Sonic Waveforms Research Articles

A matched filter for S-wave sonic imaging

A matched filter is developed and applied to sonic waveforms for extracting reflected signals and minimizing borehole mode waves for S-wave sonic imaging using dipole sources. Sonic imaging uses waveforms acquired by a sonic logging tool in a fluid-filled borehole. Signals from a source are reflected from geological interfaces and recorded by arrays of receivers of the same tool. The reflected signals in the recorded waveforms are extracted and migrated to image geological structures. Because the reflected signals are covered by the strong borehole mode waves, they often are difficult to extract. To overcome this difficulty, a new matched filter has been developed. In the new matched filter, the second derivative of the driving waveform of the dipole source is used for the reference signal, and an average of spectral densities of array data is used to approximate those of the borehole mode waves, which are treated as noise. Because the spectral density of sonic waveforms generally varies for different time intervals, the matched filter is applied using a windowed approach. The matched filter effectively suppresses the borehole mode waves, and reflected signals are well conserved. Application of the matched filter is followed by additional filtering to further remove remaining borehole mode waves. The resulting reflected signals indicate high signal-to-noise ratio compared with those by existing filtering procedures without the matched filter. The use of the matched filter significantly increases applicability of sonic imaging by extracting weaker reflected signals that cannot be obtained by the existing filtering procedures.

Evaluation of Open Fractures-Sonic Velocity Relation in Fractured Carbonate Reservoirs

Summary Rock physics evaluation of heterogeneous fractured reservoirs where such fractures are the main fluid conduits is necessary for production optimization and field development. In this regard, fracture parameters (i.e., aperture, density, etc.) play a significant role in acoustic wave velocities and propagation, which should be investigated. However, the effect of fracture parameters on sonic wave velocities and their propagation is relatively vague in fractured reservoirs. This study aims to evaluate the mentioned relation by integration of velocity deviation logs, image logs, conventional logs, sonic waveforms, core data, and thin sections in the four studied wells. The results of the velocity deviation logs and sonic waveforms indicate that fracture density and fracture aperture directly control their fluctuations. For example, results showed that zones with higher fracture aperture and lower fracture density, mainly in the dolomitic facies, exhibited more deviations (<−500) compared with the zones with higher fracture density and lower fracture aperture. Hence, the negative deviations on the velocity deviation log are not directly controlled by fracture density, and fracture aperture is the most effective factor on the negative deviation of the velocity deviation log. So, it seems that this parameter controls sonic waves scattering and reservoir behavior. It was found that open fractures and zones with higher porosity (>10%) would have a great impact on sonic wave velocities, attenuation, and other rock physics properties. Ultimately, it was confirmed that a simple but robust combination of the velocity deviation log with sonic waveforms can be used to delineate fractured zones with high fracture aperture (>0.1 mm) in the wells where core samples or image logs are not available.

Do dipole sonic logs measure group or phase velocity (revisited)?

We have revisited the debate about whether flexural waves from dipole sonic tools and standard processing algorithms measure group or phase velocities in anisotropic formations. We observe that much of the confusion arises from a failure to understand the different meanings of group and phase velocities. Using a transversely isotropic medium with a vertical axis of symmetry that exhibits a triplication in its S-wave group slowness surface, we generate synthetic flexural sonic waveforms corresponding to boreholes at angles of 0°–90° with respect to the anisotropy symmetry axis in 1° increments. We processed these synthetic data using standard time- and frequency-domain semblance methods. The results conclusively demonstrate that dipole sonic logs measure the group slowness for the group angle corresponding to the angle between the borehole and the anisotropic symmetry axis. In addition, data that we have evaluated suggest that current tool geometries and semblance processing may not always be sensitive enough to resolve all branches of the group slowness triplication surface.

Atmospheric vertical wind effects and their impact on sonic boom

The one-dimensional augmented Burgers' equation has been generally used for nonlinear lossy sonic boom propagation. This equation models the effects of nonlinearities, loss mechanisms such as absorption and dispersion due to molecular relaxation and thermoviscous dissipation, and geometric spreading through ray tube areas including Blokhintzev scaling as well as atmospheric stratification. Traditionally, atmospheric winds are accounted for by using horizontal wind components varying in magnitude as the sonic boom pressure waveform propagates from near the aircraft toward the ground. Vertical wind components, though present in the real atmosphere, are generally ignored because they are usually much weaker compared to the horizontal components. However, for long propagation distances, such as in the case of secondary booms or speeds at or slightly below Mach cut-off, vertical winds could play a role in changing the location and intensity of sonic booms. This work will update sBOOM, an augmented Burgers' equation solver, to include the vertical component of atmospheric winds. The sonic boom ground signatures and other relevant data will be compared against those obtained ignoring vertical wind components. Such differences will be discussed and documented for cases which may include shaped low-boom as well as strong shock signatures.

