Echo Types Research Articles

HF echo types revisited: aspect angle attenuation effects

Abstract. The high-spatial-resolution observations of the auroral E-region echoes by the Super Dual Auroral Radar Network (SuperDARN) HF radar at Pykkvibaer, Iceland considered in a previous study are re-examined. Both the spectral power and Doppler velocity of the E-region HF echoes exhibit strong dependence on the slant range and expected off-perpendicular magnetic aspect angle (aspect angle attenuation). We consider the aspect angle attenuation effects in identification of spectral HF echo types. It is argued that echoes with Doppler velocities close to the nominal value of the ion-acoustic speed Cs (~350 m/s) and echoes with velocities greatly exceeding the Cs (up to 600 m/s) are more related in their generation mechanisms than previously thought. It is proposed to treat the echoes near the Cs as aspect-angle-attenuated counterparts of the high-velocity echoes and that both types are generated directly by the modified two-stream instability with aspect angle attenuation resulting in the preferential phase velocity saturation at the Cs.

E-region decameter-scale plasma waves observed by the dual TIGER HF radars

Abstract. The dual Tasman International Geospace Environment Radar (TIGER) HF radars regularly observe E-region echoes at sub-auroral magnetic latitudes 58°–60° S including during geomagnetic storms. We present a statistical analysis of E-region backscatter observed in a period of ~2 years (late 2004–2006) by the TIGER Bruny Island and Unwin HF radars, with particular emphasis on storm-time backscatter. It is found that the HF echoes normally form a 300-km-wide band at ranges 225–540 km. In the evening sector during geomagnetic storms, however, the HF echoes form a curved band joining to the F-region band at ~700 km. The curved band lies close to the locations where the geometric aspect angle is zero, implying little to no refraction during geomagnetic storms, which is an opposite result to what has been reported in the past. The echo occurrence, Doppler velocity, and spectral width of the HF echoes are examined in order to determine whether new HF echo types are observed at sub-auroral latitudes, particularly during geomagnetic storms. The datasets of both TIGER radars are found to be dominated by low-velocity echoes. A separate population of storm-time echoes is also identified within the datasets of both radars with most of these echoes showing similar characteristics to the low-velocity echo population. The storm-time backscatter observed by the Bruny Island radar, on the other hand, includes near-range echoes (r<405 km) that exhibit some characteristics of what has been previously termed the High Aspect angle Irregularity Region (HAIR) echoes. We show that these echoes appear to be a storm-time phenomenon and further investigate this population by comparing their Doppler velocity with the simultaneously measured F- and E-region irregularity velocities. It is suggested that the HAIR-like echoes are observed only by HF radars with relatively poor geometric aspect angles when electron density is low and when the electric field is particularly high.

Comparison of sedimentary processes on adjacent passive and active continental margins offshore of SW Taiwan based on echo character studies

ABSTRACTEcho character recorded on Chirp sub‐bottom sonar data from offshore of SW Taiwan was analysed to examine and compare the sedimentary processes of adjacent passive and active continental margin settings. Seafloor echoes in the study area are classified into four types: (1) distinct echoes, (2) indistinct echoes, (3) hyperbolic echoes and (4) irregular echoes. Based on the mapped distribution of the echo types, the sedimentary processes offshore of SW Taiwan are different in the two tectonic settings. On the passive South China Sea (SCS) margin, slope failure is the main process on the upper continental slope, whereas turbidite deposits accumulate in the lower continental slope. In contrast, the submarine Taiwan orogenic wedge is characterized by fill‐and‐spill processes in intraslope basins on the upper slope, and mass‐transport deposits are observed in the canyons and on the lower Kaoping slope. This difference is largely caused by the huge influx of terrigenous sediments into the submarine Taiwan orogenic wedge compared with the passive SCS continental margin. In the latter, loading and movement of the Taiwan orogenic wedge has had a significant effect on the seafloor morphology and has triggered retrogressive failures. Gas hydrate dissociation may have enhanced the slope failure processes at some locations.

