Signal-generated Noise Research Articles

Data-oriented constraint on the interpretation of S receiver function and its application to observations of seismic discontinuities in the lithosphere–asthenosphere system

SUMMARYThis paper aims to improve the robustness of interpretation in the S receiver function (SRF), a technique commonly used to retrieve forward scattering of S-to-P converted waves (Sdp) originated from the lithosphere–asthenosphere system (LAS) beneath the stations. Although the SRF does not suffer interferences from backward scattering waves such as the first multiples from the Moho, one major drawback in the method is that Sdp phases can interfere with P coda waves and it is conceivable that these signal-generated noise may be misinterpreted as Sdp phase from the LAS beneath seismic stations. Through systematic analysis of full-waveform synthetics and SRFs from catalogued source parameters, we find that the strong P coda waves before the S wave in the longitudinal-component waveforms result in unwanted signal-generated noise before the S wave in the synthetic SRFs. If the mean amplitude of SRFs after the S wave is large, dubious signal-generated noise before the S arrival are strong as well. In this study, we honor the level of these unwanted signal-generated noise and devise data-oriented screening criteria to minimize the interference between P coda waves and genuine S-to-P converted waves. The first criterion is LQR, a direct measure of the amplitude ratio between longitudinal P coda waves and radial S wave in the waveform data. The second criterion is AMP, the amplitude of SRFs after the S arrival. We illustrate that these criteria effectively measure the energy level of mantle waves such as the SP wave. With synthetics and real data, we demonstrate the effectiveness of LQR and AMP criteria in minimizing these unwanted signal-generated noise in the stacked SRFs down to 1–2 per cent, improving detection threshold and interpretation of Sdp phases from seismic discontinuities in the LAS.

On the robustness of attenuation measurements on teleseismic P waves: insights from micro-array analysis of the 2017 North Korean nuclear test

SUMMARY Despite their importance as a fundamental constraint on Earth properties, regional-scale measurements of body-wave seismic attenuation are scarce. This is partially a result of the difficulty in producing robust estimates of attenuation. In this paper, we focus on measuring differential attenuation on records of teleseismic P waves. We examine a unique data set of five records of the North Korean nuclear test of 2017 measured at five broad-band seismic stations deployed within a few metres of each other but using different installation procedures. Given their extreme proximity, we expect zero differential intrinsic attenuation between the different records. However, we find that different attenuation measurement methods and implementation parameters in fact produce significant apparent differential attenuation (Δt*). Frequency-domain methods yield a wide range of Δt* estimates between stations, depending on measurement bandwidth and nuances of signal processing. This measurement instability increases for longer time windows. Time domain methods are largely insensitive to the frequency band being considered but are sensitive to the time window that is chosen. We determine that signal-generated noise can affect measurements in both the frequency and time domain. In some cases, the range of results amounts to a significant fraction of the range of differential attenuation across the conterminous United States as determined by a recent study. We suggest some approaches to manage the inherent instability in these measurements and recommend best practices to confidently estimate body wave attenuation.

Receiver function deconvolution using transdimensional hierarchical Bayesian inference

S U M M A R Y Teleseismic waves can convert from shear to compressional (Sp) or compressional to shear (Ps) across impedance contrasts in the subsurface. Deconvolving the parent waveforms (P for Ps or S for Sp) from the daughter waveforms (S for Ps or P for Sp) generates receiver functions which can be used to analyse velocity structure beneath the receiver. Though a variety of deconvolution techniques have been developed, they are all adversely affected by background and signal-generated noise. In order to take into account the unknown noise characteristics, we propose a method based on transdimensional hierarchical Bayesian inference in which both the noise magnitude and noise spectral character are parameters in calculating the likelihood probability distribution. We use a reversible-jump implementation of a Markov chain Monte Carlo algorithm to find an ensemble of receiver functions whose relative fits to the data have been calculated while simultaneously inferring the values of the noise parameters. Our noise parametrization is determined from pre-event noise so that it approximates observed noise characteristics. We test the algorithm on synthetic waveforms contaminated with noise generated from a covariance matrix obtained from observed noise. We show that the method retrieves easily interpretable receiver functions even in the presence of high noise levels. We also show that we can obtain useful estimates of noise amplitude and frequency content. Analysis of the ensemble solutions produced by our method can be used to quantify the uncertainties associated with individual receiver functions as well as with individual features within them, providing an objective way for deciding which features warrant geological interpretation. This method should make possible more robust inferences on subsurface structure using receiver function analysis, especially in areas of poor data coverage or under noisy station conditions.

