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The Dunedin earthquake of 9 April, 1974 was of magnitude 5.0, and is the largest known to have originated in eastern Otago. The shock was centred about 10 km south of the city centre, with a shallow focus, probably about 20 km deep. Damage to a value of about $250,000 occurred, and many brick chimneys were broken or damaged in the worst affected areas, where the intensity reached MM VII. The areas of greatest damage were the southern suburbs, on alluvial soils, where results from a survey of natural seismic noise show strong amplification effects due to the unconsolidated nature of the ground.

In September and October, 1972 the U. S. Geological Survey made an investigation of seismic noise associated with the known geothermal phenomena in Yellowstone National Park. Eighty‐four stations, each recording for at least 48 hours, were operated. All major geyser basins were covered by the experiment. L-shaped three‐element arrays, three‐component stations, and single vertical component stations were operated. Four eight‐element mobile arrays were operated to study propagation characteristics of the noise. Preliminary analysis of data shows that high noise levels are associated with all the major thermal areas in the park. An elongated band of high noise envelops Lower and Upper Geyser Basins; noise levels are high around Norris Basin, Mammoth Hot Springs, Sulphur Mountain, and Mud Volcano; and a strong noise field exists around Lower and Upper Falls of the Yellowstone River. The seismic waves generated by the waterfalls have very different spectral characteristics from the waves associated with geothermal activity. The geothermal noise is predominantly in the spectral band of 2–8 hz, whereas the waterfall noise is predominantly around 2 hz. A mobile array operated near Norris Basin showed coherent wave trains radiating from seismic sources in the basin. Seismic noise measured around 50 m from Old Faithful Geyser showed amplitude fluctuations that followed the eruption cycles of the geyser. A few minutes after each eruption, the noise level starts rising slowly in ramplike fashion. Twenty to thirty minutes before the next eruption, sharp bursts of noise activity occur with increasing rapidity and continue for a few minutes after the eruption. The predominant energy of seismic noise generated by Old Faithful is at frequencies well above 8 hz. We postulate that only such high frequency noise is generated by the surface activity of geysers and hot springs and that the lower frequency noise found in and around the geyser basins is generated by a deeper convection system associated with the geothermal activity.

A seismic noise survey at Solfatara Crater, Phlegraean Fields, was carried out during March 1973 using a portable tape seismograph. The shape of the spectra and the maximum amplitude values of the ground velocity point out that the seismic noise is strongly influenced by the surface thermal activity and mechanical properties of outcropping rocks.

The effect of vibrations on the accuracy of measurements with aCG-2 gravimeter was investigated on a vibrating table with a vertical, harmonic motion of an amplitude range of 0.1–4 µ and frequency range of 10–100Hz (Tabs 1, 2). The error in determining the gravity is a non-linear function of the amplitude and frequency (Figs 1–4), and within the range of motion of the table amounts to as much as tens of milligals. A permanent seismic noise of average intensity does not generate an error in the gravimeter reading. The effect of wind and close sources of vibrations can only be evaluated by means of concurrent seismic observations of ground movement.

A decision-theoretical model for seismic discrimination between earthquakes and underground nuclear explosions is developed. The influence of long- and short-period detection thresholds on the probabilities to identify explosions and to make false alarms is discussed. The model includes identification by negative evidence. This kind of identification is considered in the case when only short-period data are available and when the long-period signal amplitude is supposed to be less than or equal to the seismic noise amplitude. For one set of data—obtained from North American events at the Hagfors Observatory in Sweden—the applicability of the m(M)-method is increased about half a magnitude by including identification by negative evidence.