Deposition processes from echo-character mapping along the western Algerian margin (Oran–Tenes), Western Mediterranean

The westernmost Algerian margin (south Algero-Provençal basin) depicts a few offshore active faults, moderate to rare seismicity, and generally very steep slopes (>16°). We classified and mapped 12 echo types according to their sub-bottom acoustic facies observed on this margin on 2–5.2 kHz Chirp echo-sounder data (MARADJA 2003 cruise). The echo-character maps are interpreted in terms of sedimentary processes: the B1 echo type (parallel to subparallel high- to low-amplitude sub-bottom reflections), mainly in the deep basin, corresponds to hemipelagic sedimentation; R1 (prolonged single echo with no sub-bottoms) and R2 (small irregular overlapping hyperbolae) echo types, generally near or in canyon systems, are associated with turbidity currents or more rarely to contour currents or mass-transport deposits such as slumps, slides and debris flows; the transparent echo types (T1–T5) and R3 (chaotic lens of low-amplitude reflections on top of higher amplitude), often located at the foot of the slope or canyons walls, typically indicate mass-transport deposits (like slides) or turbidites. Large zones that display a large variety of echo types are evidenced in the study area and are generally associated with turbidity currents, but could also be associated with bottom currents. It appears that active tectonics plays a significant role in this part of the margin which presents a few active faults offshore but also a strong and relatively frequent seismicity onland. The general pattern of the distribution of mass-transport deposits is particular – i.e. many but small slides all along the margin – and suggests a probable triggering by recurrent earthquake shakings. However, active tectonics is not the only factor influencing the deposition pattern, as some zones seem characterized by predominant strong turbidity currents transporting sediments far away from the foot of the margin, whereas others depict retrogressive erosion features on the slope, i.e. small slides scarps in gullies rather than transport by turbidity currents. In particular, the rivers sediment discharge fluxes and the geomorphologic characteristics of the margin seem to be very important factors too.

Estimation of leaf area index and canopy openness in broad-leaved forest using an airborne laser scanner in comparison with high-resolution near-infrared digital photography

We estimated leaf area index (LAI) and canopy openness of broad-leaved forest using discrete return and small-footprint airborne laser scanner (ALS) data. We tested four ALS variables, including two newly proposed ones, using three echo types (first, last, and only) and three classes (ground, vegetation, and upper vegetation), and compared the accuracy by means of correlation and regression analysis with seven conventional vegetation indices derived from simultaneously acquired high-resolution near-infrared digital photographs. Among the ALS variables, the ratio of the “only-and-ground” pulse to “only” pulse (OGF) was the best estimator of both LAI (adjusted R2 = 0.797) and canopy openness (adjusted R2 = 0.832), followed by the ratio of the pulses that reached the ground to projected lasers (GF). Among the vegetation indices, the normalized differential vegetation index (NDVI) was the best estimator of both LAI (adjusted R2 = 0.791) and canopy openness (adjusted R2 = 0.764). Resampling analysis on ALS data to examine whether the estimation of LAI and canopy openness was possible with lower point densities revealed that GF maintained a high adjusted R2 until a fairly low density of about 0.226 points/m2, while OGF performed marginally when the point density was reduced to about 1 point/m2, the standard density of high-density products on the market as of February 2008. Consequently, the ALS variables proposed in the present study, GF and OGF, seemed to have great potential to estimate LAI and canopy openness of broad-leaved forest, with accuracy comparable to NDVI, from high-resolution near-infrared imagery.

Sediment echo types and acoustic characteristics of gas-related acoustic anomalies in Jinhae Bay, southern Korea

