Résumé De préférence, les sismographes seront disposés dans un lieu sans bruit, oú il n'y aurait pas besoin de patternes. Par contre, si les bruits sont tels qu'une patterne est nécessaire, l'objectif de la patterne est d'améliorer le rapport signal/bruit et donc de rendre le signal le plus utile possible. La patterne sera dessinée en fonction des charactéristiques du signal, des charactéristiques du bruit, et de la géologie du site. Il a été démontré que, dans le sens pratique, l'opération optimum sur les données d'une patterne est représentée par le proces “delai et somme”. L'augmentation du nombre N de senseurs sur une surface donnée diminue les espacements entre les senseurs et peut augmenter la coherence entre les bruits enrigistrés aux senseurs voisins, donnant donc moins d'amélioration au rapport signal/bruit que la √N anticipée. L'augmentation du nombre de senseurs par l'augmentation proportionnelle de la surface de la patterne peut resulter en la détérioration du signal, et peut également donner moins d'amélioration que la √N Ces deux effets, ainsi que l'élément économique, combinent à limiter le nombre de senseurs qui peuvent être employés. Bien que les données sur lesquelles l'on a basé ces conclusions soient tirées de la sismologie conventionnelle, ces principes sont également valables pour l'exploration sismique et pour d'autres mesures géophysiques qui emploient des dispositifs dans lesquelles plusieurs senseurs sont requis.

Amplitude-distance curves are different in the EUS and WUS; P decays-2.5-3.0 max as r and r in EUS and WUS, while for the maximum after 3.6 km/sec on-2-3 the vertical component (termed Lg) the decay rates are r and r. The EUS results are in general agreement with the literature and with the data presented by Nersesov and Rautian (1964) for events on the northern margin of tectonic regions in the Soviet Union suggesting that discrimination results in the EUS are relevant to NSS stations within the Soviet Union. Using these distance amplitude relations, network mean amplitudes at 1000 km were computed for L and P for earthquakes and explosions in the g max EUS and WUS and a separation of 0.6 magnitude units was observed thus forming a regional discriminant. The L is larger for earthquakes than for explosions. g This conclusion is somewhat uncertain in the WUS because of the large scatter in the explosion population. However, a reasonable explanation for this large scatter is that the small events at NTS are at such shallow depths in dry alluvium that the medium is weak, resulting in a low corner frequency. This decreases the ratio P /L since P contains comparatively more high frequency than L. The scatter probably would not be a problem in a true test ban situg ation since shots will be well buried to avoid surface collapse.

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.

AD820291 Approved for public release; distribution is unlimited. Document partially illegible. Distribution authorized to U.S. Gov't. agencies and their contractors; Critical Technology; SEP 1967. Other requests shall be referred to Air Force Technical Application Center, Washington, DC 20330. Document partially illegible. This document contains export-controlled technical data.

LASA short-period recordings of 8 teleseismic earthquakes were prefiltered and beamsteered on P-wave arrivals across the 200 km. aperture to establish the relationship between sensor spacing and beam efficiency in terms of noise reduction, signal loss, and S/N ratio improvement. Results show that the combined effect of increasing the number of elements in a beam v,hile simultaneously reducing inter-sensor spacing is to produce progressively less rms noise reduction and S/N gain relative to N. The study further thows that beamforming each of the events in two ways, e.g., with 51 and 525 inputs, produces an average signal loss of ~H db. Moreover, beaming the smaller number of traces reduces rms noise and improves S/N only about 1 db less than the 525-element beam. For the 51-element beam, the minimum sensor spacing was 6 km., the distance at which the short-period noise at LASA becomes incoherent.