1m occurs at 11.5 kHz. This optimized design is fabricated and molded with polyurethane of 3 mm thickness. The prototype was tested at the Acoustic Test Facility (ATF) of National Institute of Ocean Technology (NIOT) for its underwater performances. Based on the result, the fundamental resonance was determined to be 2.18 kHz and the peak value of TCR of 182 dB re 1 μPa/A at 1m occurs at 14 kHz. The maximum value of the RS was found to be −190 dB re 1V/μPa at 1m at a frequency of 2.1 kHz.

Acoustic Sub-Bottom Profiling Surveys of Flood Control Reservoirs

Research Article| May 01, 2013 Acoustic Sub-Bottom Profiling Surveys of Flood Control Reservoirs JOHN A. DUNBAR; JOHN A. DUNBAR 1 Department of Geology, Baylor University, One Bear Place 97354, Waco, TX 76798-7354 1Corresponding author email: john_dunbar@baylor.edu. Search for other works by this author on: GSW Google Scholar PETER M. ALLEN; PETER M. ALLEN Department of Geology, Baylor University, One Bear Place 97354, Waco, TX 76798-7354 Search for other works by this author on: GSW Google Scholar PAUL D. HIGLEY; PAUL D. HIGLEY Specialty Devices, Inc., 2905 Capital Street, Wylie, TX 75098 Search for other works by this author on: GSW Google Scholar SEAN J. BENNETT SEAN J. BENNETT Department of Geography, University at Buffalo, Buffalo, NY 14261-0055 Search for other works by this author on: GSW Google Scholar Author and Article Information JOHN A. DUNBAR 1 Department of Geology, Baylor University, One Bear Place 97354, Waco, TX 76798-7354 PETER M. ALLEN Department of Geology, Baylor University, One Bear Place 97354, Waco, TX 76798-7354 PAUL D. HIGLEY Specialty Devices, Inc., 2905 Capital Street, Wylie, TX 75098 SEAN J. BENNETT Department of Geography, University at Buffalo, Buffalo, NY 14261-0055 1Corresponding author email: john_dunbar@baylor.edu. Publisher: Association of Environmental & Engineering Geologists First Online: 02 Mar 2017 Online ISSN: 1558-9161 Print ISSN: 1078-7275 © 2013 Association of Environmental & Engineering Geologists Environmental & Engineering Geoscience (2013) 19 (2): 135–148. https://doi.org/10.2113/gseegeosci.19.2.135 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JOHN A. DUNBAR, PETER M. ALLEN, PAUL D. HIGLEY, SEAN J. BENNETT; Acoustic Sub-Bottom Profiling Surveys of Flood Control Reservoirs. Environmental & Engineering Geoscience 2013;; 19 (2): 135–148. doi: https://doi.org/10.2113/gseegeosci.19.2.135 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract Since the 1940s, the U.S. Department of Agriculture (USDA) has built over 11,000 flood-control reservoirs (FCR) throughout the United States. Most were designed to hold 50 years' worth of sedimentation. Many have reached that age, but the amount of sediment they contain is unknown. Conventionally, reservoir sedimentation is monitored by measuring the change in reservoir capacity over time relative to an initial survey. However, the vast majority of FCR in the United States have never been surveyed. We evaluate a new method for surveying FCR in which an acoustic sub-bottom profiler is used to measure both water depth and sediment thickness in one survey. From these measurements both the current reservoir capacity and post-impoundment sediment volume are determined. We evaluate the method by conducting surveys of 21 FCR to find the frequency with which the base of post-impoundment sediment can be mapped throughout the reservoir. In 18 of the 21 FCR surveyed, both the water bottom and the base of post-impoundment sediment could be mapped throughout, allowing both the current and initial reservoir capacity to be determined. Comparing our estimates of initial reservoir capacities with as-built capacities estimated prior to construction we find that only seven of the 18 agree to within 10 percent. Some disagree by more than a factor of 2. We conclude that as-built capacities of USDA FCR should be used with caution for estimating post-impoundment sediment volumes and that in the majority of cases more accurate estimates can be made by direct measurement using sub-bottom acoustic profiling. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

First geomorphological record and glacial history of an inter-ice stream ridge on the West Antarctic continental shelf