Analysis of Chirp profiles from Jinhae Bay, southern Korea, revealed three sediment echo types (echo type I–III) and acoustic anomalies associated with shallow gas such as acoustic blanking and turbidity. Echo type I, characterized by low-to-high amplitude internal reflectors, almost parallel to the seafloor and seen away from the Gadeog Channel, probably represent mud from the Nakdong River that moved into the area by bayward transport. Echo type II, characterized by truncation at the seafloor and seen only in the Gadeog Channel, is interpreted to be tidal-channel deposits. Echo type III is acoustically transparent and interpreted as distal prodelta deposits. The acoustic blanking and turbidity were further divided into AB-I-III and AT-I-II, respectively. AB-I has a very strong, shallow top reflection that is parallel to the seafloor, crosscutting the stratigraphy. Depths of AB-I may correspond to the sulphate-methane transition that remains shallow probably due to large supply of methane. The top reflection of AB-II is of high amplitude and follows the bedding planes, probably controlled or limited by lithology. AB-III is characterized by a very irregular or highly diffractive and strong top reflection, which may be due to very closely-spaced patches of high-concentration gas. AT-I exhibits a weak and highly irregular top reflection, suggesting low gas concentration. AT-II, comprised of isolated weak diffractions, is likely due to isolated accumulations of low concentration gas. C/N ratios (6.8–8.6) of the organic matter in the cored sediments in the study area suggest mixed marine and terrestrial sources.

Recent sedimentary processes in the Prestige site area (Galicia Bank, NW Iberian Margin) evidenced by high-resolution marine geophysical methods

An echo-character analysis of the oil tanker Prestige wreck area was undertaken using high-resolution marine geophysical methods (TOPAS and airgun seismic-reflection profiles, multibeam echosounder and TOBI sidescan sonar). Integration and comparison of the results using all these methods is presented given some practical applications for indirect near-surface and seafloor interpretations. Ten different echo types were identified and grouped into four main classes: I) distinct; II) indistinct; III) irregular and IV) hyperbolic. Echo-character distribution enabled us to determine recent sedimentary processes in the area. Two major depositional systems can be found through the identification of these sedimentary processes: a) a slope depositional system (SDS) located in the eastern and central area, where mass-movement processes are dominant and b) a turbiditic Main Channel depositional system (TDS) located in the western area where channel-related processes are dominant. Both of these interact over the half-graben structure of the southwestern sector of the Galicia Bank, where the Prestige wreck is located. Within the SDS, erosive and depositional mass-movement processes characterised a complex depositional system. Erosive processes occur on the fault scarp, channels head, inter-lobe channels and distal part of the sedimentary lobes. Moreover, depositional processes take place on the top of the fault scarp, sedimentary wedges, sedimentary lobes, and on the west flank of the Main Channel. Both depositional systems interact, but the SDS should be more active during fault-scarp reactivation periods, through relief rejuvenation and new exposed deposits. Microearthquake activity would favour the available materials, being weathered, eroded and transported by mass-movements. In such a situation, the TDS acts as the main collector of eroded sediment derived from the fault scarp throughout the SDS being responsible for its evacuation into the Iberian Abyssal Plain. However, outside of fault reactivation periods, the SDS is less active (such as during the present situation).

Schooling behaviour of small pelagic fish: phenotypic expression of independent stimuli

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 334:263-272 (2007) - doi:10.3354/meps334263 Schooling behaviour of small pelagic fish: phenotypic expression of independent stimuli Patrice Brehmer1,*, François Gerlotto2, Charline Laurent1, Pascal Cotel3, Alina Achury4, Birane Samb5 1Centre de Recherche Halieutique Méditerranéenne et Tropicale (IRD; IFREMER and University of Montpellier II), 1 Avenue Jean Monnet, BP 171, 34203 Sète, France 2Institut de Recherche pour le Développement, c/o Instituto del Mar del Peru, Esq. Gamarra y Gral Valle s/n, Chucuito, La Punta, Callao, Peru 3Institut de Recherche pour le Développement, Sainte Clotilde, 97490, Île de La Réunion, France 4Fundación La Salle, Estación de Investigaciones Marinas Isla Margarita, Isla de Margarita, Punta de Piedras 6318, Venezuela 5Institut Sénégalais de Recherche Agronomique, Centre de Recherche Océanographique de Dakar-Thiaroye, BP 2241, Senegal *Email: patrice.brehmer@ifremer.fr ABSTRACT: We compared echo sounder measurements of fish schools on Venezuelan and Senegalese coastal shelves. In both areas Sardinella aurita was the predominant aggregative species; the stocks are known to be genetically independent. Persistent school typology is described by analysing 7 acoustic surveys in Venezuela and comparing with pioneer observations in Senegal. The echo types encountered in both countries were comparable, with S. aurita and their associated species having comparable schooling behaviour. School morphology appears to be more related to species than to global characteristics of their ecosystems, as fish school descriptors were similar for both stocks. Nevertheless, S. aurita may adopt different school morphologies. Differentiation between day and night fish schools was mainly due to morphological variables, as determined by fractal dimension. Echo types were different between day and night in both Venezuela and Senegal. This analysis shows that several independent parameters can influence school morphology. However, although schooling behaviour is temporally highly variable, it shows several constant, typical 2D morphological structures, i.e. echo types. The observation of small pelagic fish schools occurring during the night is not unusual according to the literature, although a strict diurnal schooling behaviour, with scattering during the night, is often reported. Schooling behaviour appears to be a phenotypic expression depending on biotic and abiotic stimuli, intrinsic characteristics and motivation. KEY WORDS: Behaviour · Diel cycle · Echo type · In situ observations · Phenotypic expression · Schooling · Small pelagic fish Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 334. Online publication date: March 26, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Estimation of LAI and fractional cover from small footprint airborne laser scanning data based on gap fraction

