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Measured time series were generated by small omnidirectional explosive sources in a shallow water area. A bottom-mounted hydrophone recorded sound signals that propagated over a sloping bottom. The time series in the 250-500 Hz band were analyzed with a broad-band adiabatic normal mode approach. The measured waveforms contain numerous bottom interacting multipaths that are complicated by the subbottom structure that contains high-velocity layers near the water-sediment interface. Several of the details of the geoacoustic structure and the depth of the water column at the receiver are inferred from comparisons of the measured data to simulated time series. The sensitivity of broad-band matched-field ambiguity surfaces in the range-depth plane for a single receiver to selected waveguide parameters is examined. A consistent analysis is made where the simulated time series are compared to the measured time series along with the single-receiver matched-field localization solutions for ranges out to 5 km. In this range interval, it was found that the peak cross-correlation between the measured and simulated time series varied between 0.84 and 0.69. The difference between the GPS range and the range obtained from the matched-field solution varied from 0 to 63 m. The geoacoustic structure obtained in the analysis consists of an 8-m low-velocity sediment layer over an 8-m high-velocity layer followed by a higher velocity, infinite half-space 相似文献
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Stotts S. A Koch R. A. Joshi S. M. Nguyen V. T. Ferreri V. W. Knobles D. P. 《Oceanic Engineering, IEEE Journal of》2010,35(1):79-102
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BJÖRN HOLMER SOFIA THORSSON INGEGÄRD ELIASSON 《Geografiska Annaler: Series A, Physical Geography》2007,89(4):237-248
Intra‐urban cooling rates were monitored for a year in the centre of Göteborg on the Swedish west coast. Five sites with different building geometry ‐ from dense canyon to open space ‐ within a radius of 300 m were analysed. Results showed two modes of cooling during the night. In the first mode, the cooling was site‐dependent ‐the open space cooled the most and the dense canyon the least. In the second mode, which began about 3–4 hours after sunset, all sites cooled at the same rate. Our hypothesis is that in the early part of the evening both radiative divergence and sensible heat dominate the cooling. However, as the evening progresses, a spatially homogeneous inversion is established which controls the radiative cooling, making the cooling independent of both surface geometry and surface type. From April to November the sites cooled rapidly around sunset and the cooling slowly decreased during the rest of the night. However, between December and March, the cooling was less intense, with an almost constant rate during the entire night. It has been suggested that this might be the result of the draining of stored solar heat during the summer and a continuous flux of anthropogenic heating during the winter. 相似文献
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KARL-DAG VORREN MAARTEN BLAAUW STEFAN WASTEGÅRD JOHANNES VAN DER PLICHT CHRISTIN JENSEN 《Boreas: An International Journal of Quaternary Research》2007,36(3):253-277
From the Sellevollmyra bog at Andøya, northern Norway, a 440‐cm long peat core covering the last c. 7000 calendar years was examined for humification, loss‐on‐ignition, microfossils, macrofossils and tephra. The age model was based on a Bayesian wiggle‐match of 35 14C dates and two historically anchored tephra layers. Based on changes in lithology and biostratigraphical climate proxies, several climatic changes were identified (periods of the most fundamental changes in italics): 6410–6380, 6230–6050, 5730–5640, 5470–5430, 5340–5310, 5270–5100, 4790–4710, 4890–4820, 4380–4320, 4220–4120, 4000–3810, 3610–3580, 3370–3340 (regionally 2850–2750; in Sellevollmyra a hiatus between 2960–2520), 2330–2220, 1950, 1530–1450, 1150–840, 730? and c. 600? cal. yr BP. Most of these climate changes are known from other investigations of different palaeoclimate proxies in northern and middle Europe. Some volcanic eruptions seemingly coincide with vegetation changes recorded in the peat, e.g. about 5760 cal. yr BP; however, the known climatic deterioration at the time of the Hekla‐4 tephra layer started some decades before the eruption event. 相似文献
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DANIEL VERES SIWAN M. DAVIES BARBARA WOHLFARTH FRANK PREUSSER STEFAN WASTEGÅRD LINDA AMPEL ANNE HORMES GÖRAN POSSNERT JEAN-PAUL RAYNAL GÉRARD VERNET 《Boreas: An International Journal of Quaternary Research》2008,37(3):434-443
A new tephra has been identified within a long core (EC 3) sequence recovered from Les Echets, near Lyon, France. This visible tephra was discovered as part of a high resolution multiproxy re-investigation of the Les Echets sequence. Independent chronological information suggests that the tephra is c . 2 000–45 000 years old, and geochemical analysis indicates that it is of basanitic composition. The latter suggests a possible origin in the Eifel; however, as yet, no other volcanic events or deposits can be correlated to the Les Echets tephra. New sedimentological and chronological data are presented indicating that the tephra falls within an interval that most likely correlates with Dansgaard–Oeschger events 12-9. Thus, this tephra could potentially be an important middle MIS 3 marker horizon in central Europe if it can be traced in other palaeorecords. 相似文献