Removing source-side scattering for virtual deep seismic sounding (VDSS)

We present a method that extends both the applicability and the quality of virtual deep seismic sounding (VDSS)—a technique for estimating crustal thickness that is robust even if the crust–mantle transition is complex or the crustal thickness is large. The results are important for studies of crustal contributions to isostasy and for understanding dynamic topography due to mantle convection. VDSS uses S-to-P conversions beneath seismic stations as virtual sources for large, post-critical reflections off the Moho, that is, the seismic phase SsPmp. Original applications of VDSS rely on deep earthquakes as sources of illumination to circumvent strong, near-source scattering (e.g. depth phases) and are, therefore, limited by the uneven distribution of deep seismicity. The method presented here effectively removes effects of the earthquake source wavelet (SW, including complexities arising from long, complicated source time functions and near-source scattering) and can be applied to signal from shallow and deep earthquakes. It involves two steps. First, based on analyses of particle motion, we separate ‘pseudo-P’ and ‘pseudo-S’ wave trains from the vertical and the radial component of ground motion. The latter is then used as the appropriate reference time-series for the deconvolution of the vertical and the radial component of ground motion. Since the reference time-series contains both the SW and S-type signals due to scattering near the receiver, the deconvolution also effectively removes S-type multiples, such as the phase SsPms and related reverberations. Applying this method to synthetic seismograms verifies that it is robust in removing complex SWs, even in the presence of random or signal-generated noise. The method is further validated using data recorded by the Hi-CLIMB array from both deep and shallow earthquakes. Impulsive signals are now routinely achieved, significantly improving both the quality and quantity of results from VDSS.

Extrapolation of Time-Dependent Waveforms along their Path of Propagation

Summary Time-dependent waveforms are commonly extrapolated in space by means of rays and occasionally by means of diffraction integrals. It is possible to extrapolate time-dependent waves in space with a partial differential equation derived from the wave equation. There are stable numerical approximations. An example illustrates a mechanism for ' signal-generated noise ' which is consistent with observations. When a wave propagates in an inhomogeneous medium the waveform changes. Given that the wave has been observed at a suitable number of points in space we may attempt to solve two types of problems. First we may attempt to ascertain the nature of material inhomogeneity along the wave paths, and second, we may attempt to extrapolate the disturbance back to the source in an attempt to discover the nature of the source. With few exceptions, the methods used during the past decade for doing this kind of geophysical work may be summarized as follows: when wave equations are to be used, separability is achieved by considering cases in which the material inhomogeneity is a function of only one spatial co-ordinate. When higherdimensional inhomogeneity is so severe that it cannot be ignored, then the wave equations are almost always specialized to ray theory. Ray theory is especially useful when only the travel time is required. Although the amplitude may also be obtained by ray theory it is often of marginal utility because amplitude measurement is made ambiguous by changing waveforms. What we develop in this paper is a finite difference approach to the wave equation which tracks the time dependent waveform of a travelling wave in two-dimensionally inhomogeneous material. This is an extension of earlier work done by one of the authors in the frequency domain. Although the Fourier transform relates time-domain solutions to frequency-domain solutions, there are several compelling practical factors which give impetus to this study. When a waveform is small at certain times of interest and large at times which are not of interest, then a satisfactory approximation to the Fourier integral may be difficult to obtain even if values are obtained with good accuracy at many frequencies: an example is the head wave. Another example occurs in reflection seismology where the most interesting part of the waveform is the late-arriving weak echoes. Another example is when the time function is of long duration but only a small portion of it is of interest; this is usually the case with short-period earthquake seismograms where there is never any hope of interpreting more than the wave packets which come from identifiable phases.

Improved imaging with phase-weighted common conversion point stacks of receiver functions

SUMMARY Broad-band array studies frequently stack receiver functions to improve their signal-to-noise ratio while mapping structures in the crust and upper mantle. Noise may produce spurious secondary arrivals that obscure or mimic arrivals produced by P-to-S conversions at large contrasts in seismic impedance such as the Moho. We use a Hilbert transform to calculate phase-weights, which minimize the constructive stacking of erroneous signal in receiver function data sets. We outline this approach and demonstrate its application through synthetic data combined with different types of noise, a previously published example of signal-generated noise, and a large data set from the Sierra Nevada EarthScope Project. These examples show that phase-weighting reduces the presence of signal-generated noise in receiver functions and improves stacked data sets.