Inter-ice stream areas cover significant portions of Antarctica's formerly glaciated shelves, but have been largely neglected in past geological studies because of overprinting by iceberg scours. Here, we present results of the first detailed survey of an inter-ice stream ridge from the West Antarctic continental shelf. Well-preserved sub- and proglacial bedforms on the seafloor of the ridge in the eastern Amundsen Sea Embayment (ASE) provide new insights into the flow dynamics of this sector of the West Antarctic Ice Sheet (WAIS) during the Last Glacial cycle. Multibeam swath bathymetry and PARASOUND acoustic sub-bottom profiler data acquired across a mid-shelf bank, between the troughs of the Pine Island-Thwaites (PITPIS) and Cosgrove palaeo-ice streams (COPIS), reveal large-scale ribbed moraines, hill-hole pairs, terminal moraines, and crevasse-squeeze ridges. Together, these features form an assemblage of landforms that is entirely different from that in the adjacent ice-stream troughs, and appears to be unique in the context of previous studies of Antarctic seafloor geomorphology. From this assemblage, the history of ice flow and retreat from the inter-ice stream ridge is reconstructed. The bedforms indicate that ice flow was significantly slower on the inter-ice stream ridge than in the neighbouring troughs. While terminal moraines record at least two re-advances or stillstands of the ice sheet during deglaciation, an extensive field of crevasse-squeeze ridges indicates ice stagnation subsequent to re-advancing ice, which deposited the field of terminal moraines in the NE. The presented data suggest that the ice flow behaviour on the inter-ice stream ridge was substantially different from that in the adjacent troughs. However, newly obtained radiocarbon ages on two sediment cores recovered from the inter-ice stream ridge suggest a similar timing in the deglaciation of both areas. This information closes an important gap in the understanding of past WAIS behaviour in the eastern ASE. Our newly-documented bedforms will also serve as an important diagnostic tool in future studies for interpreting ice-sheet histories in similar inter-ice stream areas.

Streaming flow of an Antarctic Peninsula palaeo-ice stream by both basal sliding and deformation of substrate

Acoustic sub-bottom profiler surveys on the northeast Antarctic Peninsula shelf indicate that parts of the seabed are underlain by an acoustically transparent layer that is thin on the inner shelf and becomes thicker and more extensive towards the outer shelf. Sedimentological and geophysical data are combined to construct a bed model where streaming ice flow, by both deformation and basal sliding, took place within cross-shelf troughs. The model suggests only limited deformation contributed to fast flow on the inner shelf, i.e. in the onset zone of ice streaming, where the bed was predominantly underlain by a stiff till. Thus, fast ice flow in this area might have been by basal sliding, with deformation confined to discontinuous patches of soft till <40 cm thick. Towards the middle and outer shelf, extensive, thick sequences of soft till suggest a change in the dominant subglacial process towards widespread deformation. This downstream change from basal sliding to subglacial deformation is manifest in the transition from stiff-till dominance to soft-till dominance, while a downstream increase in ice flow velocity is evident from the complex geomorphic imprint on the inner shelf evolving to the more restricted set of bedforms on the outer shelf.

Late Quaternary grounded ice extent in the Filchner Trough, Weddell Sea, Antarctica: new marine geophysical evidence

The Last Glacial Maximum (LGM; ca 23–19 ka BP) extent of grounded ice in the Filchner Trough, a major cross shelf trough extending seaward from the Filchner Ice Shelf in the southern Weddell Sea, has been much debated. Here we present data from the first extensive multibeam swath bathymetry and sub-bottom acoustic profiling surveys in the Filchner Trough that include several parallel survey lines with overlapping swaths. We interpret these new data, combined with published observations and radiocarbon dates from sediment cores, as indicating that the grounding line in the Filchner Trough during the LGM advanced beyond the middle shelf, probably to within 40 km of the shelf break, and possibly reached the shelf break. Three different hypotheses are discussed that could reconcile this interpretation with interpretations, based on ice coring and surface exposure age data, that LGM ice surface elevations in areas draining into the Filchner and Ronne ice shelves were no more than a few hundred metres higher than today: (1) ice plain conditions extended along most of the Filchner Trough; (2) the ice shelf advanced and thickened so that it “touched down” on the continental shelf for a short period; (3) LGM ice drainage pathways in the interior of the Weddell Sea embayment were different from those observed today.