Compensating elastic transmission losses for P‐wave attenuation estimation from sonic logs

ABSTRACTThe decay of seismic amplitude is caused by a variety of physical phenomena that can be divided broadly into elastic transmission losses (including geometrical spreading, interface transmission losses and scattering attenuation) and intrinsic attenuation, where wave energy is converted into heat due to viscous friction. The so‐called statistical averaging method is currently considered as the most advanced sonic wave attenuation estimation method, and there exist various implementations of this method. But the way elastic transmission losses – that mask the true intrinsic attenuation – are compensated for appears to be an issue and in some cases this correction has been overlooked. In this paper, we revisit the statistical averaging method for intrinsic attenuation estimation with particular focus on the role of elastic transmission losses. Through synthetic examples, we demonstrate the importance of compensating for elastic transmission losses even if the variation of velocity and density with depth is not notable. Our implementation of the method uses finite‐difference simulations thereby providing a versatile and accurate way to generate synthetic seismograms. We use a combination of elastic and viscoelastic finite‐difference simulations to demonstrate the significant error without accurate compensation of the elastic transmission losses. We apply our implementation of the method to sonic waveforms acquired in an exploration well from Browse basin, Australia. The resulting intrinsic attenuation estimates are indeed indicative of gas‐saturated zones identified from petrophysical analysis in which viscous friction are thought to be of importance.

High-resolution acoustic imaging from a borehole to detect a nearby well

A new imaging application is presented using sonic waveform data for ranging to locate a nearby borehole. The challenge of locating a nearby well from a borehole is commonly addressed with electromagnetic (EM) or passive magnetic ranging methods, which can suffer from poor resolution and penetration and require the presence of a conductive or magnetic casing. In addition, the deeper reading active EM methods do not work well when the two boreholes are orthogonal. In contrast, acoustic imaging requires only an impedance contrast between the target object and the surrounding formation, which is adequately provided by the presence of a fluid-filled borehole, even without casing. Here we use acoustic full-waveform sonic data to identify the location, distance, and direction to a target vertical borehole from a nearby highly deviated observation well. The target borehole was located at a distance of about 9 ft with an accuracy of less than 0.5 ft, which was subsequently confirmed by drilling. In addition, acoustic waveform data made it possible to image up to 100 ft above and below the observation well at a resolution comparable to gamma ray and electric logs from the target well.

Simulation and interpretation of sonic waveforms in high-angle and horizontal wells

We have developed a new method to process and interpret sonic waveforms acquired in high-angle (HA) and horizontal (HZ) wells in the vicinity of layer boundaries. Numerical simulations are examined for HA/HZ fluid-filled borehole models, in which sonic tools with monopole and dipole sources operate across a horizontal bed boundary in hard and soft formations. Simulations are performed with a 3D time-domain finite-difference method assuming a sonic logging tool, in which each receiver station consists of eight azimuthal receivers. Instead of conventional processing for monopole and dipole waveforms, which takes the average and difference, respectively, of waveforms acquired with azimuthal receivers, we analyze the waveforms acquired individually by each azimuthal receiver. For dipole sources, we refer to the dispersive signals recorded by individual receivers as pseudo-flexural waves, and we first process them with a weighted spectral semblance method to obtain the frequency dispersion curves. We then apply a correction to the dispersion curves of receivers with specific azimuths to estimate the formation shear slowness. Close to a horizontal bed boundary, we select two azimuths: One of them is the azimuth with the corresponding receiver positioned within the top layer and having minimal sensitivity to the bottom formation. The second azimuth is such that the corresponding receiver is positioned within the bottom formation and has minimal sensitivity to the top formation. Simulations show that the presence of a bed boundary significantly alters the propagation of P-, S-, and flexural waves. Rather than by borehole guided waves, in HA wells, receiver signals generated with a monopole source are mostly influenced by converted P-P and S-S waves induced by the bed boundary. Apparent slownesses of these converted waves are determined by their direction of propagation and the corresponding true formation slowness. Also, values of apparent slownesses are consistently lower than the true formation slownesses. In soft formations, borehole guided pseudo-flexural waves generated by a vertical dipole source show good azimuthal resolution for the HA and HZ wells. Processing the pseudo-flexural waveforms separately for the top and bottom receivers yields unbiased shear slownesses for formations above and below the well, respectively. This procedure enables the accurate evaluation of formation heterogeneity due to the bed-boundary effects.

