Echo Traces Research Articles

Infrasonic sounds excited by seismic waves of the 2011 Tohoku‐oki earthquake as visualized in ionograms

AbstractAfter the M 9.0 Tohoku‐oki earthquake in 2011, strong deformation of ionogram echo traces, forming multiple cusp signatures (MCSs), were observed at three stations 790–1880 km from the epicenter. The vertical structure of the ionospheric disturbances was determined by true height analysis and compared with broadband seismograph records at stations close to the ionosondes. These ionospheric disturbances were caused by vertically propagating acoustic waves excited by the up/down ground motion of seismic waves. Numerical simulations have shown that acoustic waves with a period of 15–40 s and amplitude of order 1 mm/s at the ground level were sufficient to create MCSs as sharp as those observed. These acoustic wave parameters are consistent with the seismic records if the motion of the air mass on the ground level is assumed to be the same as the ground motion. The travel time diagram of the seismic records along the line connecting the epicenter and ionosondes showed that the first MCS ionogram detected at each station was caused by P waves, while the others were caused by Rayleigh waves.

Equatorial range spread F echoes from coherent backscatter, and irregularity growth processes, from conjugate point digital ionograms

Radio wave returns from spread F plasma structures as received by ionosondes can originate from total/specular reflection, partial reflection or coherent backscattering. The dominant mechanism to account for the spread F traces in equatorial ionograms is still an open question. Depending upon the precise mechanism, ionosondes are sensitive to irregularity scale sizes of tens of meters to several tens/hundreds of kilometers. In this paper we analyze signatures of range spreading F layer traces in Digisonde ionograms, taken at Fortaleza (3.9°S, 38.45°W, dip: −9°) and Sao Luis (2.33 S, 44.2W, dip angle: −0.5°), and at a dip equatorial site Cachimbo (9.5°S, 54.8°W, dip: −4.2°) and at its two conjugate sites Bova Vista (02.8°N, 60.7°W, dip: 22.0°) and Campo Grande (20.5 S, 54.7 W, dip −22.3°) in Brazil, to determine the dominant process/mechanism of echo returns from the irregularity structures. A significant component of the ESF trace structures is found to be consistent with the echoes originating from coherent backscattering at field line perpendicular directions. The degree of range spreading of the echoes is found to increase linearly with the top frequency of the echo trace, which is shown to be a more precise indicator of the irregularity strength. Further, the irregularity strength exhibits a significant increase from the equator toward the EIA crests, as well as a strong asymmetry between the conjugate sites.

Rayleigh wave signature in ionograms induced by strong earthquakes

After the magnitude 9.0 earthquake off the Pacific coast of Tohoku (near the east coast of Honshu, Japan), which occurred on 11 March 2011, an unusual multiple‐cusp signature (MCS) was observed in ionograms at three ionosonde stations across Japan. To investigate the general characteristics of MCSs, we examined ionograms obtained during the period 1957 to 2011 from five ionosonde stations in Japan after earthquakes with a seismic magnitude of 8.0 or greater. A total of 43 such earthquakes occurred after 1957, as recorded in the database of the United States Geological Survey. MCSs were observed in 8 of 43 earthquakes, allowing us to investigate the specific conditions of MCS appearance. The appearance of MCSs at different epicentral distances exhibits traveling characteristics at a velocity of ∼4.0 km/s, which is in the range of Rayleigh waves. There is a ∼7 min offset in delay time after the earthquake at each epicentral distance in the travel‐time diagram. This offset is consistent with the propagation time of acoustic waves from the ground to the ionosphere launched by Rayleigh waves. MCSs in ionograms provide a snapshot of vertical disturbances induced by a wave because the sweep time for the whole ionospheric echo traces is much shorter than the propagation time of acoustic waves. The vertical scale of the disturbances is 10∼30 km, which corresponds to an acoustic wave period of 20∼50 s at ionospheric heights.

