Low-frequency Emission Research Articles

Prediction and suppression of HIFU-induced vessel rupture using passive cavitation detection in an ex vivo model.

BackgroundOcclusion of blood vessels using high-intensity focused ultrasound (HIFU) is a potential treatment for arteriovenous malformations and other neurovascular disorders. However, attempting HIFU-induced vessel occlusion can also cause vessel rupture, resulting in hemorrhage. Possible rupture mechanisms include mechanical effects of acoustic cavitation and heating of the vessel wall.MethodsHIFU exposures were performed on 18 ex vivo porcine femoral arteries with simultaneous passive cavitation detection. Vessels were insonified by a 3.3-MHz focused source with spatial-peak, temporal-peak focal intensity of 15,690–24,430 W/cm2 (peak negative-pressure range 10.92–12.52 MPa) and a 50% duty cycle for durations up to 5 min. Time-dependent acoustic emissions were recorded by an unfocused passive cavitation detector and quantified within low-frequency (10–30 kHz), broadband (0.3–1.1 MHz), and subharmonic (1.65 MHz) bands. Vessel rupture was detected by inline metering of saline flow, recorded throughout each treatment. Recorded emissions were grouped into ‘pre-rupture’ (0–10 s prior to measured point of vessel rupture) and ‘intact-vessel’ (>10 s prior to measured point of vessel rupture) emissions. Receiver operating characteristic curve analysis was used to assess the ability of emissions within each frequency band to predict vessel rupture.Based on these measurements associating acoustic emissions with vessel rupture, a real-time feedback control module was implemented to monitor acoustic emissions during HIFU treatment and adjust the ultrasound intensity, with the goal of maximizing acoustic power delivered to the vessel while avoiding rupture. This feedback control approach was tested on 10 paired HIFU exposures of porcine femoral and subclavian arteries, in which the focal intensity was stepwise increased from 9,117 W/cm2 spatial-peak temporal-peak (SPTP) to a maximum of 21,980 W/cm2, with power modulated based on the measured subharmonic emission amplitude. Time to rupture was compared between these feedback-controlled trials and paired controller-inactive trials using a paired Wilcoxon signed-rank test.ResultsSubharmonic emissions were found to be the most predictive of vessel rupture (areas under the receiver operating characteristic curve (AUROC) = 0.757, p < 10-16) compared to low-frequency (AUROC = 0.657, p < 10-11) and broadband (AUROC = 0.729, p < 10-16) emissions. An independent-sample t test comparing pre-rupture to intact-vessel emissions revealed a statistically significant difference between the two groups for broadband and subharmonic emissions (p < 10-3), but not for low-frequency emissions (p = 0.058).In a one-sided paired Wilcoxon signed-rank test, activation of the control module was shown to increase the time to vessel rupture (T- = 8, p = 0.0244, N = 10). In one-sided paired t tests, activation of the control module was shown to cause no significant difference in time-averaged focal intensity (t = 0.362, p = 0.363, N = 10), but was shown to cause delivery of significantly greater total acoustic energy (t = 2.037, p = 0.0361, N = 10).ConclusionsThese results suggest that acoustic cavitation plays an important role in HIFU-induced vessel rupture. In HIFU treatments for vessel occlusion, passive monitoring of acoustic emissions may be useful in avoiding hemorrhage due to vessel rupture, as shown in the rupture suppression experiments.

Macroscopic Correlation of Elementary Deformation Events in a Low-Stability Crystal Lattice of FCC-Metals

A discrete pattern of the low-frequency acoustic emission spectrum under conditions of high-temperature plastic deformation of aluminum is analyzed. It is attributed to re-distribution of vibrational energy of the primary acoustic signal over resonant vibrations of standing waves of the resonators. In a low-stability crystal medium, standing-wave oscillations initiate elementary deformation displacements in a certain material volume. The linear dimensions of this volume are related to the length of the standing wave, thus determining the macroscopic scale of correlation. The correlated deformation displacements in turn generate acoustic signals, whose interference results in the formation of a single acoustic signal of abnormally high amplitude. In a low-stability state of the crystal lattice, activation of the elementary plastic shears could result from a combined action of static forces, thermal fluctuations and dynamic forces of standing acoustic waves.