Marine geological constraints for the grounding-line position of the Antarctic Ice Sheet on the southern Weddell Sea shelf at the Last Glacial Maximum

The history of grounded ice-sheet extent on the southern Weddell Sea shelf during the Last Glacial Maximum (LGM) and the timing of post-LGM ice-sheet retreat are poorly constrained. Several glaciological models reconstructed widespread grounding and major thickening of the Antarctic Ice Sheet in the Weddell Sea sector at the LGM. In contrast, recently published onshore data and modelling results concluded only very limited LGM-thickening of glaciers and ice streams feeding into the modern Filchner and Ronne ice shelves. These studies concluded that during the LGM ice shelves rather than grounded ice covered the Filchner and Ronne troughs, two deep palaeo-ice stream troughs eroded into the southern Weddell Sea shelf. Here we review previously published and unpublished marine geophysical and geological data from the southern Weddell Sea shelf. The stratigraphy and geometry of reflectors in acoustic sub-bottom profiles are similar to those from other West Antarctic palaeo-ice stream troughs, where grounded ice had advanced to the shelf break at the LGM. Numerous cores from the southern Weddell Sea shelf recovered sequences with properties typical for subglacially deposited tills or subglacially compacted sediments. These data sets give evidence that grounded ice had advanced across the shelf during the past, thereby grounding in even the deepest parts of the Filchner and Ronne troughs. Radiocarbon dates from glaciomarine sediments overlying the subglacial deposits are limited, but indicate that the ice grounding occurred at the LGM and that ice retreat started before ∼15.1 corrected 14C kyrs before present (BP) on the outer shelf and before ∼7.7 corrected 14C kyrs BP on the inner shelf, which is broadly synchronous with ice retreat in other Antarctic sectors. The apparent mismatch between the ice-sheet reconstructions from marine and terrestrial data can be attributed to ice streams with very low surface profiles (similar to those of “ice plains”) that had advanced through Filchner Trough and Ronne Trough at the LGM. Considering the global sea-level low-stand of ∼130 m below present, a low surface slope of the expanded LGM-ice sheet in the southern Weddell Sea can reconcile grounding-line advance to the shelf break with limited thickening of glaciers and ice streams in the hinterland. This scenario implies that ice-sheet growth in the Weddell Sea sector during the LGM and ice-sheet drawdown throughout the last deglaciation could only have made minor contributions to the major global sea-level fluctuations during these times.

Submarine landforms and the behavior of a surging ice cap since the last glacial maximum: The open-marine setting of eastern Austfonna, Svalbard

Multi-beam swath bathymetry data and Topographic Parametric Sonar (TOPAS) 3.5 kHz sub-bottom acoustic profiles were acquired from the inlet of Hartogbutka, an open-marine setting offshore of Austfonna, northeastern Nordaustlandet, Svalbard, the largest ice cap in the Eurasian Arctic. Using these marine geophysical data, the seafloor morphological and sedimentary evidence of former ice extent and dynamics of the three drainage basins (Basins 3, 4, and 5) adjacent to Hartogbukta were examined. Submarine landforms and landform assemblages that provide evidence of past ice dynamics are described and interpreted in the context of previous studies of surge-affected submarine environments elsewhere in Svalbard. A sequence of four distinct ice-flow episodes in Hartogbukta, from the late Weichselian deglaciation to the present, is identified to explain the morphological evolution of the seafloor. A full-glacial ice sheet extended over Hartogbukta during the late Weichselian, flowing ESE. At intervals throughout the Holocene, the drainage basins adjacent to Hartogbukta probably underwent repeated surges. Most recently, Basins 3 and 4 probably surged as one cohesive ice mass in the late 1800s and Basin 5 most likely underwent a less extensive surge in the 20th century. These ice-flow events are interpreted in the context of the glaciology and known glacial history of Austfonna and the NW Barents Sea. The 200 km-long ice cliffs of eastern Austfonna provide an unusual and important analog for open-marine deposition that would have been much more prevalent under full-glacial conditions where ice sheets in many parts of the Arctic advanced from fjord systems onto the adjacent continental shelves.