On the inner-outer core density contrast from PKiKP/PcP amplitude ratios and uncertainties caused by seismic noise

The inner core boundary (ICB) of the earth is characterized by a discontinuous change in elastic properties between the liquid outer and solid inner core. In the ray theory approximation, a measure of the density contrast at the ICB is given by the amplitude ratio of P waves reflected from the core—mantle boundary (CMB; PcP waves) and the ICB (PKiKP waves), since that ratio conveniently appears in an explicit form in the transmission/reflection coefficient equations. The results for inner—outer core density contrast derived from direct amplitude picks of these waves in the time domain have varied significantly among different authors. The transmission/reflection coefficients on the liquid—solid and solid—liquid boundaries derived from ground displacements enable a direct comparison between the amplitude measurements on displacement seismograms in the time domain and theoretical values. A new approach is proposed and applied to integrate effects of microseismic and signal-generated noise with the amplitude measurements, thus providing a direct maximal uncertainty measure. To suppress the effects of varying radiation pattern and distinctively different ray paths at longer epicentral distances, this new method was applied to high-quality arrivals of PcP and PKiKP waves from a nuclear explosion observed at epicentral distances 10°–20° from recording stations. The resulting uncertainties are high precluding precise estimates of the ICB density contrast, but provide a robust estimate of an upper bound from body waves of about 1100 kg m−3. Median values of two amplitude ratios observed around 17° epicentral distance indicate a small density contrast of 200–300 kg m−3 and suggest the existence of zones of suppressed density contrast between the inner and the outer core, a density contrast stronger than 5000 kg m−3 at the CMB, or a combination of both.

Crustal Scattering and Some Artifacts in Receiver Function Images

The existing 2D and 3D depth imaging approaches using multichannel teleseismic receiver functions (RFs) can result in significant artifacts caused by misinterpretation of signal-generated noise resulting from broadside scattering within the crust. A synthetic example using acquisition geometry of the 1993 Cascadia experiment shows that scattering of the primary P -wave arrivals from a sedimentary accretionary wedge could produce RF events close to those observed in real data. Quasi-linear crustal structures with significant velocity contrasts near Moho depths, such as subduction zones, should create particularly strong and coherent noise arrivals. Conventional interpretations of these arrivals based on assumptions of their origin in mode conversions could lead to images of spurious dipping structures of landward dips steeper than ∼10° that may be very difficult to distinguish from the true structures. Detailed analysis of prestack, premigration RF data is required for correct identification of the signal-generated noise and validation of the resulting depth images. Identification of scattered events in RF records could also help constrain major crustal structures, such as crustal sutures and subduction fault zones, and particularly those with strong Moho expressions. Manuscript received 10 June 2003.

Receiver Functions from Multiple-Taper Spectral Correlation Estimates

Teleseismic P waves are followed by a series of scattered waves, particularly P -to- S converted phases, that form a coda. The sequence of scattered waves on the horizontal components can be represented by the receiver function (RF) for the station and may vary with the approach angle and azimuth of the incoming P wave. We have developed a frequency-domain RF inversion algorithm using multiple-taper correlation (MTC) estimates, instead of spectral division, using the pre-event noise spectrum for frequency-dependent damping. The multitaper spectrum estimates are leakage resistant, so low-amplitude portions of the P -wave spectrum can contribute usefully to the RF estimate. The coherence between vertical and horizontal components can be used to obtain a frequency-dependent uncertainty for the RF. We compare the MTC method with two popular methods for RF estimation, time-domain deconvolution (TDD), and spectral division (SPD), both with damping to avoid numerical instabilities. Deconvolution operators are often biased toward the frequencies where signal is strongest. Spectral-division schemes with constant water-level damping can suffer from the same problem in the presence of strong signal-generated noise. Estimates of uncertainty are scarce for TDD and SPD, which impedes developing a weighted average of RF estimates from multiple events. Multiple-taper correlation RFs are more resistant to signal-generated noise in test cases, though a “coherent” scattering effect, like a strong near-surface organ-pipe resonance in soft sediments, will overprint the Ps conversions from deeper interfaces. The MTC RF analysis confirms the broad features of an earlier RF study for the Urals foredeep by Levin and Park (1997a) using station ARU of the Global Seismographic Network (GSN), but adds considerable detail, resolving P -to- S converted energy up to f = 4.0 Hz.