Attenuation modes from Vertical Seismic Profiling and sonic waveform in a carbonate reservoir, Abu Dhabi, United Arab Emirates

ABSTRACTIn this study, we derived accurate and high‐resolution attenuation profiles using spectral ratio, centroid frequency shift, and seismic interferometry methods. We utilized high‐quality vertical seismic profiling and sonic waveform data acquired in a carbonate reservoir located in Abu Dhabi, United Arab Emirates. The scattering profile of vertical‐seismic‐profiling data contributes significantly to wave attenuation that can be explained by high heterogeneity of the carbonate rocks. The scattering profile also correlates well with the reservoir lithology and fractured zones imaged by the Formation MicroImager. A tar mat zone occurs within the lower part of Arab D reservoir. This zone corresponds with a decrease in scattering attenuation. The tar mat may have filled the pores and made this zone less heterogeneous. Therefore, a decrease in scattering attenuation can be considered a potential parameter for tar mat detection. After removing the scattering effect, nonphysical negative intrinsic attenuation values still exist at certain depths. The most probable explanation for this is the three‐dimensional scattering effect, which is not taken into account in this paper, and short‐period upgoing waves. Seismic interferometry is less sensitive to the remaining scattered upgoing wave, which is why seismic interferometry method shows fewer negative values than the spectral ratio and centroid frequency shift methods. Compared with vertical‐seismic‐profiling attenuation, scattering attenuation estimated from sonic waveforms recorded in the reservoir zones is insignificant, and the intrinsic attenuation is almost equivalent to the total attenuation. We attribute this underestimation of the scattering attenuation to the sparse spatial sampling of the sonic logging data at 0.1524 m, which is not sufficient to appropriately estimate the scattering effect in heterogeneous media. The cross‐plots between sonic attenuation and various petrophysical properties show slight dependence between the sonic attenuation and neutron porosity and resistivity in the reservoir zones. However, we can highlight from these plots two zones belonging to the Arab reservoirs. The lower zone corresponds to Arab D reservoir and displays higher sonic intrinsic attenuation than the upper zone (Arab A–C reservoirs) due to higher oil saturation. This highlights the sensitivity of the intrinsic attenuation to the oil saturation.

Analytic Simulation of the Elastic Waves Propagation in the Neighborhood of Fluid Filled Wells with Monopole Sources

An analytic formulation to understand the scattering, diffraction and attenuation of elastic waves at the neighborhood of fluid filled wells is presented. An important, and not widely exploited, technique to carefully investigate the wave propagation in exploration wells is the logging of sonic waveforms. Fundamental decisions and production planning in petroleum reservoirs are made by interpretation of such recordings. Nowadays, geophysicists and engineers face problems related to the acquisition and interpretation under complex conditions associated with conducting open-hole measurements. A crucial problem that directly affects the response of sonic logs is the eccentricity of the measuring tool with respect to the center of the borehole. Even with the employment of centralizers, this simple variation, dramatically changes the physical conditions on the wave propagation around the well. Recent works in the numerical field reported advanced studies in modeling and simulation of acoustic wave propagation around wells, including complex heterogeneities and anisotropy. However, no analytical efforts have been made to formally understand the wireline sonic logging measurements acquired with borehole-eccentered tools. In this paper, the Graf’s addition theorem was used to describe monopole sources in terms of solutions of the wave equation. The formulation was developed from the three-dimensional discrete wave-number method in the frequency domain. The cylindrical Bessel functions of the third kind and order zero were re-derived to obtain a simplified set of equations projected into a bi-dimensional plane-space for displacements and stresses. This new and condensed analytic formulation allows the straightforward calculation of all converted modes and their visualization in the time domain via Fourier synthesis. The main aim was to obtain spectral surfaces of transfer functions and synthetic seismograms that might be useful to understand the wave motion produced by the eccentricity of the source and explain in detail the new arising borehole propagation modes. Finally, time histories and amplitude spectra for relevant examples are presented and the validation of time traces using the spectral element method is reported.