Geomorphic setting and geohazard-related features along the Ionian Calabrian margin between Capo Spartivento and Capo Rizzuto (Italy)

The aim of this work is to describe the morpho-dynamic processes of coastal and submarine areas based on morpho-bathymetric data and sub-bottom acoustic profiles from the Calabrian Ionian margin (from Capo Spartivento to Capo Rizzuto). We describe some of the most significant geo-hazard features and show some examples of their complex interactions and related potential risk. The relationship between these geo-hazard features and the active tectonics and some severe historical earthquakes and tsunamis is also discussed. The studied area is characterized by several well developed canyon systems. The retrogressive erosion of the canyon heads affects the continental shelf and interacts with the coastal morpho-sedimentary processes, evidencing the intense geomorphic activity of this sector of Ionian margin. The widespread evidence of mass wasting processes and slope instability, together with the historical seismicity, makes this area also an ideal case study for investigating on the possible relationship between earthquakes and landslide-triggered tsunamis and assessing the potential risk.

Short-Term Uplift Rates and Surface Deformation along the Reelfoot Fault, New Madrid Seismic Zone

Abstract This study of variability in the amount and rate of deformation along the intraplate Reelfoot fault of the New Madrid seismic zone (NMSZ) utilizes diverse research methods, including geophysics, geomorphology, and fluvial sedimentology. The transpressional Reelfoot fault deforms late Holocene Mississippi River sediment into the Tiptonville dome and adjacent Reelfoot basin. Because Reelfoot Lake submerges a portion of the basin, subbottom acoustic profiling of the lake was used to quantify the relief on the uplift. At the natural levee of the abandoned Tiptonville meander, the most appropriate landform along the scarp for measuring cumulative structural relief, a maximum relative vertical uplift of approximately 11.3 m was measured. To identify spatial and temporal variability in surface deformation using geologic methods, we relate fluvial sedimentation to three documented earthquake events during the past 2300 yr (Tuttle et al. , 2002, 2005), which is the approximate age of the oldest deformed floodplain sediment (Guccione et al. , 2002). Radiocarbon dates from organic material in fluvial and lacustrine sediment indicate that most of the uplift occurred during the last two major seismic events. A minimum of 1.6–3.9 m of uplift was associated with the seismic episode at A.D. 1450±150 and a maximum of 5.9–8.2 m of uplift was associated with the 1812 episode. We estimate a mean short-term uplift rate of 1.2 cm/yr (0.4–2.1 cm/yr with 80% probability) or a mean short-term slip rate of 1.3 cm/yr (0.5–2.3 cm/yr with 80% probability). These values are approximately 2.5 times higher than the long-term rates based on the total amount of uplift averaged over the past 2300 yr (Mueller et al. , 1999; Van Arsdale, 2000). Estimating deformation rates associated with individual earthquake recurrences provides information on fluctuating activity of the NMSZ.

High-resolution geophysical observations of the Yermak Plateau and northern Svalbard margin: implications for ice-sheet grounding and deep-keeled icebergs

High-resolution geophysical evidence on the seafloor morphology and acoustic stratigraphy of the Yermak Plateau and northern Svalbard margin between 79°20′ and 81°30′N and 5° and 22°E is presented. Geophysical datasets are derived from swath bathymetry and sub-bottom acoustic profiling and are combined with existing cores to derive chronological control. Seafloor landforms, in the form of ice-produced lineations, iceberg ploughmarks of various dimensions (including features over 80 m deep and down to about 1000 m), and a moat indicating strong currents are found. The shallow stratigraphy of the Yermak Plateau shows three acoustic units: the first with well-developed stratification produced by hemipelagic sedimentation, often draped over a strong and undulating internal reflector; a second with an undulating upper surface and little acoustic penetration, indicative of the action of ice; a third unit of an acoustically transparent facies, resulting from debris flows. Core chronology suggests a MIS 6 age for the undulating seafloor above about 580 m. There are several possible explanations, including: (a) the flow of a major grounded ice sheet across the plateau crest from Svalbard (least likely given the consolidation state of the underlying sediments); (b) the more transient encroachment of relatively thin ice from Svalbard; or (c) the drift across the plateau of an ice-shelf remnant or megaberg from the Arctic Basin. The latter is our favoured explanation given the evidence currently at our disposal.