Field Comparison of High-frequency Seismic Sources for Imaging Shallow (10–250m) Structures

Several high-frequency seismic sources (i.e., sledgehammer, pipegun, weightdrop, and small- and intermediate-size vibrators) have been tested at a site characterized by a surface layer of very recent sediments overlying unconsolidated glacial and glaciolacustrine units. Reflections from very shallow [Formula: see text] to moderate [Formula: see text] depths were recorded. Evaluation of the different sources involved analyzing prominent features (e.g., reflections and signal-generated noise that includes complex patterns of interfering direct, guided, refracted, and surface waves) on source gathers, frequency spectra and stacked sections. Our experiments demonstrated that minor changes in local conditions (e.g., changes in ground-water table depth) may influence significantly the characteristics of the resulting data. Of the tested sources, the simple sledgehammer provided the highest resolution images at shallow depths [Formula: see text] and comparably good images at greater depths [Formula: see text]. Local changes in surficial sediments affected the quality of data generated by the small ([Formula: see text] reaction mass) vibrator; at many locations, the input energy was insufficient to illuminate the important geological structures. By comparison, the intermediate-size ([Formula: see text] reaction mass) vibrator provided high quality images to depths as great as [Formula: see text] (corresponding to the deepest reflection from near the base of unconsolidated sediments), but relatively strong airwaves and imprecise vibroseis correlation precluded the resolution of reflections from depths shallower than [Formula: see text]. Despite recent advances in vibratory systems, the humble, but cost-effective sledgehammer continues to be an attractive source for high-resolution seismic experiments.

Near-surface scattering effects observed with a high-frequency phased array at Pinyon Flats, California

Abstract Analysis of data collected by a high-frequency array experiment conducted at Pinyon Flat in southern California provides strong evidence that the high-frequency wave field from local earthquakes at this hard-rock site are strongly distorted by near-surface scattering. The seismic array we deployed consisted of 60, 2-Hz natural frequency, three-component sensors deployed in a three-dimensional array. Two of the sensors were located in boreholes at 150 and 275 m depth. The other 58 sensors were deployed in an areal array above these boreholes. Thirty-six of these were deployed in a 6-by-6 element grid array with a nominal spacing of 7 m centered over the borehole sensors. The remaining 22 seismometers were laid out in two 11-element linear arrays radiating outward from the grid. Coherence calculations reveal a rapid loss of coherence at frequencies over 15 Hz at all but the shortest length scales of this array. Three-dimensional visualization techniques were used to closely examine the spatial stability of particle motions of P and S waves. This reveals systematic variations of particle motion across the array in which the particle motion tracks tilt drastically away from the backazimuth expected for an isotropic medium. These variations, however, are frequency dependent. Below around 8 Hz, the particle motions become virtually identical for all stations. At progressively higher frequencies, the wave-field particle motion becomes increasingly chaotic. Frequency-wave-number analysis of these data provide quantitative measures of the same phenomena. We find that direct wave f-k spectra are bathed in a background of signal-generated noise that varies from 10 to 30 dB down from the direct arrival signal. This signal-generated noise appears to be nearly white in wavenumber indicating the wavelength of this “noise” on the scale of tens of meters and less. Refraction measurements we made on two lines crisscrossing the array reveal that the weathered layer velocities are highly variable and define a very strong wave guide. Measured surface P-wave velocities varied from 400 to 1300 m/sec, and velocities at depth of approximately 15 m varied from 1600 to 2700 m/sec. Previous measurements in the boreholes showed that the intact granite below about 65 m depth has a velocity of approximately 5400 m/sec. These results demonstrate the extreme velocity contrast and degree of velocity heterogeneity of the near surface at this site. We conclude that all the observations we made can be explained by strong scattering of incident body-wave signals into a complex mishmash of body-wave and surface-wave modes in this heterogeneous near-surface wave guide.