Frequency-domain simulation of logging-while-drilling borehole sonic waveforms

Numerical simulation of sonic logging-while-drilling (LWD) borehole measurements is challenging because of significant wave propagation effects due to the massive drilling collar occupying a large portion of the borehole. In addition, the internal structure of the LWD tool can have a significant impact on the measured dispersions of Stoneley and quadrupole modes. The collar is typically constructed with a set of inner periodic grooves, which act as a mechanical filter to attenuate undesirable collar modes. Reliable numerical simulation and interpretation of LWD sonic waveforms requires that all features and dimensions of the drilling collar be included in the simulation model. Furthermore, the presence of the drilling collar can prompt numerical instabilities due to backward propagating modes in the perfectly matched layer (PML) commonly used to truncate the computational domain. This problem can be circumvented with the implementation of artificial viscoelastic attenuation in the collar whenever the simulations are intended to reproduce only wave propagation within the surrounding rock formations. In addition, reliable modeling of borehole wave propagation in the presence of high contrasts in material properties and the internal structure of the LWD collar requires a numerical method capable of accurately and stably resolving all spectral scales present in the model. We implemented an automatic [Formula: see text]-adaptive finite-element method in the frequency domain combined with a PML technique to simulate LWD sonic logging measurements. Examples of the application verified the accuracy and reliability of the simulated borehole and formation propagation modes in the presence of casing and internal structures in the LWD collar. The presence of steel casing and quality of casing/formation bond significantly influence the propagation modes excited in a borehole. However, it is still possible to estimate the formation shear slowness using monopole and quadrupole sources regardless of the quality of cement bond in fast formations. Assessment of the formation compressional slowness was significantly impeded by the strong pipe mode. Estimation of formation shear slowness in slow formations is practically impossible due to the presence of casing and a strong annulus mode when the quality of casing bond is poor.

Quantifying uncertainties in attenuation estimation at methane-hydrate-bearing zones using sonic waveform logs

Application of seismic attenuation estimation using sonic waveform data is limited because the estimation methods have not yet been fully developed. Although the median frequency shift method is considered to be effective and robust compared to the conventional spectral ratio method, we demonstrated that the median frequency shift methods strongly depend on reference data under lower signal-to-noise ratios. We modified an existing median frequency shift method not to depend on arbitrarily choosing a reference value and to quantify the uncertainties in attenuation estimation. Furthermore, we implied the optimum selection of receiver pairs used for more stable attenuation analysis. Our numerical experiments supported the advantage of the proposed method. Although our main findings by applying the proposed methods in methane hydrate-bearing sediments are almost consistent with past field sonic logging measurements, we find some differences in the magnitude of attenuation values compared to existing sonic attenuation measurements and discuss various possible factors. We believe that more stable and reliable attenuation results can lead to clarifying various factors affecting attenuation estimation, such as the effect of scattering, near-field effects, and source-coupling effects. Furthermore, we emphasized the importance of scattering effect caused by the heterogeneity of the formation and demonstrated the limitation of characterizing the 1D heterogeneity using the sonic logging data spatially sampled at 0.15 m to adequately estimate the effect of scattering attenuation.

Frequency-domain finite-element simulations of 2D sonic wireline borehole measurements acquired in fractured and thinly bedded formations

We simulated wireline borehole sonic waveforms to appraise modal frequency dispersions across fractured and thinly bedded formations. Simulations included monopole and dipole sources of excitation and explicitly took into account the borehole, the mandrel tool, and casing whenever present. Calculations were performed in the frequency domain with a highly accurate finite-element method that automatically generates optimal grids for each problem/frequency combination. The method guarantees solutions at significantly reduced computational time with relative energy errors below 0.5% (even in the presence of singularities in the solution that can originate from complex geometries, high material contrasts, and simultaneous presence of large and fine structures). Such a high accuracy in the simulation of sonic waveforms is necessary to accurately quantify the effects of fractures and thin beds on acoustic logs. Simulations indicate that fractures mainly influence propagation modes related to the formation shear velocity. In slow thinly bedded formations, effective properties are similar to those of average layer properties, as predicted by the Reuss lower bound. However, presence of intralayer interfaces gives rise to multiple reflections that can deleteriously affect the estimation of elastic properties with dispersion processing. Casing effectively functions as a low-pass frequency filter of sonic waveforms, significantly distorting the original borehole modes, hence the estimation of elastic properties.