The nature of the low-frequency emission of M 51

Context. Low-frequency radio continuum observations (<300 MHz) can provide valuable information on the propagation of low-energy cosmic ray electrons (CRE). Nearby spiral galaxies have hardly been studied in this frequency range because of the technical challenges of low-frequency radio interferometry. This is now changing with the start of operations of LOFAR. Aims. We aim to study the propagation of low-energy CRE in the interarm regions and the extended disk of the nearly face-on spiral galaxy Messier 51. We also search for polarisation in M 51 and other extragalactic sources in the field. Methods. The grand-design spiral galaxy M 51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarisation. This observation covered the frequencies between 115 MHz and 175 MHz with 244 subbands of 8 channels each, resulting in 1952 channels. This allowed us to use RM synthesis to search for polarisation. Results. We produced an image of total emission of M 51 at the mean frequency of 151 MHz with 20′′ resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300 MHz so far. The integrated spectrum of total radio emission is described well by a power law, while flat spectral indices in the central region indicate thermal absorption. We observe that the disk extends out to 16 kpc and see a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are greater at 151 MHz, and the break is smoother at 151 MHz than those observed at 1.4 GHz. The arm-interarm contrast is lower at 151 MHz than at 1400 MHz, indicating propagation of CRE from spiral arms into interarm regions. The correlations between the images of radio emission at 151 MHz and 1400 MHz and the FIR emission at 70 μm reveal breaks on scales of 1.4 and 0.7 kpc, respectively. The total (equipartition) magnetic field strength decreases from about 28 μG in the central region to about 10 μG at 10 kpc radius. No significant polarisation was detected from M 51, owing to severe Faraday depolarisation. Six extragalactic sources are detected in polarisation in the M 51 field of 4.1◦ × 4.1◦ size. Two sources show complex structures in Faraday space. Conclusions. Our main results, the scale lengths of the inner and outer disks at 151 MHz and 1.4 GHz, arm-interarm contrast, and the break scales of the radio-FIR correlations, can be explained consistently by CRE diffusion, leading to a longer propagation length of CRE of lower energy. The distribution of CRE sources drops sharply at about 10 kpc radius, where the star formation rate also decreases sharply. We find evidence that thermal absorption is primarily caused by H ii regions. The non-detection of polarisation from M 51 at 151 MHz is consistent with the estimates of Faraday depolarisation. Future searches for polarised emission in this frequency range should concentrate on regions with low star formation rates.

LIMITS ON GAMMA-RAY BURST PROMPT RADIO EMISSION USING THE LWA1

As a backend to the first station of the Long Wavelength Array (LWA1) the Prototype All Sky Imager (PASI) has been imaging the sky $>$ -26$^{\circ}$ declination during 34 Gamma Ray Bursts (GRBs) between January 2012 and May 2013. Using this data we were able to put the most stringent limits to date on prompt low frequency emission from GRBs. While our limits depend on the zenith angle of the observed GRB, we estimate a 1$\sigma$ RMS sensitivity of 68, 65 and 70 Jy for 5 second integrations at 37.9, 52.0, and 74.0 MHz at zenith. These limits are relevant for pulses $\geq$ 5 s and are limited by dispersion smearing. For pulses of length 5 s we are limited to dispersion measures ($DM$s) $\leq$ 220, 570, and 1,600 pc cm$^{-3}$ for the frequencies above. For pulses lasting longer than 5s, the $DM$ limits increase linearly with the duration of the pulse. We also report two interesting transients, which are, as of yet, of unknown origin, and are not coincident with any known GRBs. For general transients, we give rate density limits of $\leq$ $7.5\times10^{-3}$, $2.9\times10^{-2}$, and $1.4\times10^{-2}$ yr$^{-1}$ deg$^{-2}$ with pulse energy densities $>1.3\times 10^{-22}$, $1.1\times 10^{-22}$, and $1.4\times 10^{-22}$ J m$^{-2}$ Hz$^{-1}$ and pulse widths of 5 s at the frequencies given above.

Black hole spin measurements through the relativistic precession model: XTE J1550-564

Abstract We present a systematic analysis of the complete set of observations of the black hole (BH) binary XTE J1550-564 obtained by the Rossi X-ray Timing Explorer. We study the fast time variability properties of the source and determine the spin of the BH through the relativistic precession model. Similarly to what is observed in the BH binary GRO J1655-40, the frequencies of the QPOs and broad-band noise components match the general relativistic frequencies of particle motion close to the compact object predicted by the relativistic precession model. The combination of two simultaneously observed quasi-periodic oscillation (QPO) frequencies together with the dynamical BH mass from optical/infrared observations yields a spin equal to a = 0.34 ± 0.01, consistent with previous determinations from X-ray spectroscopy. Based on the derived BH parameters, the low-frequency QPO emission radii vary from ∼30 gravitational radii (Rg) to the innermost stable orbit for this spin (∼5 Rg), where they sharply disappear as observed for the case of GRO J1655-40.