Observational study of daytime ionospheric irregularities associated with typhoon

Spread-F is a manifestation of ionospheric irregularities and generally takes place at nighttime. However, it can also be observed seldom at daytime. It is recognized that acoustic gravity waves (AGWs) play an important role in triggering plasma instability which results in spread-F in the ionosphere. The typhoon is a main source of the AGWs. In this paper, two cases of ionospheric daytime spread-F in the period of typhoon were analyzed. One case was on July 29, 1988 and the other was on August 01, 1989. The results showed the following: 1) There were some wave-like disturbances appearing in the HF Doppler records firstly, consequently the Doppler echo traces became scattered, which indicated that the ionospheric spread-F was triggered; 2) the blurred echo traces in the both two cases appeared in the morning (08:30–11:30 Beijing time) and lasted for more than two hours; 3) with the blurred echoes gradually weakening, the traveling ionospheric disturbances (TIDs) still existed and became clearer; 4) the frequency shifts in the two cases were both positive, implying the effective reflecting surface of the radio wave in the ionosphere moved downwards. These results provide good observational evidence for daytime spread-F during the typhoon period in Asian region.

Grazing by sprat schools upon zooplankton within an enclosed marine lake

We have investigated grazing by sprat schools upon zooplankton within Lough Hyne (Ireland), a marine lake with only a narrow and shallow connection to the sea. Acoustic surveys showed the presence of large numbers of sprat (Sprattus sprattus), preyed upon by mackerel Scomber scombrus. The sprat formed dense schools during the day and dispersed at night. Zooplankton were widely distributed within the lough, although absent below the oxythermocline. However, echo traces showed clear volumes, indicating an absence of zooplankton, surrounding the daytime sprat schools. Pumped sampling of the zooplankton at different depths, close to and away from the sprat schools, confirmed that those volumes of water that appeared acoustically clear were largely devoid of macro zooplankton (92.7% reduction overall). Comparison of clear areas with adjacent areas showed that almost all decapod larvae and calanoid copepods and most bivalve larvae were absent. The differences for cladocerans were also significant but for gastropod larvae they were not significant. Large calanoid copepods were much less abundant in the clear areas and their size distribution had changed. We conclude that the sprat schools had rapidly depleted their surroundings of zooplankton, suggesting that fish within the schools may be substantially food-limited. Dispersal at night, when visual predators are less efficient, may enable the sprat to feed more effectively. Thus, although schooling may confer benefits to individual fish there are concomitant disadvantages in terms of food depletion. The reduction in zooplankton as a result of heavy grazing by sprat within the enclosed lough may affect the phytoplankton, with significant effects upon the ecology of Lough Hyne.

Ionospheric multiple stratifications and irregularities induced by the 2011 off the Pacific coast of Tohoku Earthquake

A strong earthquake with a magnitude of 9.0 occurred at 1446:23 JST on March 11, 2011, in Japan. Ionospheric disturbances were detected at 1500 JST at four ionosonde stations. An irregular distortion of echo trace was observed at Kokubunji, which is the nearest station to the epicenter and is 440 km from it. Multiple-cusp-type trace indicating extra stratification was observed at Wakkanai and Yamagawa, which are 870 and 1410 km away from the epicenter. A small wavy fluctuation was observed at Okinawa 1910 km away from the epicenter. The real height analysis of the ionograms showed a vertical structure with a scale size of 20~30 km.

Exploratory and Instantaneous Swimming Speeds of Amphidromous Fish School in Shallow-Water Coastal Lagoon Channels

Fish school swimming speeds is essential for ecological and management studies. The multibeam sonar in horizontal beaming provided dynamic echo traces of mobile fish schools. We used two school swimming speed indicators: the average of a series of instantaneous speed values, and the exploratory speed. These swimming speeds were estimated for each fish school observed on the basis of their Euclidian position within the sonar beams. The average ISS values per school ranged from 0.15 m s−1 to 4.46 m s−1, while the ESS values per school were lower, ranging from 0.04 m s−1 to 3.77 m s−1. Multibeam sonar technology makes it possible to measure fish school swimming speeds in their natural habitat at small spatio-temporal scales. This methodology can therefore be used to analyse in situ their movements, and has a wide range of applications in behavioural studies and management purposes.