Simulation of borehole sonic waveforms in dipping, anisotropic, and invaded formations

A numerical simulation study has been made of borehole sonic measurements that examined shoulder-bed, anisotropy, and mud-filtrate invasion effects on frequency-dispersion curves of flexural and Stoneley waves. Numerical simulations were considered for a range of models for fast and slow formations. Computations are performed with a Cartesian 3D finite-difference time-domain code. Simulations show that presence of transverse isotropy (TI) alters the dispersion of flexural and Stoneley waves. In slow formations, the flexural wave becomes less dispersive when the shear modulus (c44) governing wave propagation parallel to the TI symmetry axis is lower than the shear modulus (c66) governing wave propagation normal to the TI symmetry axis; conversely, the flexural wave becomes more dispersive when c44 > c66. Dispersion decreases by as much as 30% at higher frequencies for the considered case where c44 < c66. Dispersion of Stoneley waves, on the other hand, increases in TI formations when c44 > c66 and decreases when c44 < c66. Dispersion increases by more than a factor of 2.5 at higher frequencies for the considered case where c44 < c66. Simulations also indicate that the impact of invasion on flexural and Stoneley dispersions can be altered by the presence of TI. For the case of a slow formation and TI, where c44 decreases from the isotropic value, separation between dispersion curves for cases with and without the presence of a fast invasion zone increases by as much as 33% for the flexural wave and by as much as a factor of 1.4 for the Stoneley wave. Lastly, presence of a shoulder bed intersecting the sonic tool at high dip angles can alter flexural dispersion significantly at low frequencies. For the considered case of a shoulder bed dipping at 80°, ambiguity in the flexural cutoff frequency might lead to shear-wave velocity errors of 8%–10%.

Wavefield modeling in exploration seismology using the discontinuous Galerkin finite-element method on HPC infrastructure

Seismic and acoustic measurements in a broad sense, including surface seismic, borehole seismic and sonic waveforms, play an important role in improving our knowledge of hydrocarbon reservoirs in both exploration and production phases. In that context, accurate modeling of wave propagation, in particular when considering irregular free-surface effects, complicated subsurface geological structures, or the physics of a sonic tool deployed downhole, is one of the major challenges geophysicists are facing.

Estimation of Dry-Rock Elastic Moduli Based on the Simulation of Mud-Filtrate Invasion Effects on Borehole Acoustic Logs

Summary Reliable estimates of dry-rock elastic properties are critical to the accurate interpretation of the seismic response of hydrocarbon reservoirs. We describe a new method for estimating elastic moduli of rocks in-situ based on the simulation of mud-filtrate invasion effects on resistivity and acoustic logs. Simulations of mud-filtrate invasion account for the dynamic process of fluid displacement and mixing between mud-filtrate and hydrocarbons. The calculated spatial distributions of electrical resistivity are matched against resistivity logs by adjusting the underlying petrophysical properties. We then perform Biot-Gassmann fluid substitution on the 2D spatial distributions of fluid saturation with initial estimates of dry-bulk (kdry) modulus and shear rigidity (µdry) and a constraint of Poisson's ratio (?d) typical of the formation. This process generates 2D spatial distributions of compressional and shear-wave velocities and density. Subsequently, sonic waveforms are simulated to calculate shear-wave slowness. Initial estimates of the dry-bulk modulus are progressively adjusted using a modified Gregory-Pickett (1963) solution of Biot's (1956) equation to estimate a shear rigidity that converges to the well-log value of shear-wave slowness. The constraint on dynamic Poisson's ratio is then removed and a refined estimate of the dry-bulk modulus is obtained by both simulating the acoustic log (monopole) and matching the log-derived compressional-wave slowness. This technique leads to reliable estimates of dry-bulk moduli and shear rigidity that compare well to laboratory core measurements. Resulting dry-rock elastic properties can be used to calculate seismic compressional-wave and shear-wave velocities devoid of mud-filtrate invasion effects for further seismic-driven reservoir-characterization studies.