We evaluate the potential of deriving fractional cover (fCover) and leaf area index (LAI) from discrete return, small footprint airborne laser scanning (ALS) data. fCover was computed as the fraction of laser vegetation hits over the number of total laser echoes per unit area. Analogous to the concept of contact frequency, an effective LAI proxy was estimated by a fraction of first and last echo types inside the canopy. Validation was carried out using 83 hemispherical photographs georeferenced to centimeter accuracy by differential GPS, for which the respective gap fractions were computed over a range of zenith angles using the Gap Light Analyzer (GLA). LAI was computed by GLA from gap fraction estimations at zenith angles of 0–60°. For ALS data, different data trap sizes were used to compute fCover and LAI proxy, the range of radii was 2–25 m. For fCover, a data trap size of 2 m radius was used, whereas for LAI a radius of 15 m provided best results. fCover was estimated both from first and last echo data, with first echo data overestimating field fCover and last echo data underestimating field fCover. A multiple regression of fCover derived from both echo types with field fCover showed no increase of R 2 compared to the regression of first echo data, and thus, we only used first echo data for fCover estimation. R 2 for the fCover regression was 0.73, with an RMSE of 0.18. For the ALS LAI proxy, R 2 was lower, at 0.69, while the RMSE was 0.01. For LAI larger radii (∼ 15 m ) provided best results for our canopy types, which is due to the importance of a larger range of zenith angles (0–60°) in LAI estimation from hemispherical photographs. Based on the regression results, maps of fCover and LAI were computed for our study area and compared qualitatively to equivalent maps based on imaging spectrometry, revealing similar spatial patterns and ranges of values.

Optimising estimates of mesospheric neutral wind using the TIGER SuperDARN radar

Super Dual Auroral Radar Network (SuperDARN) HF backscatter radars scan 16 beam directions over a field of view of ∼52°. In the common mode of operation, data is collected using 45-km range gates and 7-s integrations on each beam. Application of a beam-swinging algorithm permits mesospheric neutral winds to be estimated from the line-of-sight (LOS) Doppler velocity of meteor echoes detected at near ranges (<600 km). Larger meteor echo detection rates better constrain the solutions and thereby increase the accuracy of wind estimates. Greater rates also lead to wind estimates with better time and height resolution. In this study, meteor echo detection rates were increased by running dedicated radar control programs on the Tasman Geospace Environment Radar (TIGER) Tasmania radar (147.2°E, 43.4°S). This involved the use of shorter 15-km range gates and 2-s integration times. The Doppler characteristics of different echo types at meteor echo ranges were identified. The echoes were then filtered according to these characteristics, and their suitability for estimating neutral winds investigated. One echo type was clearly of ionospheric origin, forming thin, continuous traces decreasing in group range from ∼1200 to ∼300 km before midnight. These “descending plasma streams” (DPS) merged into and contaminated the meteor scatter observed by TIGER. However, they will be less of a problem for the planned network of “storm time” SuperDARN radars to be deployed at mid-latitudes for the study of major substorms and storms which occur less frequently.