Earth motion in response to atmospheric pressure changes is a major contributor to the ambient seismic background in the 20- to 100-s period range. During windy intervals at the surface the quasi-static deformations caused by wind-generated pressure changes are often quite large in comparison to ambient vibrations from other sources. Recent studies [Sorrells et al., 1971; Ziolkowski, 1973] have demonstrated that the wind-related noise can be eliminated by installing the sensors at depths of the order of several hundred meters. When such action is taken, it is found that the noise levels obtained approximate those observed at the surface during calm intervals. The calm interval noise level thus constitutes the current minimum threshold. The calm interval noise level in the 20- to 100-s period range is largely the result of earth motion from atmospheric pressure sources of undetermined origin [Savino et al., 1972]. If noise levels are to be reduced below the currently realizable minimums, it will be necessary to establish the origin of the atmospheric pressure variations with some precision. In this paper, evidence is presented that supports the hypothesis that much of the earth noise observed during calm intervals in the 20- to 100-s period range is the result of atmospheric infrasonic waves. If this hypothesis is borne out by additional studies, then efforts to reduce further the current long-period noise threshold by increasing installation depths will not yield satisfactory results unless depths of the order of 1–3 km are reached. The relatively high costs of installing systems at such depths would rule out this particular method for obtaining further reductions in the long-period seismic noise threshold. Thus if the current noise threshold is largely the result of infrasonic waves, then the practical limits of using depth of installation for the suppression of seismic noise have already been reached.

Short‐period vertical seismometers are used in deep holes at several sites to obtain the change with depth in amplitude and phase spectra of short‐period seismic noise. Although the observed spectra can be explained by an arbitrary combination of several Rayleigh modes, an alternative theory is suggested here. An attempt is made to explain both amplitude and phase spectra of observed microseisms of period less than 6 sec in terms of standing‐wave phenomena caused by steady‐state plane harmonic compressional waves propagating vertically through a horizontally stratified medium. At most sites, the observed data indicate satisfactory agreement with the expected results. A considerable fraction of the short‐period noise may, therefore, be regarded as P‐wave noise propagating vertically from below.

Seismic refraction investigations in two “noisy” areas differing in geologic environment and field conditions indicate the reliability of such a technique for foundation engineering problems. Shothole conditions and generating media are of great importance. Dense media, clay, and materials within the water‐table are most favorable; fill debris is the least suitable. Seismic noise in the city, usually transient in character, is not an insurmountable problem; a patient instrument operator is a key to success. Several shot locations along an individual spread and a geophone spacing not to exceed 50 ft are standard field procedure for achieving a maximum amount of data.

For the past two years we have been engaged in a program of seismic noise measurements in deep boreholes; noise in a frequency range of approximately 0.2 to 5.0 cps has been studied. A special deep‐well seismometer designed by The Geotechnical Corporation has been used for recording noise at depths down to 14,000 ft in cased holes. The seismometer, which has a response almost identical to the typical short‐period Benioff, was clamped to the casing of the deep hole by a mechanical locking arm during observation periods. An identical seismometer has been used as a reference in a 500‐ft hole adjacent to the deep well. A surface array of four vertical Benioff seismometers was also recorded, with one seismometer near the well and three others at distances ranging from 0.5 to 1.0 km. Two horizontal seismometers were stationed near the wellhead.

Undesirable seismic noise of a nondeterministic type must be destroyed by making use of its statistical properties. Averaging of one sort or another provides methods for performing this noise removal. Our purpose here is to present a method for direct estimation of signal strength versus seismogram time, with stepout as a parameter. After describing the method and its expected behavior to some extent, we illustrate its application to a set of three noisy records.

A problem that is common to all phases of seismic data processing is the lack of knowledge concerning the input or seismic source signal axnd the noise characteristics of the recorded waveforms from the seismic sensors.

Spectrograms of underwater seismic noise between 0.2 and 2.0 cps from the bottom of relatively shallow ice-covered seas show a line structure which indicates that vertical resonances occur within layers bounded at depth by the Mohorovičić discontinuity. The line structure appears with most clarity when recognizable transients are excluded. Under these quasi-stationary noise conditions, measurements of pressure spectra indicate that the seismic noise of the observed level can be generated by a vertical bobbing of the ice field as a whole.