Variance estimate in frequency-domain deconvolution for teleseismic receiver function computation

SUMMARY In this study, the problem of estimating the receiver function variance is discussed. The receiver function computation is performed through a frequency-domain deconvolution (Oldenburg 1981) of the vertical seismogram from the radial and tangential seismograms. This technique requires a knowledge of the noise contaminating the seismic signals, and provides an estimate of the receiver function variance. Oldenburg (1981) modelled this noise as additive; Amnion (1992) proposed to estimate the noise level from the power-spectral density in a pre-signal time window. In this paper, this approach is used jointly with an additional inversion, which yields a sequence of spikes as a model for the true receiver function. The resulting residual in a time window preceding the direct P pulse is used in order to evaluate the receiver function variance and the actual noise level involved in the frequency-domain deconvolution. This variance estimate is compared with the one provided by Oldenburg's original method. The application of this method to teleseismic recordings of earthquakes (5.1 ≤mb≤ 6.1) with similar backazimuths and distances shows that considering the additive noise only is not sufficient to explain the uncertainty measured from the pre-signal in the receiver function. Moreover, the estimated uncertainty seems to be independent of the seismic event magnitude, suggesting a signal-generated noise affecting the receiver function. The proposed approach provides a variance estimate for a single receiver function, allowing one to assess the statistical accuracy of its amplitudes in agreement with the uncertainty involved in a stacking procedure. This is particularly useful when poor-quality waveforms from relatively small earthquakes (mb∼ 5.1) are analysed and data from nearly co-located events are not available for stacking.

Variance estimate in frequency-domain deconvolution for teleseismic receiver function computation

In this study, the problem of estimating the receiver function variance is discussed. The receiver function computation is performed through a frequency-domain deconvolution (Oldenburg 1981) of the vertical seismogram from the radial and tangential seismograms. This technique requires a knowledge of the noise contaminating the seismic signals, and provides an estimate of the receiver function variance. Oldenburg (1981) modelled this noise as additive; Amnion (1992) proposed to estimate the noise level from the power-spectral density in a pre-signal time window. In this paper, this approach is used jointly with an additional inversion, which yields a sequence of spikes as a model for the true receiver function. The resulting residual in a time window preceding the direct P pulse is used in order to evaluate the receiver function variance and the actual noise level involved in the frequency-domain deconvolution. This variance estimate is compared with the one provided by Oldenburg's original method. The application of this method to teleseismic recordings of earthquakes (5.1 ≤mb≤ 6.1) with similar backazimuths and distances shows that considering the additive noise only is not sufficient to explain the uncertainty measured from the pre-signal in the receiver function. Moreover, the estimated uncertainty seems to be independent of the seismic event magnitude, suggesting a signal-generated noise affecting the receiver function. The proposed approach provides a variance estimate for a single receiver function, allowing one to assess the statistical accuracy of its amplitudes in agreement with the uncertainty involved in a stacking procedure. This is particularly useful when poor-quality waveforms from relatively small earthquakes (mb∼ 5.1) are analysed and data from nearly co-located events are not available for stacking.

Synthetic seismograms through synthetic Franciscan: Insights into factors affecting large‐aperture seismic data

In spite of an order of magnitude increase over the past 15 years in spatial sampling of the wavefield, a major uncertainty in the analysis of active source seismic data remains phase identification. This uncertainty results in part from the wide range of spatial scales of velocity heterogeneity in the crust. Smaller scale variations than those which can be deterministically resolved given the design of a particular seismic experiment can be modeled statistically using geologic constraints. Here we present synthetic seismograms generated from several different realizations of a stochastic model describing the velocity heterogeneity of Franciscan terrane rocks. We compare the results to observed data and to synthetic seismograms generated for a model derived from tomographic inversion of the data in order to obtain qualitative insights into the relative importance of large and small scale velocity heterogeneity. Not surprisingly, the synthetic data for the tomographic model best reproduce observed small‐scale variations in first arrival time, which only occur for particular realizations of the stochastic model. The synthetic seismograms generated for the stochastic models best reproduce the level of signal‐generated noise and suggest that the amplitude of velocity variation locally within the Franciscan is approximately 1 km/s. They also illustrate the effect of a strongly heterogeneous upper and mid‐crust on the amplitude‐versus‐offset pattern of arrivals from the lower crust and upper mantle. These effects may sometimes be interpreted deterministically, leading to biased models or an overly optimistic estimate of lower crustal resolution.