Depositional processes of late Quaternary sediments in the Yellow Sea: a review

This review focuses on the depositional processes of late Quaternary sediments in the eastern Yellow Sea, an epicontinental sea with a flat and broad seafloor (less than 100 m in water depth) and extensive tidal flats along the southeastern coast. The Yellow Sea was subaerially exposed during the last glacial period when sea level was about 120 m below the present level. During erosional retreat of shorefaces and river mouths, sedimentation was largely controlled by high-amplitude rise in sea level, forming transgressive sheets (echo type 1–3a) and sediment ridges (echo types 2-1 and 2–2) with extensive development of ravinement surfaces. The distribution of surface sediments reflects an interplay of sediment input from the surrounding landmass and the hydrodynamic regime in response to sea-level rise. Muddy sediments in the central part represent the Huanghe-River source and form a highstand sheet (echo type 1–3b). Large birdfoot-like sand bodies off the Jiangsu coast also represent highstand deposits when sea level reached the present position at about 6 ka. In the southeastern part of the Yellow Sea, sediments are dominated by sand ridges (echo types 2-1, 2–2 and 2–3), largely shaped by tidal currents. The southwestern corner of the Korean Peninsula is dominated by a thick deposit of mud, the Heuksan mud belt. The muds largely originate from the Geum River, whose distribution is controlled by strong southward coastal currents. In the coastal regions of the southeastern Yellow Sea, sedimentation is controlled by a combined effect of waves and tides with distinctive season-alities in sedimentary facies owing to the monsoonal climate: tidedominated mud deposition in summer and wave-dominated sand deposition/erosion in winter: Winter storms play a role in sedimentation on intertidal flats. Quantitative monitoring of sediment transport suggests that the textural variation results from the overwhelming role of winter waves superimposed on tidal currents in pulling sands and resuspending muds. Due to low sedimentation rate, the tidal flats formed retrogradational, coarsening-upward pattern during the Holocene sea-level rise.

Distinctive sedimentary processes in Guanabara Bay - SE/Brazil, based on the analysis of echo-character (7.0 kHz)

Guanabara Bay bottom sediments and seabed characteristics were analysed using high-resolution (7 kHz) sub-bottom profiles associated with particle size analyses of 92 bottom sediment samples. Eight types of echo-characters were identified revealing the strong relation with the particle size distribution. Sandy bottom areas presented strong echo reflections, without sub-bottom penetration (Echo types I and III), while in muddy areas sub-bottom reflections showed the acoustic basement delineating buried sugar-loaf hills and infilled-valley features (Echo type IV). The presence of shallow gas within the sediments is indicated by acoustic blanket and a series of bottom-multiple reflections (Echo types Va and Vb). Erosion by bottom currents and artificial mechanical dredging are suggested by truncations of sub-bottom reflections and a wrinkled seabed surface (Echo types VI and VII). Crystalline basement outcrops on the seabed are recognized by multiple or single hyperbolae with varying elevations above the bay bottom (Echo type II).

남극 남쉐틀랜드 북부 대륙주변부 및 남스코시아해 지역의 고해상(3.5 kHz)음향 특성

High-resolution (3.5 kHz) subbottom profiles were analyzed in order to reveal sedimentation pattern of late Quaternary in the northern South Shetland continental margin and the South Scotia Sea, Antarctica. On the basis of clarity, continuity and geometry of surface and subbottom echoes together with seafloor topography, high-resolution echo characters are classified into eight echo types which represent rock basements (echo type III-1), coarse-grained subglacial till or moraine (echo type I-1), slides/slumps (echo type IV), debris-flow deposits (echo types II-3 and III-2), and bottom-current deposits (echo types I-2, II-1 and II-2). Subglacial till or moraine (echo type I-1) is mostly present in the lower continental shelf and upper continental slope of the northern South Shetland continental margin, which changes downslope to slides/slumps (echo type IV) and debris-flow deposits (echo types II-3 and III-2) in the middle to lower continental slope. This distribution suggests that the continental slopes of the northern South Shetland continental margin were mostly affected by downslope gravitational processes. Further downslope, bottom-current sediments (echo type I-2) deposited by the southwestward flowing Antarctic Deep Water (ADW) occur at the South Shetland Trench, reflecting an Interaction between mass flows and bottom currents in the area. In contrast to the northern South Shetland continental margin, the South Scotia Sea is dominated by bottom-current deposits (echo types II-1 and II-2), indicating that the sedimentation was mostly controlled by the westward flowing ADW. Flow intensity of the ADW has increased in the relative topographic highs, forming thin covers of coarse-grained contourites (echo type II-1), whereas it has decreased in the relative topographic lows, depositing thick, fine-grained contourites (echo type II-2). The poor development of wave geometry in the fine-grained bottom-current deposits (echo type II-2) is suggestive of the unsteady nature of the ADW flow.