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Cable P.G. Yudichak T.W. Dorfman Y. Knobles D.P. Renhe Zhang Zhaohui Peng Fenghua Li Zhenglin Li 《Oceanic Engineering, IEEE Journal of》2006,31(1):145-155
Determinations of bottom scattering strength in the decade below 1 kHz under downward refracting conditions have been made using acoustic reverberation and transmission data from the 2001 East China Sea Asian Seas International Acoustic Experiment (ASIAEX). The measurements were performed using explosive sources and receiving hydrophones in ship-suspended vertical-line arrays. The focus of this paper has been the dependence of bottom scattering strength on the frequency and characterization of the uncertainties associated with the extraction of scattering strength from reverberation. The derived bottom scattering strength gradually rises with frequency from 100-300 Hz and then more rapidly above 300 Hz. A potential explanation suggests that the frequency variation results from two scattering mechanisms, rough layer scattering at the low end of the band and sediment near-surface volume scattering at the high end. The spatial extrapolation of these results is explored by comparing them with similarly derived scattering strengths using data obtained under the Navy's Harsh Environments Program at a somewhat separated site (56 km) under environmental conditions similar to those during ASIAEX. In the ASIAEX analysis, it has been found that the largest source of uncertainty in the scattering-strength frequency dependence arises from persistence of finite-amplitude effects associated with the source signal. 相似文献
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RD Hodgkiss A Grant JHR McClelland R Quatre B Rademakers C Sanchez 《African Journal of Marine Science》2017,39(2):225-232
This study produced the first data on population parameters for sicklefin lemon shark Negaprion acutidens at the granitic Seychelles Islands, adding to a limited catalogue of species information. Juveniles at a site in the Curieuse Marine National Park were tagged over two periods, in 2014–2015 and 2015–2016, with measurements of length (precaudal, fork and total) and weight, sex, and state of the umbilical opening recorded. Recapture data were used to calculate growth rates and population size. Mean annual growth rate was 5.40 cm y–1. A mean size at birth of 62.5 cm TL is in line with previous assessments of the species. Population-size estimates were similar for the two sampling periods (comprising 311 and 255 individuals) and higher than anticipated. Movements to outside the lagoon were recorded for early ages, and this, combined with weight loss and a reduction in condition factor as displayed in several individuals, suggests that the study area alone could not support the large number of sharks pupped yearly. 相似文献
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Expanding spread profile at the northern Jan Mayen Ridge 总被引：1，自引：0，他引：1
An expanding spread seismic profile at the central northern Jan Mayen Ridge, ESP-5, has yielded a crustal seismic velocity distribution which is similar to observations from the thinned continental crust at the Norwegian continental margin. The profile reveals a post-early Eocene sedimentary sequence, about 1. 5 km thick, overlying 1 km of volcanic extrusives and interbedded sediments. Below, there are about 3 km of pre-opening sediments above the seismic basement. The results indicate that the main ridge block is underlain by a thinned crust, possibly only 13.5 km thick. The results are compatible with a continental nature for the main ridge complex. 相似文献
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Seismic evidence of up to 200 m lake‐level change in Southern Patagonia since Marine Isotope Stage 4
ANDREA CATALINA GEBHARDT CHRISTIAN OHLENDORF FRANK NIESSEN MARC DE BATIST FLAVIO S. ANSELMETTI DANIEL ARIZTEGUI PIERRE KLIEM STEFAN WASTEGÅRD BERND ZOLITSCHKA 《Sedimentology》2012,59(3):1087-1100
Maar lake Laguna Potrok Aike is located north of the Strait of Magellan (south‐eastern Patagonia). Seismic reflection profiles revealed a highly dynamic palaeoclimate history. Dunes were identified in the eastern part of the lake at approximately 30 to 80 m below the lake floor, overlying older lacustrine strata, and suggest that the region experienced dry conditions probably combined with strong westerly winds. It is quite likely that this can be linked to a major dust event recorded in the Antarctic ice cores during Marine Isotope Stage 4. The dunes are overlain by a series of palaeo‐shorelines indicating a stepwise water‐level evolution of a new lake established after this dry period, and thus a change towards wetter conditions. After the initial, rapid and stepwise lake‐level rise, the basin became deeper and wider, and sediments deposited on the lake shoulder at approximately 33 m below present‐day lake level point towards a long period of lake‐level highstand between roughly 53·5 ka cal. bp and 30 ka cal. bp with a maximum lake level some 200 m higher than the desiccation horizon. This highstand was then followed by a regressional phase of uncertain age, although it must have happened some time between approximately 30 ka cal. bp and 6750 yrs cal. bp . Dryer conditions during the Mid‐Holocene are evidenced by a dropping lake level, resulting in a basin‐wide erosional unconformity on the lake shoulder. A second stepwise transgression between ca 5·8 to 5·4 ka cal. bp and ca 4·7 to 4 ka cal. bp with palaeo‐shorelines deposited on the lake shoulder unconformity again indicates a change towards wetter conditions. 相似文献