Recent transfer of coastal sediments to the Laurentian Channel, Lower St. Lawrence Estuary (Eastern Canada), through submarine canyon and fan systems

Multibeam sonar data, acoustic sub-bottom profiles and box cores were used to study the activity of submarine canyons and fans near the city of Les Escoumins, on the North Shore of the Lower St. Lawrence Estuary (Eastern Canada). The multibeam data were used to generate a high-resolution digital terrain model that reveals the presence of a large number of canyons and fans along the northern slopes of the Laurentian Channel. This paper focuses on two of the larger canyons, and their associated submarine fans. The sub-bottom profiles on the fans reveal high-amplitude reflections at the sediment/water interface and near the seafloor surface, indicating the occurrence of layers of coarse material. A turbidite was observed in a box core sampled in one of the fans, confirming the nature of the coarse layer. Geophysical and sedimentological data indicate that the canyons and fans play an important role in transferring coastal sandy sediments to the deeper marine environments by longshore drift-initiated turbidity flows, and thereby contribute to the negative sediment budget along the coast. The morphology of the canyons indicates that they were produced by a combination of erosive turbidity flows and retrogressive failures. The two box cores sampled on the fans reveal a recent (~last 60 years) quasi-exponential increase in sand content near the surface of the cores, possibly reflecting recent deforestation and/or increased coastal erosion.

Pockmarks in the inner Oslofjord, Norway

Multibeam bathymetric surveys of the Inner Oslofjord, Norway have revealed a high density of pockmarks in the 179-km2 inner fjord area, which contains over 500 pockmarks of varying size, typically 20–50 m in diameter and 2–10 m deep. These pockmarks have been investigated with a variety of techniques, including acoustic subbottom profiling, sedimentological and geochemical analyses of cores, remotely operated vehicle observation, and morphometry. Both the distribution and shapes of the pockmarks suggest that they are related to structures in the bedrock underlying relatively thin (<50 m) unconsolidated glacial and postglacial sediments. The data provide no direct indication of a particular mode of pockmark formation, but release of large amounts of biogenic, shallow methane seems unlikely. Several lines of evidence point to a continuous process of pockmark formation followed by inactivity, with some pockmarks recently active whereas others have been inactive for a considerable time. Some pockmarks are characterised by coarse sediment in their centres. The density, variety and easy access make this pockmark field an ideal model area for pockmark research.

Biogeochemical processes and microbial diversity of the Gullfaks and Tommeliten methane seeps (Northern North Sea)

Abstract. Fluid flow related seafloor structures and gas seeps were detected in the North Sea in the 1970s and 1980s by acoustic sub-bottom profiling and oil rig surveys. A variety of features like pockmarks, gas vents and authigenic carbonate cements were found to be associated with sites of oil and gas exploration, indicating a link between these surface structures and the underlying, deep hydrocarbon reservoirs. In this study we performed acoustic surveys and videographic observation at Gullfaks, Holene Trench, Tommeliten, Witch's Hole and the giant pockmarks of the UK Block 15/25, to investigate the occurrence and distribution of cold seep ecosystems in the Northern North Sea. The most active gas seep sites, i.e. Gullfaks and Tommeliten, were investigated in detail. At both sites, gas bubbles escaped continuously from small holes in the seabed to the water column, reaching the upper mixed surface layer. At Gullfaks a gas emitting, flat area of 0.1 km2 of sandy seabed covered by filamentous sulfur-oxidizing bacteria was detected. At Tommeliten, we found a patchy distribution of small bacterial mats indicating sites of gas seepage. Below the patches the seafloor consisted of sand from which gas emissions were observed. At both sites, the anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) was the major source of sulfide. Molecular analyses targeting specific lipid biomarkers and 16S rRNA gene sequences identified an active microbial community dominated by sulfur-oxidizing and sulfate-reducing bacteria (SRB) as well as methanotrophic bacteria and archaea. Stable carbon isotope values of specific, microbial fatty acids and alcohols from both sites were highly depleted in the heavy isotope 13C, indicating that the microbial community incorporates methane or its metabolites. The microbial community composition of both shallow seeps shows high similarities to the deep water seeps associated with gas hydrates such as Hydrate Ridge or the Eel River basin.