Geophysical signatures of mud mounds at hydrocarbon seeps on the Louisiana continental slope, northern Gulf of Mexico

Mud mounds are common features at cold seeps in many areas of the world. They are abundant in the northern Gulf of Mexico, where they occur at the numerous hydrocarbon seeps on the continental margin. Although the mounds are widely known, their geophysical signatures and geology have not been widely studied in detail. In this study, we examined mounds in two areas of the upper Louisiana slope using geophysical data of several types: 11–12 kHz side-scan sonar, 3.5 kHz echo-sounder profiles, near-bottom 2–12 kHz chirp sonar profiles, and three-dimensional (3D) multichannel seismic data. In side-scan images the mud mounds appear as ovoid high backscatter spots associated with larger, irregular high backscatter zones along faults and fault complexes. Three different sonar responses were noted: (1) a more-or-less uniform appearance, (2) a central higher backscatter spot surrounded with a lower return halo (bulls-eye), and (3) a more-or-less uniform high backscatter area with an acoustic dead-spot near its center. Near-bottom chirp sonar records indicate high lateral variability in seafloor reflection characteristics, implying that the long-range acoustic signature of a given mound results from the integration of seafloor characteristics. In these records, anomalous seafloor echo types include strong seafloor reflectors (hardbottom) or buried hardbottom, acoustic wipe-out (attenuation), and acoustic turbidity (prolonged echoes). Cores from acoustically anomalous seafloor areas indicate that a variety of seep-related material has been introduced into the near-seafloor sediments. The prevalence of strong bottom return signatures implies that the formation of hard grounds or crusts is an integral part of hydrocarbon seep evolution and suggests that much of the ovoid high backscatter signatures may result from such features. Multichannel seismic reflection data from several of the active mounds show columnar subsurface chaotic reflection zones and strong reflections in sediments directly underneath. Seafloor seismic amplitude maps from these mounds are either generally uniform, with a positive or negative anomaly, or display a ring of higher reflectivity around the mound periphery. Positive or negative anomalies may depend on the degree of seafloor lithification and sediment gas charging. The ring feature is thought to result from the preferential concentration of seep-debris in the mound periphery relative to the center. Some mounds, which show no seismic reflection anomaly from the seafloor, appear inactive. Altogether, observations from this study are consistent with the mud mounds being formed by sedimentary volcanism. The data imply they are Holocene features that have a life-cycle that includes hardbottom formation, which progressively crusts over the fluid vent, ending its activity.