Processing of array sonic logging data with multi-scale STC technique

It is desirable to develop new signal processing techniques for effectively extracting reflected waves under the strong interferences of borehole guided waves. We presented a multi-scale semblance method for the separation and velocity (slowness) analysis of the reflected waves and guided waves in borehole acoustic logging. It was specially designed for the newly developed tools with ultra-long source-receiver spacing for acoustic reflection survey. This new method was a combination of the dual tree complex wavelets transform (DT-CWT) and the slowness travel time coherence (STC) method. Applications to the 3D finite difference (FD) modeling simulated data and to the field array sonic waveform signals have demonstrated the ability of this method to appropriately extract the reflected waves under severe interference from the guided waves and to suppress noise in the time-frequency domain.

Sonic logging in deviated boreholes penetrating an anisotropic formation: Laboratory study

Development of deepwater fields requires drilling deviated or horizontal wells. Many formations are highly anisotropic, that is, the P- and S-wave velocities vary with propagation direction. Sonic logs acquired in these wells need to be corrected for anisotropy effects before the logs can be used in formation evaluation and seismic applications. In this study, we use a laboratory model made of an orthorhombic Phenolite block to study acoustic logging in deviated wells. We first measure the qP-, qSV-, and SH-wave group velocities by using body waves at angles of 0°, 15°, 30°, 45°, 60°, 75°, and 90° relative to the slowest P-wave principal axis of the Phenolite block. We then drill holes at the same angles in the block. We record monopole and dipole sonic waveforms in these holes and extract the qP-, qSV-, SH-, and Stoneley-wave velocities by using the slowness-time semblance method. The velocities measured through the use of monopole logging and dipole logging vary with borehole deviations. We find that an equivalent transversely isotropic (TI) model can fit the measured qP-, qSV-, and Stoneley-wave velocities very well. The S-wave velocities at low to medium borehole deviations can be used to differentiate an orthorhombic material from a TI one.

Amplitude loss of sonic waveform due to source coupling to the medium

In contrast to hydrate‐free sediments, sonic waveforms acquired in gas hydrate‐bearing sediments indicate strong amplitude attenuation associated with a sonic velocity increase. The amplitude attenuation increase has been used to quantify pore‐space hydrate content by attributing observed attenuation to the hydrate‐bearing sediment's intrinsic attenuation. A second attenuation mechanism must be considered, however. Theoretically, energy radiation from sources inside fluid‐filled boreholes strongly depends on the elastic parameters of materials surrounding the borehole. It is therefore plausible to interpret amplitude loss in terms of source coupling to the surrounding medium as well as to intrinsic attenuation. Analyses of sonic waveforms from the Mallik 5L‐38 well, Northwest Territories, Canada, indicate a significant component of sonic waveform amplitude loss is due to source coupling. Accordingly, all sonic waveform amplitude analyses should include the effect of source coupling to accurately characterize a formation's intrinsic attenuation.

Modeling atmospheric turbulence as a filter for sonic boom propagation

Atmospheric turbulence is known to alter the pressure waveform created by supersonic flight. It is challenging to predict how a particular sonic boom waveform will be affected by a particular atmospheric realization without performing intense atmospheric modeling calculations. This paper attempts to approximate the effects of turbulence as a linear FIR filter. Such a filter is estimated from real sonic boom data measured on the ground and at altitude.

Realism assessment of sonic boom simulators

The perceived realism of booms reproduced in three simulators, at NASA, Lockheed Martin, and Gulfstream Aerospace, was studied. The NASA and Lockheed facilities are loudspeaker‐driven, airtight concrete structures that enable reproduction of the entire spectrum, including the very lowest frequencies. The Gulfstream facility is enclosed within a trailer and consists of a folded horn as well as loudspeakers. This creates the sonic boom waveform as a traveling wave but is unable to reproduce the lowest frequencies. In the tests, subjects experienced in listening critically to real sonic booms heard reproductions in the simulators of sonic booms recorded outdoors. Tests comparing the NASA/Gulfstream simulators and the Lockheed/Gulfstream simulators used the same outdoor sonic boom waveforms, though each simulator uses its own preprocessing to equalize the waveforms for presentation. The tests showed that the presence of ground reflections, very low frequencies, or recording hiss were not factors in realism ra...