Classification of fish schools based on evaluation of acoustic descriptor characteristics

Acoustic surveys were conducted from 2002 to 2006 in the East China Sea off the Japanese coast in order to develop a quantitative classification typology of a pelagic fish community and other co-occurring fishes based on acoustic descriptors. Acoustic data were postprocessed to detect and extract fish aggregations from echograms. Based on the expert visual examination of the echograms, detected schools were divided into three broad fish groups according to their schooling characteristics and ethological properties. Each fish school was described by a set of associated descriptors in order to objectively allocate each echo trace to its fish group. Two methods of supervised classification were employed, the discriminant function analysis (DFA) and the artificial neural network technique (ANN). We evaluated and compared the performance of both methods, which showed encouraging and about equally highly correct classification rates (ANN 87.6%; DFA 85.1%). In both techniques, positional and then morphological parameters were most important in discriminating among fish schools. Fish catch composition from midwater trawling validated the fish group classification through one representative example of each grouping. Both methods provided the essential information required for assessing fish stocks. Similar techniques of fish classification might be applicable to marine ecosystems with high pelagic fish diversity.

“Hook” structure in MARSIS ionogram and its interpretation

[1] MARSIS/MEX is a low frequency radar used to sound the ionosphere of Mars. In general the echo trace associated with reflections in nadir indicates an ionosphere with a main maximum electron density at typically about 130 km altitude, and a monotonously decreasing density with increasing altitude. Another maximum is sometimes detected at higher altitudes, indicating a double layer in the upper ionosphere. Sometimes a special structure (here called a “hook”) is observed at the low frequency end of the trace: as the sounding frequency decreases the delay time of sounding signal increases rapidly. It is here proposed that this signature indicates a secondary maximum with density which is so low that it can be only partly resolved by the radar. An analytical inversion approach coping with the “hook” case is then proposed. It is further suggested that the “hook” is caused by vertical wavelike structures in the Martian topside ionosphere.

Simultaneous generation of large‐scale density irregularities and geomagnetic pulsations via filamentation instability

Results of experiment conducted at Gakona, Alaska, using intensity‐modulated HF heating waves of 3.2 MHz to generate geomagnetic pulsations and large‐scale field‐aligned density irregularities (FAI), are reported. The echo traces of o‐ and x‐mode sounding pulses from 3.53 to 4 MHz were recorded during heater on/off periods. The ionograms showed that only x‐mode echo traces were significantly affected by the HF heater. The returns from 3.53 to ∼3.8 MHz disappeared during heater on period and reappeared after heater was turned off. Ray tracings are performed to explore the effect of FAI on the backscattering of o‐ and x‐mode sounding pulses. The drastic difference between the FAI effects on o‐ and x‐mode backscattering trajectories provides a theoretical interpretation of the observation. Geomagnetic pulsations as large as 2.5 nT were also recorded by a Fluxgate Magnetometer. Thermal filamentation of the HF heater leads to the simultaneous generation of FAI and geomagnetic pulsations.

Estimating Fish Densities from Single Fish Echo Traces

Estimating mean fish density per unit area has been done by counting single fish echo traces with a split beam echo sounder system in the open sea. A data program has been written that scans the acoustic signals for echo peaks and classify these into echo traces. The program counts the number of echo traces from fish that cross the beam within given distances from the acoustic axis over a known sailing distance, and gives an absolute estimate of fish density. This is ob- tained by using the off acoustic axis angles of fish that generate resolved echoes to compute the athwartship distance from the vertical acoustic axis to the fish. The program is described and demonstrated on selected files of raw data recorded by the SIMRAD EK 60 split beam system. Estimated fish densities for these files are also obtained by the echo integration method, and the echo trace method gives estimates that agree well with these estimates under sufficiently good condi- tions.