Уровень помех на ссйсмических станциях СССР исследовался для периодов от 0,1 до 5–7сек при помощи специальной аппаратуры с увеличением до 20 000. Слектральное распределение помех характеризуется одним или двумя максимумами па периодах от 0,1 до 0,6сек, резким минимумом на периодах от 0,6 до 2,0сек и большим максимумом в области сущестнования штормовых микросейсм (3–8сек). Для сейсмографов высокой чувствительности наиболее выгодной оказывается кривая увеличения с максимумом на периодах около 1сек. В каждом конкретном случае, однако, желателен спецнальный расчет онтималяной кривой увеличения. Исходными данными дли расчета являются зависимость амплитулы помех от периода и требование, чтобы амплитуда записп помех на сейсмограмме не превышала заданной величины. По совокупности этих данных может выть выбрана кривая увеличения, удовлетворяюшая требованиям теории сейсмографа и дающая наибольшсе увеличение при заданном уровне помех иа записи. Необходимые параметры сейсмографа с достаточной точностью расчитываются по нескольким характерным точкам кривой увелнчепия.

Ruapehu is an andesite volcano in the North Island of New Zealand whose activity is being monitored by two new instruments: a seismic power recorder described in this paper, and a differential magnetometer described in a companion paper. On April 5, 18, and 26, May 29, and June 9, 1968, episodes of activity commenced with volcanic tremor of frequency 2 c/s and radiated seismic power up to 100 kw. Between 10 hours and 7 days after the commencement, when the tremor power had decreased, strong hydrothermal eruptions occurred in the crater lake. The largest eruption, which occurred at 0749 NZST on April 27, 1968, had a peak seismic power of 9 Mw and a seismic energy of 1015 ergs; it ejected nearly 105 cubic meters of mud and water from the lake.

In recent years there has been increasing interest in the role of geothermal steam as a source of energy. Only recently has geophysical exploration begun to play a major role in the exploration for geothermal resources; heat flow, gravity, and resistivity measurements have been the techniques most used. One recent development is the discovery that high surface‐noise levels are associated with the presence of geothermal reservoirs below the surface. Field surveys using short‐period seismographs have been conducted in the Imperial Valley of California in areas where heatflow measurements or drilling have indicated the presence of a geothermal deposit. In all three surveys abnormally high noise levels were found above the reservoir in the frequency range of 0.5 to 5.0 hz. The ground‐noise anomalies show a more complex pattern than the associated heat flow and gravity measurements. A theoretical model has been developed in which small, random pressure variations in a convecting geothermal reservoir are suggested as the source of the noise. Using this model, the noise level above one of the anomalies was duplicated, using a reasonably sized reservoir with pressure variations of less than 1 millibar.

Median values of seismic noise in the period range of 0.3 to 1.3 sec were obtained from recordings at vaults of the Pole Mountain and WMSO arrays. Interquartile ranges were used to measure dispersions about the medians. Medians of the noise at Pole Mountain ranged from 0.91 mµ to 2.20 mµ in November 1962. The former value was obtained for a vault that was located in dense granite at the base of a massive granite outcrop; the latter value was obtained for a vault in a slab of dense granite located on a grassy plain. This indicated that topographic shielding from wind rather than density of bedrock affected noise. As a test of this idea, wind protection numbers were assigned to vaults Z1 through Z9 of the WMSO array based on comparative topographic shielding with respect to a known wind direction. Noise values increased as wind numbers decreased. Topographic protection and vault construction limited wind noise at WMSO.

Operation of short-period vertical seismometers at depths down to 3000 m in abandoned oil wells provides a new method of studying seismic surface waves. Power spectral density functions and the cross-products of simultaneous noise samples at the surface and at depth are used to obtain the change in amplitude and phase with depth. The vertical component of the noise is shown to be caused mainly by fundamental and higher mode Rayleigh waves. The fundamental, first, and third Rayleigh modes are identified in the noise. Each higher mode can be identified by its unique variation in displacement with depth and the 180-deg phase shifts that occur at the nodal points. The experimentally determined displacement of the different Rayleigh modes with depth is in good agreement with the theoretical displacement.

Methods are developed to analyze the effects of thermal noise and seismic noise in masking small earth vibrations. The procedure is applied to a simple seismometer, seismometers with electronic and galvanometer amplifiers, and a seismometer with a shunt capacity. A numerical example is worked out for the Benioff one-second instrument. In the 0.1 second to 10 second band, thermal noise is not limiting.