First results of simultaneous recording of VLF emissions at two closely located points at auroral latitudes

For the first time, simultaneous observations of very low frequency (VLF) emissions at auroral latitudes (L = 5.3) are carried out at two points located at close geomagnetic latitudes and spaced in longitude by 400 km: the Finnish Kannuslehto station (Φ = 64.2°) and the Russian Lovozero observatory (Φ = 64.1°). A recording equipment with similar frequency responses was used. The first results of a comparison of simultaneous observations showed that, in the overwhelming majority of cases, bursts of VLF emission appeared at both points synchronously with an identical (more often right-handed) polarization of the magnetic field of VLF waves, which can be evidence in favor of large dimensions of the ionosphere exit region of VLF waves. A simultaneous burst of quasi-periodic VLF emissions that occurred on February 02, 2013, during a substorm at 23–24 UT is discussed in detail. Additionally, VLF bursts were recorded which were observed only at one point, e.g., the appearance of left-hand polarized periodic emissions (PEs) in band 2.5–4.0 kHz with a repetition period of 3–4 s.

Quasiperiodic VLF emissions: Analysis of periods on different timescales

The properties of quasiperiodic (QP), extremely low-frequency (ELF), and very low-frequency (VLF) emissions detected at a ground-based station in northern Finland between 15:00 UT and 22:00 UT on December 24, 2011 were studied. Characteristic periods in the spectra of QP emissions at different timescales and their dynamics during the considered event were analyzed. Two types of period variations of the QP emissions were observed: a regular increase of periods on timescales of 1 min to 2.5 min and appreciable period variations on a timescale of 1–10 min during substorms. These period variations are explained based on the properties of a self-oscillating cyclotron-instability regime in the magnetosphere. The analysis of the fine structure of the spectra of QP elements indicated that short-periodic modulations with periods of approximately 3 and 6 s were detected in the low-frequency part of the spectrum (f 2.5 kHz), respectively. The 6 s period corresponded to the period of two-hop propagation of whistling atmospherics. The presence of this short-periodic modulation may be related to the passive mode locking in a magnetospheric cyclotron maser.

ELECTRON-POSITRON FLOWS AROUND MAGNETARS

The twisted magnetospheres of magnetars must sustain a persistent flow of electron-positron plasma. The flow dynamics is controlled by the radiation field around the hot neutron star. The problem of plasma motion in the self-consistent radiation field is solved using the method of virtual beams. The plasma and radiation exchange momentum via resonant scattering and self-organize into the "radiatively locked" outflow with a well-defined, decreasing Lorentz factor. There is an extended zone around the magnetar where the plasma flow is ultra-relativistic; its Lorentz factor is self-regulated so that it can marginally scatter thermal photons. The flow becomes slow and opaque in an outer equatorial zone, where the decelerated plasma accumulates and annihilates; this region serves as a reflector for the thermal photons emitted by the neutron star. The e+- flow carries electric current, which is sustained by a moderate induced electric field. The electric field maintains a separation between the electron and positron velocities, against the will of the radiation field. The two-stream instability is then inevitable, and the induced turbulence can generate low-frequency emission. In particular, radio emission may escape around the magnetic dipole axis of the star. Most of the flow energy is converted to hard X-ray emission, which is examined in the accompanying paper.

The lower ionospheric perturbation as a precursor to the 11 March 2011 Japan earthquake

It is found that clear lower ionospheric perturbations appeared as a precursor to the 11 March 2011 Japan earthquake. This study is based on the observation of two completely different phenomena: (1) subionospheric very low frequency/low frequency propagation anomaly on the NLK (Seattle, USA) – Chofu propagation path, and (2) depression of magnetospheric ultra low frequency emissions observed on the ground (Kakioka, etc.). But, both effects are suggested to be interpreted by a unified phenomenon of seismo-lower ionospheric perturbation because they occurred on the days of 5 and 6 March, 2011.