Meteor‐induced transient sporadic E as inferred from rapid‐run ionosonde observations at midlatitudes

Observations were made with a rapid‐run ionosonde during a 1‐month period from the end of July to the end of August 2002 that included the Perseid meteor shower. Ionograms were obtained at 1‐minute intervals. All the echo traces below a virtual height of 200 km were hand‐scaled for detecting weak and broken traces as well as well‐determined sporadic E. Among the scaled E‐region traces, a distinct meteor echo persisting for 40 min was observed. This long‐duration meteor event was similar to that observed during the Leonid meteor shower in 2001. However the apparent height of the echo traces in the current event first decreased and then increased after equaling the height of a persistent sporadic E layer, while the apparent height of the long‐duration meteor echo trace observed in Leonids 2001 remained nearly constant. The descending rate of the meteor echo was approximately 22 m/s, which was considerably greater than that of the tidal ion layer trapped in a downward‐moving wind shear node. We ascribe the long‐duration meteor echo to a transient and horizontally drifting sporadic E patch generated from meteor trail plasma.

Morphological features of ionospheric response to typhoon

Couplings between the ionosphere and meteorological events have been studied widely. However, most of them are individual case studies or correlation analyses, and few are aiming at the full morphological features of the ionospheric response processes. In this paper, complete records of 24 strong typhoons from 1987 to 1992 were collected, and comparison was made with corresponding ionospheric HF Doppler shift data. The main purpose of the present work is to find the temporal evolution of these responses and their common features by the merit of the continuities of HF Doppler shift observation in time. On the basis of the statistical analyses, this paper reveals the common features of ionospheric responses to typhoon. A summary of these characteristics is as follows: (1) During the existing time of a typhoon, there are almost always medium‐scale traveling ionospheric disturbances (TIDs) in the ionosphere, especially when a strong typhoon is landing or near the coast of a mainland. (2) These TIDs show quite clear periodicity and their periods vary with time and gradually grow longer. (3) After sunset, the wavelike disturbances with large magnitudes often excite the midlatitude spread‐F. (4) The intense typhoon can cause the wavelike records of the Doppler shift to show the S‐shaped echo tracing, which means that the amplitudes of those waves are sufficiently large, and (5) the sunrise‐like phenomena often appear in nonsunrise time during the period the typhoon exists. The phenomena mentioned above are generally in agreement with the linear propagation theories of the acoustic‐gravity waves (AGWs) in the atmosphere. A typhoon is surely one of the important ground sources of the wavelike disturbances in troposphere; this source is very effective especially when a typhoon is landing on or near a mainland coast. Of course, the morphological details of the ionospheric response to typhoon can by no means be completely identical every time. In this study, except for TIDs that almost always appear during all the typhoon events, the other common features are not seen every time. However, we are certain that the phenomena summarized above are statistically the manifestation of the ionospheric response to typhoon since they appear much more frequently during periods influenced by typhoon.

Robust analysis of single echo trace obtained by split beam sounder

The theory and method of the echo trace analysis of single echoes (ETA), which simultaneously analyze the shape and level of single echoes, are almost established. But, the method has not been extensively used. The major reasons are high‐quality echoes with a high signal‐to‐noise ratio (SNR) necessary for the analysis are some times difficult to obtain; the error introduced by transducer motion is large; and conditions for the theory sometimes could not be fulfilled. Therefore, we apply the ETA to the split beam method, which is most advantageous for the ETA, and make the method robust by introducing the following: careful echo selection considering SNR, correction of the effect of transducer motion, smoothing of the trace by regression analyses, introduction of indexes checking reliability, etc. The useful results of our ETA are the track, the swimming speed and orientation, the target strength as a function of tilt angle, and the estimated size of individual fish. The robust ETA was applied to echoes of...