Power spectral analysis in frequency-wavenumber space and coherence studies in lag space have shown that the noise recorded by the short period Øyer subarray at NORSAR is critically dependent on the weather situation in the North Atlantic Ocean. In addition to the low-frequency noise from the west, there is observed 2-sec microseisms from the Baltic Sea. Because of the nonisotropic noise, the coherence is usually strongly azimuthal dependent, being represented in lag space by ellipses. Large time variations of the coherence are demonstrated.

Summary A study has been made of the capability of the Norwegian Seismic Array to discriminate between earthquakes and underground explosions occurring in central Asia and western Russia. The ratio of surface to body wave magnitudes (Ms,: mb) has been used exclusively, the chief application of the array being in the detection and measurement of low amplitude surface waves. Beamforming and matched filtering were the signal enhancement techniques applied. Of 34 events in central Asia studied, 10 were identified as explosions, 22 earthquakes, and two were unidentifiable because of high, long period background noise. Five events in aseismic western Russia, all presumed to be explosions, showed wide variation in Ms,: mb; two of these measurements being close to the earth-quake population of central Asia. The Ms: mb measurements of central Asia explosions made in this and another study are compared with measurements of Nevada explosions. For a given mb the latter consistently have higher Ms values. The possible causes of this are discussed and, depending on the cause, the array seems capable of detecting surface waves from an explosion of 4–16 kt yield at 40° distance in central Asia. Strong seismic noise variations in Norway make this long period capability time dependent.

The intermittent occurrence of earth tremors during the solidification of magma is considered a possible mechanism for the development of layering in intrusives. Qualitatively, at least, the effect is comparable to that of shocks being administered to supersaturated liquids in the laboratory. It should be possible to interpret the seismic history of the region during the crystallization of the intrusive from a critical and detailed analysis of the pattern of layering developed.

Seismic events can be relocated relative to a reference event by using the group-velocity dispersion curves of surface waves. Since group velocity is a function of the travel path, surface waves from two events in the same locale should show identical group velocities when viewed at any one seismograph station. A computer technique has been developed for comparing the group-velocity curves of any event with the curves of a reference event and for determining the relocation which causes the curves to best coincide. The method is evaluated by relocating eight intermediate-size nuclear explosions of the Nevada Test Site series. With precise curve fitting, the surface-wave locations are slightly more accurate in southern Nevada than the standard body-wave determinations. The surface-wave origin times are considerably more accurate. In areas of sparse station coverage or of many small earthquakes, the surface-wave method can be expected to improve seismic locations significantly.

Seismic noise has been measured in deep wells by the Geotechnical Corporation at twelve different locations and by the Shell Development Company at three deep wells. To explain the power attenuation and the coherence between vertically separated seismometers, one requires noise vibrations taken as a mixture of propagation modes, possibly surface‐wave modes and body‐wave modes. This was illustrated in observations by many authors including Archambeau et al. (1964), Douze (1964), Sax and Hartenberger (1964), Seriff et al. (1965), and Sax (1965).

By using the power density spectra of the seismic signal and the noise, it is possible to design a transfer function of an optimum (in Wiener sense) linear filter. The corresponding impulsetransfer functions dealt with in the paper were calculated as the approximation of the continuous transfer functions. This transformation can be done e.g. by the well-known partial-fraction expansion or through the bilinear transformation. The properties of the optimum discrete filtering for an accurate determination of the first arrival on records with the great background noise are discussed.

A comparison of the seismic waves generated by a nuclear explosion and an earthquake is discussed. The epicenter of the earthquake was located within the Nevada Test Site. Both events were recorded at the same station with the same type of equipment. The earthquake waves contained slightly lower frequency than the waves generated by the nuclear shot. The early P phases of the shot had larger amplitudes while the phases after Pg for the earthquake were larger. Seismic waves from collapses were generally found to be composed of lower frequencies than the waves from the original shot. Aftershocks of the Hebgen Lake earthquake were found to generate seismic waves whose frequency content was related to the magnitude of the aftershock. Spectral differences in quarry shot recordings that correlate with source duration times are also discussed.

Seismic noise attenuation as a function of depth below the earth’s surface is described in terms of surface wave modes. The energy density of the noise, which often represents a combination of modes, is computed as the arithmetic sum of the energy densities of all contributing modes. Three models are used to determine the fractional contribution of each mode. Model A stipulates that each mode contributes equally to the energy of the noise; Model B specifies that the energy fraction contributed by each mode is dependent upon the energy in those beds in which the shear velocity of the bed exceeds the phase velocity of the mode; and Model C prescribes that all modes have equal energy density at the free surface. A comparison of observations with attenuations based on the three models reveals that Model B is the best.

