The horizontal extent of lightning channels reconstructed acoustically from several storms is significantly larger than the vertical extent. Furthermore, an examination of all the reconstructed lightning structure in each of three storms, one in Arizona, one in Colorado, and one in Florida, shows that the lightning activity tends to occur in layers 2–3 km thick. In the Arizona and Florida storms, there were two layers of activity. Temperatures at the lower boundary of the layers were near 4°C and −18°C in the Arizona storm and −11°C and −39°C in the Florida storm. In the Colorado storm, there was a single layer having a lower boundary near the 0°C isotherm. Our interpretation is that each of the two layers in the Arizona and Florida storms is associated with a corresponding charged region in the dipole of the thundercloud charge distribution. We suggest that the single layer in the Colorado storm was a result of the dipolar regions being closer together in altitude.
The effects of wave front curvature, wind, and temperature on the determination of the direction of propagation of thunder at an array of microphones is examined, and it is shown that the direction of propagation can be measured at the array with an error of less than ±1° under the following conditions: the wind velocity at the array should be measured with an accuracy of ±1 m/s, the temperature at the array with an accuracy of ±2°C, and the range to the source with an accuracy of ±20%. The positions of the source points on the lightning channel, which represent the total lightning event at that location, are mapped by acoustic ray tracing techniques; this procedure requires information on the temperature and wind velocity as a function of altitude; without this information and in adverse conditions, errors of 10% for high‐altitude sources and 25% for sources near the horizon will occur. By using supportive data on wind and temperature profiles all error estimates can be reduced to 5%, which is more than sufficient to do comparative studies of lightning channel structures inside clouds with the macrophysical properties of the clouds.
Studies of the dissipating stage of a thunderstorm indicate that for the storm under investigation both cloud‐to‐ground (CG) lightning flashes and intracloud (IC) lightning flashes possess an extensive horizontal structure inside the cloud. The configurations of all lightning flashes, 17 CG events and 20 IC events, that occurred in the 30‐min time interval preceding the end of significant electrical activity of a thunderstorm have been mapped. Lightning channel reconstructions were derived from an analysis of thunder recorded by an array of microphones. Results indicate that the structure of IC lightning and the intracloud portions of CG lightning may be modeled as being contained in an ellipsoid whose long axis is parallel to the earth's surface. A typical ratio of long horizontal axis to short horizontal axis to vertical axis is 3:2:1. The results indicate that horizontal lightning structures are more persistent and extended than have been estimated from observations of the electric field. The most striking feature of the IC events and of the intracloud portions of the CG events is their marked tendency to align themselves along the same direction.
Air-gun array technology has developed in the 1970's along the general line of producing tuned arrays with source strengths in excess of 100 bar-meters (peak-to-peak) in the seismic bandwidth and with wavelets of front-to-back ratios in excess of 10:1. Most such arrays are towed in a so-called “point source configuration,” for which a significant amount of energy is beamed in undesireable directions such as forward of the vessel and to each side of the vessel. This paper examines several available air gun array geometries (the SLAG array, in which the air gun source is extended along the axis of a survey line, the SWAG array, in which the air gun source array is extended along an axis perpendicular to the survey line, and the point source array), and shows how noise problem characteristic of certain offshore areas may be reduced by beaming the array in preferred directions. It is shown that this beaming may improve the achievable signal-to-noise ratio—both by increasing power input to the water in desireable directions, and by reducing noise arriving from unwanted directions.
Air gun array technology has developed in the 1970's along the general line of producing tuned arrays with source strengths in excess of 100 bar-meters (peak-to-peak) in the seismic bandwidth and with wavelets of front-to-back ratios in excess of 15:1. Most such arrays are towed in a so-called "point-source configuration", for which a significant amount of energy is beamed in such undesirable directions as forward and to each side of the towing vessel. This paper examines several available air gun array geometries; the SLAG array, in which the air gun source array is extended along the axis of a survey line, the SWAG array, in which the air gun source array is extended along an axis perpendicular to the survey line, and the point source array .It is shown how noise problems characteristic of certain offshore areas may be reduced by beaming the array in preferred directions. It is shown that this beaming may improve the achievable signal-to-noise ratio --- both by increasing power input to the water in desirable directions, and by reducing noise arriving from unwanted directions. Air Gun Array Design Criteria (1970's) The conventional design criteria for tuned air gun arrays are:Maximize the peak-to-peak amplitude of the array signature as one would observe at normal incidence in the far-field.Maximize the ratio of the peak amplitude to the residual bubble amplitude (P/B ratio), as observed at normal incidence in the far-field.Assume that, in the seismic bandwidth, typically 10 Hz to 125 Hz, the power spectral density of the array signature is smooth, i.e., the array signature should resemble a band-limited spike.Adjust the depths of source and streamer and the recording system impulse response to place spectral notches at uninteresting frequencies, hence optimizing the frequency band desirable for the geophysical objective. Giles and Johnston (1973) and Nooteboom (1978) discuss these criteria and one example of a modern, high resolution, deep penetration array, reported by Brandsaeter, et. al (1979), has a total volume of 4165 cu. in. with a peak-to-peak amplitude of 49.5 bar-meters and a primary-to-bubble ratio of 16.7. This total array was deployed astern in a surface area 16 m × 20 m, and, while it is evident that some spatial beam forming is taking place, this array is commonly thought of as being in a "point source configuration." Non-Source Generated Noise Implicit in the design criteria mentioned above is the notion that the signal must be large compared to any additive random and incoherent noise which is not source-generated. By simply increasing the source power, such noises as electrical noise, certain types of towing noises, cultural noises and ambient sea noise may be overcome. In addition, this helps overcome the effects of absorption and other frequency-dependent loss mechanisms. Source-Generated Noise The effects, on the stacked section, CDP and shot gathers of such noises as guided waves in near-surface layers, side scatter from reefs, islands, river channels, and reflected refractions are discussed by Larner, et al,198l.
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