Acoustic Sub-bottom Profiling Research Articles

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.

Evidence for topography- and current-controlled deposition on the Reykianes Ridge between 59°N and 60°N

Sediment patterns derived from sediment sampling and acoustic subbottom profiling were mapped on the Reykjanes Ridge (North Atlantic) between 59°N and 60°N. Five discrete sediment echo patterns were distinguished and mapped on a regional scale. The prolonged and layered echo facies, which mainly reflect sediment filled basins on the ridge flanks, indicate deposition of predominantly fine-grained sediments deposited by the Iceland-Scotland Overflow Water. A combination of westward flowing currents spilling over the ridge crest due to the Coriolis force together with the existing morphology probably caused the N-S trending facies distribution pattern on the northwest flank. Furthermore, the modern surface sediment distribution is controlled by biological productivity, which is closely related to the mixing zone of cold subpolar surface water masses and the warm North Atlantic Current, and bottom water transport processes. The effect of bottom current transport is reflected in the pattern of settling velocity and sediment grain size. The clay mineral composition indicates that most of the fine-grained material is supplied predominantly from the Icelandic province by the Iceland-Scotland Overflow Water. Erosional processes are concentrated on narrow zones on top of the axial ridges and on the steep flanks of the Catalonia Seamount. Well-sorted foraminiferal sands on these exposed regions are assumed to represent residual sediments.

Documentation of earthquake-induced disturbance of lake sediments using subbottom acoustic profiling

Subbottom acoustic profiling in lakes on the Canadian Shield and in the Canadian Cordillera has revealed disruptions in deltaic and lacustrine sediments that can be attributed to large historical earthquakes. The Timiskaming earthquake of 1935 (Magnitude 6.25) and the Vancouver Island earthquake of 1946 (Magnitude 7.2) triggered extensive slumping and flow of postglacial sediments in lakes located within a few tens of kilometres of the epicentres of these events. There is some evidence in one of the surveyed lakes for in situ mobilization of sediment on relatively gentle slopes, with minimal horizontal displacement; this may be a product of liquefaction induced by coseismic shaking or displacements along faults beneath the basin. In another lake, widespread coseismic slumping resuspended large amounts of fine sediment, which then settled out onto the lake floor to produce a texturally anomalous layer. Features similar to those produced by the 1935 and 1946 earthquakes can also result from a variety of nontectonic processes, for example, the collapse of sediment over buried ice blocks, depositional oversteepening of deltas, groundwater sapping, and variations in pore-water pressures due to lake-level fluctuations. Records of sediment disruption in lakes therefore must be interpreted with caution. In general, many lakes in a region should be surveyed and a variety of different features identified before sediment disruptions are attributed to an earthquake.

Evidence of late and postglacial seismic activity in the Témiscouata–Madawaska Valley, Quebec – New Brunswick, Canada

Three types of geological phenomena independently suggest that the Témiscouata–Madawaska Valley was affected by one or more seismic events following its deglaciation:(1) Subbottom acoustic profiling of Lac Témiscouata and Grand lac Squatec revealed disturbance of bottom sediments by mass transport processes in both lakes. Erosional truncation of preexisting, acoustically laminated sediments and accumulation of hummocky, chaotic deposits over older hummocky surfaces or laminated sequences both result from mass transport processes. Unidirectional mass flows from several points in these symmetrical basins, in situ disruption of laminated sediment beneath flat bottoms, and the large area of the lake floors affected suggest strong similarities in sedimentation style with lakes that have been disturbed during strong earthquakes.(2) Southeast of Lac Témiscouata, in Saint-Jacques, New Brunswick, two separate mass flow deposits, made up largely of coarse (> 0.5 m), angular boulders of local bedrock, occur on opposite sides of the Madawaska River valley. These deposits have different source areas and transport directions, but occupy more or less the same stratigraphic position within sediments deposited in glacial Lake Madawaska.(3) At one site in Saint-Jacques, a near-vertical fault displaces a glacially striated bedrock surface at least 7 cm, suggesting a response to postglacial compressive stress similar to that observed on outcrops in the nearby epicentral region of the 1982 Miramichi earthquake.Although the Témiscouata–Madawaska Valley lacks historical evidence of seismic activity, and many of the phenomena observed could, individually, have been generated by aseismic processes, we conclude that the close proximity of diverse features related to mass transport and faulting suggest that the valley has been the locus of seismic activity from the time of its deglaciation to the recent, but prehistorical, past.