Both British work at A.W.R.E. and American work in the A.R.P.A. Vela Uniform Program have demonstrated that arrays of seismometers may be used to enhance the signal/noise ratio of distant seismic events over the single instrument signal/noise ratio. In addition, arrays may have the virtue of suppressing signal-generated noise arising in the crust below the receiver. Other Vela research has demonstrated that buried instruments frequently have a lower noise environment than surface instruments. Borehole emplacement may be particularly effective against wind-generated seismic noise. In an attempt to exploit these possible improvements to the maximum extent we are now constructing a large aperture seismological array of buried seismometers. This instrument will consist of over 500 seismometers distributed in 21 clusters over a 200 km aperture. It will exploit modem array signalprocessing techniques. If it performs as expected it will have major implications for worldwide seismological detection systems. The status of and plans for the experimental array are described along with some of the world-wide system possibilities.

When seismic signals from impulsive sources are reflected or refracted by discrete inhomogeneities in the seismic medium, ‘arrivals’ are recorded. If, however, the number of inhomogeneities becomes large and the distance between them becomes small, then interference among the arrivals takes place and source-caused ‘noise ’ is recorded. If the spacing between observatories is large compared with the spacing between and dimensions of the scatterers, the source-caused noise is incoherent. If the number of scatterers is large enough for the problem to be treated statistically, the noise has a random character. The properties of the noise can be computed by averaging statistically over all the signals due to the scattering from the ensemble of scatterers. Single scattering only is treated here. There are ‘local’ or ‘end’ effects corresponding to scattering near the source or the receiver which cannot be taken into account in the calculations. The main problem which has been treated is that of the scattering of body waves of P and S types in an unbounded inhomogeneous medium. The magnitudes of the scattered waves of all types— PP, PS, SP, SS —have been computed. In addition the phase shifts (time delays or advances) in incident P and S can be computed. It is found that body waves of either P or S type convert into scattered S waves with considerably greater ease than into scattered P waves. The comparative efficiency of these processes is about two orders of magnitude. Thus P waves show small phase shifts; S waves show large phase shifts. The waves between P and S are most likely of the character of S . The approximations in the calculations involve the assumptions of wavelength long compared with the dimensions of the scatterer and the dimensions of the scattering region long compared with wavelength. Under these conditions the approximations are those for Rayleigh scattering. Hence, in all the results, the scattering varies as the fourth power of the frequency and the mean square scattered energy is proportional to the linear dimension of the scattering region. At higher frequencies, the scattering changes from a fourth power dependence upon frequency to a second power dependence. This is a result which is obtained only for scattering by elastic media; it is not found in media without shear modulus. Experimental evidence for this high frequency effect has been found.

Approximately half the noise observed by long-period seismometers at LASA is nonpropagating; that is, it is incoherent over distances greater than a few kilometers. However, because it is often strongly coherent with microbarograph data recorded at the same site, a large proportion of it can be predicted by convolving the microbarogram with some transfer function. The reduction in noise level using this technique can be as high as 5 db on the vertical seismometer and higher still on the horizontals. If the source of this noise on the vertical seismogram were predominantly buoyancy, the transfer function would be time-invariant. It is not. Buoyancy on the LASA long-period instruments is quite negligible. The noise is caused by atmospheric deformation of the ground and, since so much of it can be predicted from the output of a single nearby microbarograph, it must be of very local origin. The loading process may be adequately described by the static deformation of a flat-earth model; however, for the expectation of the noise to be finite, it is shown that the wave number spectrum of the pressure distribution must be band-limited. An expression for the expected noise power is derived which agrees very well with observations and predicts the correct attenuation with depth. It is apparent from the form of this expression why it is impossible to obtain a stable transfer function to predict the noise without an array of microbarographs and excessive data processing. The most effective way to suppress this kind of noise is to bury the seismometer: at 150 m the reduction in noise level would be about 10 db.