Estimation of swimming behavior and backscattering pattern of Antarctic krill using echo‐trace‐analysis

Echo‐trace‐analysis was applied to single echo‐traces of Antarctic krill to estimate in situ swimming orientation, swimming speed, and backscattering pattern. Acoustic data were collected with a split‐beam echosounder operating at 70 kHz on 11 January 2005 during the Southern Ocean survey by the training vessel Umitaka‐maru of Tokyo University of Marine Science and Technology. While the acoustic data were collecting, the ship speed was 2 knots. A pulsewidth and a pulse‐repetition‐period of the echosounder were 0.6 ms and 0.27 s, respectively. Krill aggregations were observed at the depth range 15–30 m from the hull‐mounted transducer and at upper and lower edges of them some single echo‐traces were visually recognized on echograms. In this work, these echo traces were analyzed. As preliminary results, the average swimming orientation was 4.0 deg with a standard deviation of 14.2 deg and the average swimming speed was 0.33 m/s with a standard deviation of 0.16 m/s. The average swimming orientation was close to the orientation observed with a video plankton recorder by Lawson et al. [J. Acoust. Soc. Am. 119(1), 232–242 (2006)]. The estimated backscattering patterns showed peaks at incident angles around −10 to 10 deg and were similar to the patterns predicted by theoretical models.

Identification of acoustic targets off Angola using General Discriminant Analysis

Pelagic fish off Angola constitute about 80% of the total fish landed and some 75% of the animal protein in the diet of the coastal population. The primary means of estimating stock abundance is through acoustic surveys, a method that is sensitive to identification of echo traces. This paper describes commonly encountered pelagic fish aggregations in terms of acoustic properties, and morphological and spatio-temporal descriptors. A knowledge base of reference observations validated by an experienced team of regional operators was used in a stepwise General Discriminant Analysis (GDA) that sought to identify traits characteristic to common species groups. A range of descriptors provided significant discriminant power (p < 0.01), and these were used to construct a classification algorithm. The resulting overall classification success emanating from the GDA was 46% when only considering the greatest posterior probability of group membership. This success rate increased to 62% and 71% respectively when including the second and third highest group membership probabilities. For Cape horse mackerel Trachurus trachurus capensis and Sardinella spp., success rates of 80% and 75% were obtained when including classifications that fell into groups with probabilities ranging from greatest to third greatest respectively.

Recent advances in real-time analysis of ionograms and ionospheric drift measurements with digisondes

Reliable long distance RF communication and transionospheric radio links depend critically on space weather, and specifically ionospheric conditions. Modern ground-based ionosondes provide space weather parameters in real-time including the vertical electron density distribution up to ∼1000 km and the velocity components of the ionospheric F region drift. A global network of digisondes distributes this information in real-time via internet connections. The quality of the automatic scaling of the echo traces in ionograms was a continuous concern ever since first attempts have been reported. The modern low-power ionosonde with ∼100 W transmitters (compared to several kilowatt for the older ionosondes) relies on more sophisticated signal processing to enhance the signal-to-noise ratio and to retrieve the essential ionospheric characteristics. Recent advances in the automatic scaling algorithm ARTIST have significantly increased the reliability of the autoscaled data, making the data, in combination with models, more useful for ionospheric now-casting. Vertical and horizontal F region drift velocities are a new real-time output of the digisondes. The “ionosonde drift” is derived from the measured Doppler frequency shift and angle of arrival of ionospherically reflected HF echoes, a method similar to that used by coherent VHF and incoherent scatter radars.