Low-frequency otoacoustic emissions in schoolchildren measured by two commercial devices

ObjectiveClick evoked otoacoustic emissions in children are known to be good indicators of hearing function when used in the frequency range 1.5–4kHz. Using two commercial devices, the present study investigates the usefulness of responses in the lower frequency range of 0.5–1kHz evoked by 0.5kHz tone bursts. MethodsOtoacoustic emissions (OAEs) were recorded from the ears of 37 schoolchildren (age 12–13 years). OAE measurements were then made using two devices: the ILO 292 (Otodynamics) and the HearId (Mimosa Acoustics). Each device was used for two measurements: first with a standard click stimulus at 80dB pSPL (CEOAEs) and a second using a 0.5kHz tone burst at 80dB pSPL (TBOAEs). Pure tone audiometry and tympanometry were also conducted. Half-octave-band values of OAE signal to noise ratios (SNRs) and response levels were used to assess statistical differences. ResultsBoth devices provided similar SNR results for click and tone burst stimuli, although the ILO device generated slightly higher response levels for clicks. For the 0.5kHz tone bursts, both devices evoked very weak responses at 0.5kHz and the peak response occurred at 0.7–1kHz. Generally, CEOAE SNRs were about 10dB in the 1–4kHz range, while SNRs for 0.5kHz TBOAEs were about 10dB at 0.7–1kHz. Conclusions0.5kHz TBOAEs could be measured in children as effectively as CEOAEs. They can provide additional information about the 0.7–1kHz frequency range, a range over which CEOAEs do not usually contain responses above the noise floor. The main difficulty was that the maxima of the 0.5kHz TBOAEs occurred at frequencies of 0.7–1kHz, probably because of spectral splatter from the short tone burst stimulus and from rapidly falling responses of the cochlea and the recording system at low frequencies.

A study of correlation between seismicity and mental health: Crete, 2008–2010

We compared the number of M > 2 earthquakes in an area, including the island of Crete, Greece, to the number of admissions to the Psychiatric Inpatient Unit of the University of Crete (IPU/UoC), during the period 2008–2010. We found that when earthquakes with M 6.4 occurred in the region examined, the number of monthly admissions to the Acute Care Unit of the IPU/UoC, NAA, reached lowest values, whereas this number increased when the monthly number NE of small (M < 3) earthquakes increased. We also found a positive correlation between the total number of monthly admissions NA and NE (r = 0.601/P = 0.001). When a daily resolution analysis was performed for the month with the highest value of NE, we found that an abrupt increase in the number of small earthquakes was followed by an increase in the number of admissions after ∼2 days (during that month, from a total of 71 patients, 38/10 people were diagnosed with schizophrenia/bipolar disorder). We hypothesize that seismic activity might be a significant contributing factor influencing the frequency of admissions of psychotic disorders in Crete in the period 2008–2010 and that the beneficial/adverse effects are related to the anomalous electric field/extra low frequency–ultra low frequency emissions.

Detection of VLF and LF emissions of fluorescent light for efficient management of power consumption

In this research work, a detection probe of Very Low Frequency and Low Frequency (LF) emissions of fluorescent light is developed by using low cost loop antenna. The developed loop antenna is able to operate at VLF and LF bandwidth. The developed antenna is tested and measured with signal generator and oscilloscope in order to verify the usefulness of antenna. The developed antenna is subsequently used to detect the signal emitted by the fluorescent light. The antenna probe is located at different distance in order to obtain the peak voltage of received signal. Besides that, the fluorescent light is switch on and off respectively in order to verify the source of signal. From the oscilloscope, the received signal is operating at approximately 28 KHz. Hence, the developed antenna probe can be used for efficient management of power consumption as 28 KHz signal is detected if the light is on.

Feasibility of online determination of the quality of 304L stainless steel machined components using acoustic emission

This article examines the relationships between the quality of a machined component and the non-intrusive measurements that can be made during the manufacturing process. For the purposes of this study, the overall quality of a machined component is defined by the surface finish and any residual stress induced by the machining process. The focus of the work involves turning of difficult-to-machine metals and non-intrusive measurements recorded during machining. These measurements were acoustic emission, cutting forces and cutting insert temperature. An austenitic stainless steel was chosen as a commonly available metal whose high work-hardening rate and ability to strain transform to martensite make it potentially difficult to machine. Tests were carried out under different cutting conditions to promote thermally and mechanically induced residual stress and variations in the surface finish. Analysis of the mean frequency of the acoustic emission signal has made it possible to determine whether thermal or mechanical interactions dominate the machining process. The mean frequency of the machined samples provided evidence of a thermally driven process. This was confirmed by the close relationship between cutting insert temperature and component residual stress. The analysis of low-frequency acoustic emission generated during machining (below 100 kHz) identified poor surface finish derived from vibrations of the cutting insert.