Acoustic facies on the inner Kara Sea Shelf: Implications for Late Weichselian to Holocene sediment dynamics

We studied the impact of the last glacial (late Weichselian) sea level cycle on sediment architecture in the inner Kara Sea using high-resolution acoustic sub-bottom profiling. The acoustic lines were ground-truthed with dated sediment cores. Furthermore we refined the location of the eastern LGM ice margin, by new sub bottom profiles. New model results of post-Last Glacial Maximum (LGM) isostatic rebound for this area allow a well-constrained interpretation of acoustic units in terms of sequence stratigraphy. The lowstand (or regressive) system tract sediments are absent but are represented by an unconformity atop of Pleistocene sediments on the shelf and by a major incised dendritic paleo-river network. The subsequent transgressive and highstand system tracts are best preserved in the incised channels and the recent estuaries while only minor sediment accumulation on the adjacent shelf areas is documented. The Kara Sea can be subdivided into three areas: estuaries (A), the shelf (B) and (C) deeper lying areas that accumulated a total of 114 × 10 10 t of Holocene sediments.

Pattern and timing of sediment infill at glacier-fed Mud Lake: implications for lateglacial and Holocene environments in the Monashee Mountain region of British Columbia, Canada

The Holocene sedimentary infill of proglacial Mud Lake, British Columbia, Canada, was investigated using 3.5 kHz acoustic sub-bottom profiling and sediment samples. The sediment infill is a mixture of silt and clay and is divided into five stratigraphic units: massive silt (Unit 1), weakly laminated silt with very fine sand beds (Unit 2), weakly laminated silt with rippled beds of fine sand (Unit 3), weakly laminated clay with very fine sand laminations (Unit 4) and silt-clay varves (Unit 5). Acoustic reflectors correlate with stratigraphic unit boundaries. Annual accumulation rates were estimated for six age/depth intervals: prior to 9.6 kyr cal. BP, accumulation rates were very high - on the order of several centimetres per year. Early to middle Holocene sediment inputs (9.6-3.6 kyr cal. BP) were variable but low, ranging from 0.3 mm/yr to 1.2 mm/yr. The late Holocene (most recent 3.6 kyr) shows annual accumulation rates that generally exceed 2 mm/yr. Surface sediment reveals a mean of 4.3 mm/yr over the last 20 years. These results are consistent with regional Holocene chronologies and long-term paraglacial succession: (1) maximum sediment and meltwater availability and minimum stabilization by vegetation following Fraser deglaciation replaced by (2) less meltwater and sediment availability during the Hypsithermal, and (3) more stabilized sediment during the early and mid Holocene. Renewal of glacial activity, particularly in the late Neoglacial, has led to increased rates of accumulation in Mud Lake. Despite the contrasting geologic setting of Mud Lake in the Omineca belt, contemporary sediment yield is consistent with the trend of sediment yield with glacier cover in lakes of the adjacent Foreland belt.

Costa de fiordes da Patagônia central, Chile, morfologia submarina e fácies acústicas

This study interprets acoustic sub-bottom profiles and submarine 3D models of some of fjords adjacent to Campo de Hielo Patagonico Sur (Southern Patagonian Icefield), Chile. The overdeepened fjord basins show irregular morphology and distinctive landforms associated with a grounding-line, ice tongues and icebergs sedimentary system. Acoustic reflectors display two main facies: chaotic and laminated, according the internal reflection configuration and the external geometry. The sediments probably were deposited during the Holocene deglaciation, as shown by the visible reflectors of sub-bottom profiler. The geometry of the depocenters and the acoustic reflectors characteristic show evidence of pre-existing topography and bathymetry influence. Due to the great water depths of the fjord basins, terminal margins may have a mix of floating and grounded cliff behaviour, albeit the predominant temperate glacial regime.

Subbottom acoustic and sedimentary records of past surface water?groundwater exchange through sinkhole lakes in south Georgia, U.S.A

Lake Balboa is a deep sinkhole lake in south Georgia. Subbottom acoustic profiling and long sediment cores reveal four stratigraphic units within >20 m of lacustrine fill above acoustically stratified and faulted Miocene/Pliocene overburden and basement Eocene limestone which hosts the Floridan Aquifer system (FAS). Fill consists of thin, rhythmically bedded, peaty sediments (Unit I), indicative of slow sedimentation in a shallow swamp until 13,110–12,680 cal BP, a mix of desiccated and over-consolidated clay and silty sand (Unit II) implying periodic subaerial exposure of the lake bed until 9470–9025 cal BP, and gyttja associated with a rapid 8 m rise in water level by 9120–9020 cal BP (Unit III), as well as an additional rise of 10 m to modern lake levels (Unit IV). Accumulation of nearly 4.5 m of low permeability gyttja, and higher lake levels have fundamentally changed exchange between surface water and groundwater, reducing by 40% or more the rate of vertical leakage to the FAS and increasing shallow aquifer discharge to the lake.