Cenozoic tectono-sedimentary evolution of the western Iberian margin

Bathymetric, 9.5-kHz long-range sidescan sonar (OKEAN) and seismic reflection data are used to characterise the Cenozoic tectono-sedimentary evolution of the Porto, Lisbon and Alentejo continental margins, offshore Portugal, where the presence of submarine fans, slope gullies, submarine canyons and seamounts was confirmed. Sediment drifts, some of probable contouritic origin, were recognised over the upper continental slope (500–1000 m) and surrounding the Vigo seamount. Seven echo types offshore Porto (types IA-1, IB, IB-2, IC-2, IIA, IIB, IIIA) and five echo types offshore Lisbon (types IB, IIA, IIB, IB-2, IIIA) were identified on 3.5-kHz profiler data. In addition, 11 Meso–Cenozoic seismic units off Lisbon and Alentejo, plus three post-Turonian seismic packages offshore Porto were interpreted and dated by well, dredge and DSDP/ODP stratigraphic data. During the Cenozoic, the tectono-sedimentary evolution of the studied areas depended on their position in relation to the locus of compression in Iberia. Accommodation space and sediment pathways varied in relation to distinct pulses of uplift or subsidence occurring at different times in the three studied regions. The Porto and Lisbon margins record extensional collapse respectively after the early Eocene and early Chattian, interrupted by short episodes of uplift related to distinct tectonic phases. As a result, gravity flows dictated deposition on these margins in most part of the Cenozoic. Seamounts and halokinetic structures controlled deposition on the Porto margin by inducing topographic barriers to the westward progradation of slope-derived sediment. The relative proximity of the Alentejo margin to the Azores–Gibraltar Fracture Zone resulted in folding and exposure during the middle Oligocene, but subsidence after the early Chattian generated a palaeoslope buried under Neogene units. Oligocene and Burdigalian canyon incision offshore Alentejo preceded the emplacement of modern channels during the Pliocene. These presently transport sediment derived from the shelf and major onshore drainage catchments into abyssal areas.

High-resolution acoustic characteristics of epicontinental sea deposits, central–eastern Yellow Sea

Large amounts (45390 line km) of closely spaced (2.2–4.4 km) high-resolution subbottom profiles (Chirp, 2–7 kHz) made it possible to identify detailed echo types and their distribution in an epicontinental shelf environment. On the basis of seafloor morphology, surface bedforms and subbottom acoustic characters, 10 echo types were identified in the uppermost sedimentary sequence of the central-eastern Yellow Sea. Flat seafloor with sharp bottom echoes (echo types 1-1, 1-2 and 1-3a; transgressive sediment sheets or relict sands) is widespread in the offshore area and underlain to the west by an acoustically transparent wedge (echo type 1-3b; highstand muds). Mounded seafloor with either smooth surface or superposed bedforms (echo types 2-1, 2-2 and 2-3; tidal ridges) and flat seafloor with regularly spaced, wavy bedforms (echo type 1-4; large-scale dunes) are dominant in the eastern nearshore area. Large-scale mounds with continuous, inclined internal reflectors (echo type 2-4; giant mud bank) occur in the southeastern nearshore area. Various-scale eroded seafloor (echo types 3-1 and 3-2; channels) and flat seafloor with regularly spaced, wavy bedforms (echo type 1-4; large-scale dunes) are present in the northern part of the sea. The distribution pattern of echo types in the central–eastern Yellow Sea reflects depositional processes and sediment dispersal systems during the Holocene transgression and highstand period: (1) development of tidal ridges and large-scale dunes in response to strong tidal currents and waves in the eastern nearshore area; (2) construction of transgressive to highstand mud bank (Huksan mud belt) by deposition of muds derived from the Keum river in the southeastern nearshore area; (3) active erosion due to intensified currents in the northern part; and (4) highstand mud deposition derived from the Huanghe river on the transgressive sediment sheets in the offshore area.

Long term studies of equatorial spread F using the JULIA radar at Jicamarca

Jicamarca unattended long term investigations of the ionosphere and atmosphere radar observations of equatorial spread F (ESF) plasma irregularities made between August 1996 and April 2000 are analyzed statistically. Interpretation of the data is simplified by adopting a taxonomy of echo types which distinguishes between bottom-type, bottomside, topside, and post-midnight irregularities. The data reveal patterns in the occurrence of ESF in the Peruvian sector that are functions of season, solar flux, and geomagnetic activity. We confirm earlier work by Fejer et al. (J. Geophys. Res. 104 (1999) 19,859) showing that the quiet-time climatology of the irregularities is strongly influenced by the climatology of the zonal ionospheric electric field. Under magnetically quiet conditions, increasing solar flux implies greater pre-reversal enhancement amplitudes and, consequently, irregularity appearances at earlier times, higher initial altitudes, and higher peak altitudes. Since the post-reversal westward background electric field also grows stronger with increasing solar flux, spread F events also decay earlier in solar maximum than in solar minimum. Variation in ESF occurrence during geomagnetically active periods is consistent with systematic variations in the electric field associated with the disturbance dynamo and prompt penetration described by Fejer and Scherliess (J. Geophys. Res. 102 (1997) 24,047) and Scherliess and Fejer (J. Geophys. Res. 102 (1997) 24,037). Quiet-time variability in the zonal electric field contributes significantly to variability in ESF occurrence. However, no correlation is found between the occurrence of strong ESF and the time history of the zonal electric field prior to sunset.