CONSTRAINTS ON FREE-FREE EMISSION FROM ANOMALOUS MICROWAVE EMISSION SOURCES IN THE PERSEUS MOLECULAR CLOUD

We present observations performed with the Green Bank Telescope at 1.4 and 5 GHz of three strips coincident with the anomalous microwave emission features previously identified in the Perseus molecular cloud at 33 GHz with the Very Small Array. With these observations we determine the level of the low frequency (~1 - 5 GHz) emission. We do not detect any significant extended emission in these regions and we compute conservative 3\sigma upper limits on the fraction of free-free emission at 33 GHz of 27%, 12%, and 18% for the three strips, indicating that the level of the emission at 1.4 and 5 GHz cannot account for the emission observed at 33 GHz. Additionally, we find that the low frequency emission is not spatially correlated with the emission observed at 33 GHz. These results indicate that the emission observed in the Perseus molecular cloud at 33 GHz, is indeed in excess over the low frequency emission, hence confirming its anomalous nature.

Daytime VLF emissions at the Sodankyla Observatory (L ∼ 5.3) at the front of high-speed solar wind streams

We discuss the results of an analysis of digital high-sensitivity ground-based observations of very low frequency (VLF) emissions, carried out in Northern Finland (L = 5.3) in May–June 2012. During this period of time, we found that three high-speed solar wind streams approached the Earth’s magnetosphere and at the front of these fluxes long-lasting intense daytime bursts of VLF emissions were generated in two frequency bands: above and below ∼2.5 kHz. At frequencies above ∼2.5–3.0 kHz, there were VLF hiss waves, the temporal structure of which consisted of a quasi-periodic sequence of separate stronger spots of noise signals. The low-frequency band was represented by chorus waves, superimposed on intense hiss emissions at frequencies below ∼1.5 kHz. The high-frequency (f > 2.5 kHz) waves were elliptic and, predominately, left-hand polarized and the low-frequency waves were right-hand polarized. It was supposed that high-frequency VLF hiss waves were generated at L 5. We discuss a possible scenario of the generation and propagation of the VLF emissions observed.

A Review of Wind Turbine Noise Perception, Annoyance and Low Frequency Emission

Current literature concerning the perception, annoyance and emission of low-frequency noise from wind turbines are reviewed. Wind turbine noise has been shown to be annoying to people with annoyance related to noise load. Other factors, such as those related to visual, economic and psychological effects, were also shown to affect a person's annoyance of wind turbine noise. Published infrasound (noise at frequencies less than 20 Hz) measurements show that levels at typical residential set-back distances are too low to be directly audible, but may be perceived via window rattling. On the other hand, low-frequency noise levels, in the frequency range of 20–200 Hz may exceed audibility thresholds and it is postulated they may be correlated with annoyance. A review of general low-frequency noise annoyance studies is presented and highlights the similarities of many wind turbine noise complaints with those due to low-frequency noise. The paper concludes with a suggestion to develop a new methodology that can simultaneously acquire annoyance and noise data at the time a person believes they are annoyed by wind turbine noise.

1133 – Association between seismicity and mental health: Crete, 2008–2010

Several investigations have shown the influence of environmental factors, i.e. seismic activity on human health. However, no studies have focused on the effect of earthquakes on mental health. Crete is located within a rectangular region at the boundaries of Eurasian and African tectonic plates with high seismicity. To investigate the effect of seismic activity on mental health. To explore possible association between seismic activity and rates of hospital admissions of psychotic patients. We compared the seismic activity (EQs with magnitude M>2) in a geographical area of Crete, and admissions to the Psychiatric Inpatient Unit/ University General Hospital of Heraklion, Crete during the years 2008–2010. Our analysis showed (1) a very low rate of admissions of psychotic patients suffering from acute psychotic disorders, during the period with several great (M>6.4) and middle (4.5< M< 6.4) magnitude period earthquakes (EQs), (2) an increasing trend of admissions of acute psychotic disorders during a period with increasing number of relatively small EQs(r=0.667; p< 0.001), (3) a correlation between the number of total monthly admissions and the number of M>2 EQs. It appears that strong EQs have a protective effect on the relapse of major psychiatric disorders, whereas small EQs are associated with an increasing number of relapses. We hypothesized that the beneficial/adverse effects are related to anomalous electric fields/Extra Low Frequency (ELF)-Ultra Low Frequency (ULF) emissions.