Sedimentary record of Harrison Lake: implications for deglaciation in southwestern British Columbia

A sedimentary record from 60 km long Harrison Lake was constructed by using 3.5 kHz subbottom acoustic profiles and gravity cores of surface sediments. In places, the glaciolacustrine sediments exceed 70 m in thickness and represent the entire deglacial and postglacial accumulation record. An upper, acoustically transparent layer decreases in thickness from 12 to 4 m. southward from the upper lake. Cores from the upper metre of this layer demonstrate that deposition is dominated by settling of suspended sediment transported in a laterally mixed, wind-driven surface plume from the north. Depositional rates, inferred from 14C dating of organic macrofossils and counting of probable annual laminated couplets in the cores, are almost 2 mm/a in the north and decline to less than 0.1 mm/a in the south. Hence, the upper acoustic layer accounts for all postglacial (last 10 500 years BP) lacustrine deposition, with most of the sediment derived from Lillooet River. A lower, thicker (12–22 m), acoustically stratified layer is interpreted as high-energy glaciolacustrine deposits. This large volume of deglaciation sediment is derived from two sources: (i) ice retreating rapidly northwest up the Lillooet valley, which may have existed for no more than 400 years in the lower valley prior to opening of Lillooet Lake (which now traps most sediment derived from the upper basin); and (ii) inflow from the south as the late-glacial Fraser River rapidly built a delta north from the sill at Harrison Hot Springs. Despite known higher sea levels during deglaciation of the eastern Fraser Lowland, we have no evidence for a marine incursion.

Chemical and microbiological aspects of acoustically turbid sediments: Preliminary investigations

Abstract Chemical, microbiological, and geophysical measurements have been carried out on sediment cores collected from Holyhead Harbour and the Western Irish Sea, where acoustic subbottom profiling has established the presence of large areas of acoustically turbid sediments, commonly referred to as “gassy” sediments. Gas analysis of these cores have shown that the acoustic turbidity was most probably due to high concentrations (>100 nM/mL) of methane occurring at subsurface depths. Microcosm experiments on sediment slurries from Holyhead Harbour confirm that acetate and H2/CO2 are important precursors for methane generation. In sediments from Holyhead Harbour methanogenesis could be slightly stimulated by the addition of H2/CO2 and sulfate (1 mM). This suggested that in surface sediments sulfate reduction and methanogenesis can occur concurrently. Such a situation may explain the appearance of gas plumes and gas pockets detected acoustically at the sediment surface in several regions of the Western Irish...

The electrical anisotropy of sea ice in the horizontal plane

Sea ice electrical anisotropy in the horizontal plane was observed with an impulse radar technique under development for profiling ice thickness. The radar technique used can be considered the electromagnetic equivalent of acoustic subbottom profiling methods. The anisotropy phenomenon was characterized by a marked change in amplitude of the vertically propagated signal reflected from the sea ice/water interface as the linearly polarized antenna was rotated in the horizontal plane on or above the ice surface. Anisotropy for both first-year and multiyear sea ice was observed in several areas of the Canadian Arctic. Additional experimental work is necessary to determine the nature of the anisotropy mechanism.

Acoustic Subbottom Profiling Systems, A State-of-the-Art Survey

Abstract : Information from field tests, literature, and published data was gathered to establish the operating principles, capabilities and limitations, environmental restrictions, and availability of acoustic systems from the standpoint of conditions in the Lower Mississippi Valley. The acoustic systems are commonly classified according to the method by which the sound energy is produced, such as boomers, sparkers, and gas guns. Different degrees of resolution and penetration are achieved, largely because of differences in the frequency spectrum of the generated energy. However, because of inherent limitations, it is impossible to achieve both high resolution and deep penetration with any acoustic system. Either transducers or hydrophones of various designs are used to detect signals reflecting from subbottom horizons. Graphic recorders of various types are used with most acoustic profiling systems; however, magnetic-tape recording and signal processing is being accomplished with increasing frequency. Proper interpretation of acoustic subbottom profiles required an awareness of and possible corrections for scale variations caused by several factors, recognition of multiple reflections, an understanding of why reflections occur, and recognition of characteristic signatures. System selection for a particular survey requires a consideration of operational factors such as size and weight of equipment as well as environmental factors such a(Author)

Sea-Floor Spreading at the Junction between Gorda Rise and Mendocino Ridge

A Quaternary deformation pattern revealed by new sub-bottom acoustic profiles supplements previous knowledge derived chiefly from magnetic and seismic evidence on the contemporary tectonics off northern California. An inferred age for the sedimentary cover along the axis of the southern part of Gorda Rise suggests that no volcanism has occurred along that part of the rise for more than 100,000 years. Sea-floor spreading at the rise crest has been accommodated by sinking of a keystone block that forms the floor of Escanaba Trough, the median valley of the rise. Differential movement between Pacific and American tectonic plates could have caused the deformation pattern, and during this movement, the displacement offshore beyond the northwest end of the San Andreas fault is inferred to have divided at Cape Mendocino between a fault along Mendocino Ridge and a fault segment that connects with the Blanco fault farther northwest.