Chirp (2–7-kHz) echo characters of the South Korea Plateau, East Sea: styles of mass movement and sediment gravity flow

The South Korea Plateau is a complex of ridges, seamount chains, troughs and a basin. Detailed analysis of 12 019 line-km Chirp (2–7-kHz) subbottom profiles from the South Korea Plateau and the eastern continental slope of Korea reveals a distinctive zonal distribution of echo types depending systematically on seafloor morphology. Pelagites/hemipelagites (type I-2) and basement highs (type III-1) prevail on the ridge summits, seamount chains and the upper to middle part of the eastern continental slope of Korea, and are commonly bounded downslope by creeps (type III-3) and slides/slumps (type IV-1) that occur extensively over the entire slope areas of the ridges, seamount chains and eastern continental slope. The mass-movement deposits change downslope to debrites and turbidites (types II, III-2, IV-2 and IV-3) in the troughs and Onnuri Basin, suggesting successive downslope evolution from slide and slump to debris flow and turbidity current. The voluminous creeps, slides and slumps over the entire slope areas of the plateau and eastern continental slope, deeper than 300 m in water depth, were most likely generated by frequent seismic shakings.

Transforming echoes into pseudo-action potentials for classifying plants.

Animals perceive their environment by converting sensory stimuli into action potentials, or temporal point processes, that are interpreted by the brain. This paper investigates the information content of point processes extracted from echoes from in situ plants in an effort to understand how bats recognize landmarks in the field. A mobile sonar converts echoes into biologically similar temporal point processes. termed pseudo-action potentials (PAPs), whose inter-PAP interval relates to echo amplitude. The sonar forms a sector scan of an object to produce a spatial-temporal PAP field. Classifier neurons apply delays and coincidence detection to the PAP field to identify three distinct echo types, glints, blobs, and fuzz, which characterize plant features. Glints are large amplitude echoes exhibiting coherence over successive echoes in the sector scan, typically produced by favorably oriented isolated specular reflectors. Blobs are large echoes lacking coherence, typically bordering glints or formed by collections of interfering reflectors. Fuzz represents weak echoes, typically produced by collection of weak scatterers or by reflectors on the beam periphery. A small mirror reflector models a flat leaf surface and motivates the glint criteria. Classifiers are applied to experimental data from two types of tree trunks, a glint-producing sycamore (Platanus occidenatalis) and a glint-absent Norway maple (Acer platanoides) and two plants, a glint-producing rhododendron (Rhododendron maximus) and a glint-absent yew (Taxus media). We speculate that our narrow-band sonar models the activity of a single frequency bin in the frequency-modulated (FM) sweep emitted by bats, and that one function of the frequency bins in the FM sweep is to form a sector scan of the environment.

Comparison and correlation of physical properties from the plain and slope sediments in the Ulleung Basin, East Sea (Sea of Japan)

High-resolution profiles (chirp sonar, 2–7kHz) acquired from the Ulleung Basin, East Sea (Sea of Japan) revealed several discrete echo types, suggesting deposits formed mainly by mass flow processes. The Ulleung Basin plain sediments are characterized by turbidite/hemipelagic deposits, while slide, slump, and debrite deposits are mainly found in the slope area. Rock fall deposits also appear near the Ulleung Island. The core sediments show distinctive downslope variations in most physical properties studied. Over the plain and the slope, mass physical properties (water content, porosity, wet bulk density, shear strength, and velocity) show considerably different values in spite of the nearly constant sediment texture. However, the relationships among the physical properties have relatively good correlations, although the absolute values are different. Grain density in the plain is relatively lower than the slope, probably because of a tephra and calcite compensation depth (CCD). Therefore, we suggest that the slope sediments may be affected by sedimentological changes resulting from diagenesis such as a certain degree of compaction and/or consolidation.