On the effect of diffraction on traveltime measurements

SUMMARY All observed waves are of finite frequency and are sensitive to a finite volume of the medium through which they pass. Diffraction causes a loss of information about time contained in the initial front of a wavefield (often referred to as wavefront healing). This effect depends upon frequency and propagation distance and imposes a low-pass filter on the spatial resolution of time measurements. A sequence of canonical, numerical experiments that simulate the diffraction of a perturbed plane wave at a fixed distance is described. Traveltimes are measured using a variety of techniques on a range of waveforms. It is empirically verified that a single Fresnel zone describes the spatial filtering effect of the propagation of a broad-band wavefield, even in the regime where the initial time perturbation cannot be represented by a linear perturbation term. For narrow-band wavefields, more Fresnel zones come into play as the bandwidth is reduced. Measurements of time include a component of signal-generated noise coherent over a small scale which scales with the Fresnel zone. It is found that, for traveltimes measured by automated picking, the width of the Fresnel zone is described by a time delay of (&I< T/4 (here T is one period). On the other hand, the width of the Fresnel zone for traveltimes measured by correlation is wider, characterized by a time delay of ldtl < T/4.

Ocean Downhole Seismic Project.

In the fall of 1989, a broadband digital seismic sensor system (OBDS: Ocean Broadband Downhole Seismometer) was successfully emplaced in an ODP drill hole in the Japan Sea. The site is located in the northern part of the Yamato Basin about 160km west from the northern Japan west coast. The instrument was clamped within a basalt section beneath 543m of sediments at 715m below seafloor and 3, 523m below sea surface. We obtained a continuous real-time seismic recording of 60h length and an off-line and intermittent sea-floor recording of 30 days operation. The dataset includes airgun signals for studying the local crustal structure, several local earthquakes, and a teleseismic event. An array of OBS's was also deployed during the airgun shooting to study detailed local crustal structure including anisotropy. The results of our experiment may be summarized as follows. We obtained for the first time a digital long-period seismic record from beneath the seafloor. The structural environment of OBDS could be constrained from results of drilling and airgun profiling; the crust is nearly identical to that of the southern Yamato Basin, which is about 14km thick, and the upper crust possesses anisotropy in agreement with the present E-W compressive state of stress. Noise is generally lower than island or seafloor stations, and is as low as about 104 (nm/s)2/Hz at about 0.2Hz, where normally a noise peak is found. Signals from local events and airguns exhibit less reverberations as compared with OBS records; i.e. they are less affected by signal-generated noise, thus allowing better phase identifications. Surface wave dispersion from an event (Mb=5.4) at 39° distance was clearly observed. Thus, our results indicate that downhole emplacement of a seismometer beneath an ocean provides higher quality data than alternative seafloor or island observations. At frequencies lower than 0.1Hz, however, our OBDS did not have sufficient sensitivity to detect the background noise level.

Earthquake site response estimation: A weak-motion case study

AbstractUsing weak-motion recordings of aftershocks of the 1989 Loma Prieta earthquake recorded in Oakland, California, near the failed Nimitz Freeway, two methods have been applied to estimate the site response of an alluvium site and three mud-over-alluvium sites. The first estimate is the traditional spectral ratio, and the second utilizes the cross spectrum. Recordings obtained at a nearby bedrock site are used as estimates of the sediment site input motions. While the two site response estimates produce similar peaks and troughs, there is an approximate factor of 2 difference in amplitudes. This discrepancy is evidence that there is a much greater level of noise than would be expected from the pre-event ambient noise. We interpret this as signal-generated noise produced by scattering from heterogeneities, which causes the true sediment site input to differ significantly from the bedrock site recording. Given this level of noise, the cross-spectrum estimate suffers a severe downward bias (by a factor of 2 in this study) and should probably not be used when the input motion is estimated from a bedrock site recording. The spectral-ratio estimates are relatively unbiased, but the level of noise introduces a large degree of uncertainty. Therefore, inferences about site response from individual spectral ratios should probably be avoided. On the other hand, ensemble averages of the estimates significantly reduce the scatter to reveal resonances that agree quite well in frequency and overall shape with those of one-dimensional models whose parameters were determined independently. A discrepancy of higher observed amplitudes than predicted by theory remains unexplained but most likely results from the effects of boundary layer topography, which are not accounted for by the simple one-dimensional models.