Improved performance of traditional ECG for diagnosis of left ventricular hypertrophy in hypertension

In order to improve performance of traditional electrocardiography (ECG) for diagnosis of left ventricular hypertrophy (LVH), we developed a new index defined by a typical strain pattern or a modified Cornell voltage (R wave amplitude in aVL, plus [(half of the S wave amplitude in V3) x body mass index (BMI) (kg/m )]. ROC curve-based partition value for LVH is 420 mV●kg/m. Halving S wave in V3 and adjustment for BMI result from multivariate analyses of ECG variables for prediction of LV mass. We examined 2612 untreated subjects (age 50 12, 56% men) with essential hypertension and good quality ECG and echo tracings. Prevalence of LVH at echo (LV mass 51.0 g/height—) was 31.1%. The following ECG criteria for LVH were compared with the new index: (a) Romhilt-Estes score 5 points, (b) Sokolow-Lyon voltage; (c) Traditional Cornell voltage (S wave in V3 plus R wave in aVL 2.0 mV (women) and 2.8 mV (men); (d) Perugia score (typical strain pattern or Cornell voltage 2.0 mV (women) and 2.4 mV (men). As shown in the figure, the modified Cornell voltage resulted in a

Beat-to-beat evaluation of systolic time intervals by analysis of endocardial acceleration

Background Echo evaluation of cardiac timings is a widely recognized method for assessing CRT patients. Endocardial Acceleration (EA), in its systolic (PEA I) and diastolic (PEA II) components, allows estimation of cardiac timings. Aim: to evaluate the correlation between Echo and EA measurements of Aortic Pre-ejection Interval (AoPEI) and Ejection Time (ET). Methods Fifteen CRT patients (10M, age 71.2±7.0 y) (NYHA III-IV, EF 25.6±7.6%) were implanted with Sorin Biomedica Living CHF Pacemakers, able to record and transmit EA signal via telemetry. Several pacing configurations were evaluated for each patient (R, L, Biventricular at different VV and AV delays). EA and Intracardiac EGM were recorded via telemetry for 20 s in each configuration. At the same time, AoPEI and Aortic Valve Closure (AVC) times were measured by Echo. Automatic EA analysis was carried out offline. AoPEI by EA was defined as the interval from the stimulus to the end of PEA I (TPEA1); AVC by EA was defined as the interval between TPEA1 and the onset of PEA II. ET by Echo was defined as the time interval between AoPEI and AVC on the echo tracings. Results AoPEI by Echo was 202.3±24.6 ms; AoPEI by EA was 196.1±29.9 ms (p=ns); correlation between the two was r=0.82 (p>0.0001). ET by Echo was 234.5±34.7 ms; ET by EA was 253.7±37.9 ms (p=ns); correlation between the two was r=0.74 (p>0.0001). Conclusion Echographic and EAbased estimation of systolic timings are well correlated. The chronical monitoring of such parameters by an implanted device using Endocardial Acceleration is attracting and opens new possibilites for day-by-day CRT therapy optimization.

Modeling of field‐aligned guided echoes in the plasmasphere

Ray tracing modeling is used to investigate the plasma conditions under which high‐frequency (f ≫ fuh) extraordinary mode waves can be guided along geomagnetic field lines. These guided signals have often been observed as long‐range discrete echoes in the plasmasphere by the Radio Plasma Imager (RPI) onboard the Imager for Magnetopause‐to‐Aurora Global Exploration satellite. Field‐aligned discrete echoes are most commonly observed by RPI in the plasmasphere, although they are also observed over the polar cap region. The plasmasphere field‐aligned echoes appearing as multiple echo traces at different virtual ranges are attributed to signals reflected successively between conjugate hemispheres that propagate along or nearly along closed geomagnetic field lines. The ray tracing simulations show that field‐aligned ducts with as little as 1% density perturbations (depletions) and &lt;10 wavelengths wide can guide nearly field‐aligned propagating high‐frequency X mode waves. Effective guidance of a wave at a given frequency and wave normal angle (Ψ) depends on the cross‐field density scale of the duct, such that ducts with stronger density depletions need to be wider in order to maintain the same gradient of refractive index across the magnetic field. While signal guidance by field aligned density gradient without ducting is possible only over the polar region, conjugate field‐aligned echoes that have traversed through the equatorial region are most likely guided by ducting.