Changes in cochlear function during acute endolymphatic hydrops development in guinea pigs

Previous studies have injected artificial endolymph into scala media in anaesthetized guinea pigs as an acute model of endolymphatic hydrops. Here, we have injected artificial endolymph into scala media in guinea pigs at rates of 40–80 nl/min, whilst monitoring Compound Action Potential (CAP) thresholds, the Summating Potential (SP)/CAP ratio, Cochlear Microphonic (CM) distortion, low-frequency modulated Distortion Product Otoacoustic Emissions (DPOAEs), and the Endocochlear Potential (EP). We found that abrupt recovery of CAP thresholds, SP/CAP ratio, and CM and DPOAE asymmetric distortion could occur several times during a single injection of less than 3 μl, suggesting that endolymph pressure could periodically decrease while the injection was ongoing. Larger volumes are thought to produce a rupture of the membranous labyrinth, however, our results suggest that multiple injections, each larger than 3 μl and within 40 min of each other, cause multiple pressure-related changes, which are difficult to be explained on the basis of a simple labyrinth rupture. We have also examined the morphological changes of the temporal bones ex vivo using X-ray micro-tomography. Both the functional changes and the micro-CT images suggest ruptures of the membranous labyrinth may not always be responsible for abrupt changes in inner ear function. Our results provide a new insight into the changes in cochlear function occurring during acute hydrops development, which compares well to the clinical findings observed in Ménière's Disease. We suggest that hydrops development may be a continual process, yet cause discontinuous functional changes due to mechanisms other than a simple rupture of the membranous labyrinth.

New type of ensemble of quasi-periodic, long-lasting VLF emissions at the auroral zone

Abstract. A new type of the series of quasi-periodic (QP) very low frequency (VLF) emissions in frequency range of 1–5 kHz, and not associated with geomagnetic pulsations, has been discovered at auroral latitudes (L = 5.3) during the Finnish VLF campaign (held in December 2011). At least five unusually spectacular events, each with a duration of several hours, have been observed during the night under conditions of quiet geomagnetic activity (Kp = 0–1), although QPs usually occur during the daytime. Contrary to the QP emissions typically occurring during the day, the spectral structure of these QP events represented an extended, complicated sequence of repeated discrete rising VLF signals. Their duration was about 2–3 min each, with the repetition periods ranging from ~1 min to ~10 min. Two such nighttime non-typical events are reported in this paper. The fine structure of the separated QP elements may represent a mixture of the different frequency band signals, which seem to have independent origins. It was found that the periodic signals with lower frequency appear to trigger the strong dispersive upper frequency signals. The temporal dynamics of the spectral structure of the QPs studied were significantly controlled by some disturbances in the solar wind and interplanetary magnetic field (IMF). This finding is very important for future theoretical investigations because the generation mechanism of this new type of QP emissions is not yet understood.

Space-based passive microwave soil moisture retrievals and the correction for a dynamic open water fraction

Abstract. The large observation footprint of low-frequency satellite microwave emissions complicates the interpretation of near-surface soil moisture retrievals. While the effect of sub-footprint lateral heterogeneity is relatively limited under unsaturated conditions, open water bodies (if not accounted for) cause a strong positive bias in the satellite-derived soil moisture retrieval. This bias is generally assumed static and associated with large, continental lakes and coastal areas. Temporal changes in the extent of smaller water bodies as small as a few percent of the sensor footprint size, however, can cause significant and dynamic biases. We analysed the influence of such small open water bodies on near-surface soil moisture products derived from actual (non-synthetic) data from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) for three areas in Oklahoma, USA. Differences between on-ground observations, model estimates and AMSR-E retrievals were related to dynamic estimates of open water fraction, one retrieved from a global daily record based on higher frequency AMSR-E data, a second derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and a third through inversion of the radiative transfer model, used to retrieve soil moisture. The comparison demonstrates the presence of relatively small areas (&lt;0.05) of open water in or near the sensor footprint, possibly in combination with increased, below-critical vegetation density conditions (optical density &lt;0.8), which contribute to seasonally varying biases in excess of 0.2 (m3 m−3) soil water content. These errors need to be addressed, either through elimination or accurate characterisation, if the soil moisture retrievals are to be used effectively in a data assimilation scheme.