Geo-acoustic characterization of calcareous seabed in the Florida Keys

Geotechnical and acoustic measurements on a set of 35 gravity cores and 11 box cores from two calcareous seabed locations in the lower Florida Keys that are characterized by contrasting environmental settings show significant differences in terms of vertical profiles of physical, acoustic, and geotechnical properties. The lower energy study site of the two is sheltered by the adjacent Dry Tortugas platform complex and reveals a higher porosity surface interval with significant changes in water content, density, and compressional wave velocity within the upper 25 cm. Sediment cores from open-water locations, such as those collected in the study area north of the Marquesas Keys, exhibit higher, less variable densities and lower velocities within the top 25 cm. This is attributed to consolidation associated with cyclic pressure variations from surface swells and strong tidal currents. Acoustic subbottom profiles display good correlation with shell-lag deposits observed in the gravity cores, although acoustic records lack the vertical resolution to detect variations in physical and acoustic properties on the order of those measured in this study. Calculated impedances at depths below 25 cm are significantly higher in the Dry Tortugas area and hence penetration of 4- and 15-kHz acoustic signals is less than at the Marquesas study site. From a geotechnical point of view, the sediments at both sites can be considered to behave like granular materials with little or no plasticity, no significant cementation, low compressibility, permeability highly dependent on void ratio, and moderate to high friction angles. A comparison with deep-sea sediments of mixed mineralogy shows that the effect of increasing calcium carbonate with decreasing clay content is to decrease plasticity and compressibility, and to increase friction angles. In other words, sediments shift from a cohesive to a granular nature as the carbonate content increases.

Geological analysis of acoustic subbottom profiles as a tool for prediction of acoustic property distribution

Geoscientists have developed analytical techniques for extraction of geologic and acoustic data from acoustic profiles as well as prediction of how these properties may vary in areas where data are lacking and therefore these may be of interest to acousticians. Analysis of reflection termination patterns in subbottom profile data are used to identify sediment deposited contemporaneously by a set of related depositional environments. A variation in the interval that separates reflections is usually related to a change in bed thickness. Closely spaced reflections are associated with environments of deposition where the mode of deposition changes frequently and those with widely spaced reflections are associated with environments where the mode of deposition rarely changes. Continuity of reflections is related to the lateral continuity of beds. Environments that have relatively short lateral continuity, such as terrestrial environments like river valleys that have been flooded, and many marginal marine environments are associated with reflections that have relatively short lateral continuity. These variations in separation and lateral continuity of beds and termination and orientation of beds that are all linked to variations in reflection character are produced by depositional processes that are associated with depositional systems and therefore distributed in a relatively predictable manner.

Sedimentation in Chilko Lake: a record of the geomorphic environment of the eastern Coast Mountains of British Columbia, Canada

Chilko Lake is the largest and deepest lake draining the Coast Mountains of British Columbia. A sedimentary record from the 60-km long lake was constructed by using 3.5-kHz subbottom acoustic profiles and short cores of surface sediments. Sediment accumulation since deglaciation is remarkably thin, averaging less than 10 m, except in the central deep part of the lake, despite extensive activity of Holocene and modern glaciers in the 1960-km 2 contributing watershed. Sediment focusing of silts and clays in the main part of the lake occurs mainly by overflow/interflow processes from the Edmond and Nine Mile/Deschamps drainages. Sandy turbidites from these drainages and other smaller streams, however, extend for several kilometers into the lake basin. Distinct laminae in short cores from the central lake suggest that modern rates of deposition reach a maximum of 2.2 mm a −1. 210 Pb dating and inferences from the acoustic results, however, indicate a lake-wide, long-term average rate of less than 1 mm a −1, which is equivalent to a specific sediment yield of about 80 t km −2 a −1. This specific yield is amongst the lowest for glacier-fed lakes of the southern Canadian Cordillera. A low sediment supply can be related to three factors: (1) a much drier climate on the leeward side of the Coast Mountains producing lower specific discharges; (2) fewer autumn rainstorms that affect mainly lakes west of the Coast Mountains and (3) opportunities for storage of sediment in the lower 20 km of Edmond River, which drains the most heavily glacierized portion of the watershed.