Normalized Noise Level Research Articles

Microseisms in geothermal exploration—studies in Grass Valley, Nevada

Frequency(f)‐wavenumber(k) spectra of seismic noise in the bands 1 ⩽ f ⩽ 10 Hz in frequency and |k| ⩽ 35.7 cycles/km in wavenumber, measured at several places in Grass Valley, Nevada, exhibit numerous features which can be correlated with variations in surface geology and sources associated with hot spring activity. Exploration techniques for geothermal reservoirs, based upon the spatial distribution of the amplitude and frequency characteristics of short‐period seismic noise, are applied and evaluated in a field program at this potential geothermal area. A detailed investigation of the spatial and temporal characteristics of the noise field was made to guide subsequent data acquisition and processing. Contour maps of normalized noise level derived from judiciously sampled data are dominated by the hot spring noise source and the generally high noise levels outlining the regions of thick alluvium. Major faults are evident when they produce a shallow lateral contrast in rock properties. Conventional seismic noise mapping techniques cannot differentiate noise anomalies due to buried seismic sources from those due to shallow geologic effects. The noise radiating from a deep reservoir ought to be evident as body waves of high‐phase velocity with time‐invariant source azimuth. A small two‐dimensional (2-D) array was placed at 16 locations in the region to map propagation parameters. The f‐k spectra reveal shallow local sources, but no evidence for a significant body wave component in the noise field was found. With proper data sampling, array processing provides a powerful method for mapping the horizontal component of the vector wavenumber of the noise field. This information, along with the accurate velocity structure, will allow ray tracing to locate a source region of radiating microseisms. In Grass Valley, and probably in most areas of sedimentary cover, the 2–10 Hz microseismic field is predominantly fundamental‐mode Rayleigh waves controlled by the very shallow structure.

Satellite-communication system employing single-channel/carrier frequency modulation with syllabic companding

The advantages in terms of increased traffic capacity of using a single-channel/carrier companded-frequency modulation satellite system are demonstrated theoretically. These capacity advantages depend critically upon being able to achieve the predicted subjective advantages due to the syllabic compandors. An experimental circuit has been set up and results are presented of objective and subjective tests conducted under carefully controlled laboratory conditions, but simulating conditions typical of a real international telephone circuit. The latter shows that the performance of the companded f.m. system was generally as good as, and frequently superior to, that of an international satellite circuit with a normal noise level of 10000 pWOp. Advantages of the proposed system are demonstrated in both the earth and space segment for global, regional and domestic satellite communications.

Radio wave emissions in the v.l.f.-band observed near the auroral zone—I occurrence of emissions during disturbances

Abstract Very low frequency emissions in the 8 kc/s-band have been recorded at the Auroral Observatory, Tromso and bursts of enhanced emissions have been compared with geomagnetic records, absorption effects recorded on a riometer and appearance of aurorae. It is shown that bursts of enhanced radiations in the 8 kc/s-band appear regularly during disturbed conditions near the auroral zone, the energy is of the order 1–10 × 10 −16 W / m 2 c . The occurrence of the bursts exhibits a pronounced diurnal variation, their occurrence is limited to the time interval 1800–2400 hours M.E.T. The average duration of the bursts may vary from one minute to several hours, with a maximum occurrence of 10–12 min duration. The v.l.f.-emissions show a complicated dependence on geomagnetic disturbances and absorption within the ionosphere. Small dips of absorption in the riometer curves of the order 0·3–0·9 dB may be accompanied by v.l.f.-emissions; there is a positive correlation. Great values of absorption of the order 5–9 dB are accompanied by a complete cessation of v.l.f.-emissions; there is a negative correlation. These two effects indicate conclusively that the v.l.f.-emissions are strongly absorbed in a disturbed ionosphere. The moderate positive geomagnetic disturbances in H appearing early in the night (which is often followed by quiet auroral forms, arcs and bands on the Northern sky) are very often accompanied by strong v.l.f.-emissions. Sudden and strong disturbances in the geomagnetic field when these appear as a sudden commencement, are usually followed by a pronounced v.l.f.-emission. When the disturbance develops with (usually negative) peaks in H the v.l.f.-emissions are completely absorbed. It is pointed out that the close connection between the v.l.f.-emissions and the geomagnetic disturbances is limited to appear only in the time interval 1800–2400 hours M.E.T. Outside this 6-hr interval v.l.f.-emissions do not appear independently of the degree of geomagnetic disturbances, auroral luminosities or level of ionospheric absorption. The point of view that the v.l.f.-emissions are generated in the near and local part of the ionosphere by an impact of charged particles (for instance as Cerenkov-radiation) which simultaneously are producing auroral luminosities, must therefore be regarded as less probable. If this were the case one would expect an appearance of v.l.f.-emissions throughout the whole night. Records demonstrating the parallel amplitude variations in v.l.f.-emissions and 5577 A-photometer recordings are shown. Parallel records of v.l.f.-emissions have been made at Tromso and Kiruna, the latter lying 210 km to the South of Tromso. In some cases the Kiruna-records show similar changes in amplitudes for great bursts as the Tromso-records. Three cases of “noise storms” have been recorded during which the enhanced v.l.f.-radiations appeared during the whole night, the “noise storms” coinciding with large geomagnetic disturbances. The amplitude of the “storm” declined and disappeared when the ionosphere was illuminated by the sun's rays in the morning and when the normal day-time absorption was formed.—In an Appendix several cases of enhanced radiation are presented on the riometer recording frequency of 28 Mc/s. Records are shown which demonstrate coincidence between v.l.f.-bursts on 8 kc/s and sudden increases in the normal galactic noise level received on 28 Mc/s. These common bursts appear only when a sudden deflection in the geomagnetic curves appears, which may often have the character of a sudden commencement. The 28 Mc/s bursts are rare events, but it is concluded that the bursts are not produced by man-made interference.

An isolated observation of the synchrotron radiation associated with the July 9 nuclear explosion

Observation of the synchrotron radiation and radiowave absorption produced by bomb-generated electrons from off the coast of Chile is reported. The noise received increased to about 0.7 db above the normal cosmic noise level starting about 5 min after the explosion. Subsequently there was approximately 0.3 db of absorption, but within the next 20 or 30 min the trace returned to its normal level and showed neither absorption nor enhancement of the noise level afterward. (P.C.H.)