Sediment shear Q from airgun OBS data

SUMMARY Direct measurement of the sediment shear-wave quality factor, Q,j, has been hindered by the lack of an effective shear-wave source. We show that if a satisfactory horizontal component ocean bottom seismometer (OBS) is available, then sediment Q, can be determined directly by using spectral ratios of converted shear-wave reflections. Spectral ratios are formed with the PS reflection from the sediment/basement interface and the PSSS multibounce sediment shear-wave reflection. As a check, we also computed Qfi from the peak amplitudes of PS and PSSS. We applied the spectral ratio method to airgun OBS data collected over 356 m of primarily high-porosity biosiliceous clay in 5467 m of water in the northwest Pacific at 4355.44'N, 159'47.84'E (DSDP Site 581). An average sediment shear-wave velocity of about 0.2 km s-' was obtained from the PS traveltime. Effective Q,< for the sediment column was found to be 97 f 11 (a = 0.281 f 0.032 dB A-') in the frequency band 3-18 Hz. We tested the methods by applying them to reflectivity synthetic seismograms computed for various velocity profiles with both frequency-dependent Q and frequency-independent Q. The Q, estimate obtained from synthetic seismograms was within 15 per cent of the true QB for each velocity profile. (2, estimates within 25 per cent of the true Q were obtained with the addition of up to 6.5 per cent signal-generated noise, whereas the addition of only 3 per cent signal-generated noise energy makes estimates of the frequency dependence of Q unreliable using spectral ratios. We conclude that the two-octave band of the data is not wide enough to determine the frequency dependence of Q,. Tests on synthetic seismograms, computed from models containing alternating layers of high impedance contrast with realistic velocities, indicated that apparent attenuation due to intrabed multiples does not significantly affect the spectral ratio Q, estimates, although a shift in spectral content to higher frequencies for PS and PSSS phases and a delay in the apparent arrival time of PSSS were observed. However, the alternative peak amplitude ratio method gave Q, estimates more than 25 per cent lower than the true Q for multilayer sediment models. We also tested the methods on synthetic data subjected to hard and soft clipping. Spectral ratio estimates of Q,, from synthetic data with PS clipped up to 50 per cent, were within 25 per cent of the true Qs.

VLF/ULF noise on basalt and sedimented seafloor.

During project NOBS (7 Sept. 1991 to 10 Oct. 1991) four arrays of ocean bottom seismometers were deployed to record seafloor noise in the band 0.01 to 10 Hz. The arrays had 0-, 10-, 100-, and 1000-m sediment, and were all within a few hundred km of FLIP, which was located at 43° 42.63 N, 125° 59.19 W. Examination of two-dimensional wave-number spectra of waves from an earthquake and from noise shows that for frequencies from 0.1 to 0.5 Hz the wavelength is of order tens of kilometers, whereas above 0.5 Hz the wavelength of noise and signal-generated noise is of order tens of meters. Four conclusions can be drawn from the data at this time; from 0.1 to 0.5 Hz the noise is propagating at a relatively high velocity, probably as Rayleigh waves with speeds of order 3 km/s; from 0.5 to 10 Hz the noise is propagating at low speeds, probably as interface (Stonely) waves with speeds of order 0.1 km/s; shear energy undergoes a large amount of scattering from 0.5 to 10 Hz; noise from 0.5 and 10 Hz is probably produced by scattering at the seafloor and is largest where the sediment is thinnest.

Broadband observations of PS conversions from the upper mantle in Eurasia

SUMMARY The delay-and-sum method by Vinnik (1977) to detect weak PS conversions in the P coda is applied to German, Czechoslovakian and Chinese broadband data. Clear phases (a factor 4-5 above the noise level) are observed in the 3-11 s period band from the 670 and 400 km discontinuities after summing several hundred traces. The decisive step to obtain such a good signal-to-noise ratio was the summation of records from stations in different regions. The summation of a large number (many hundreds) of traces of a single station does not result in similar improvements because of coherent signal-generated noise due to scattering below the station. There are weak indications of a 520 km discontinuity in our data. The delay times relative to P of the converted phases are stable within 1 s between central Europe and China.