Critical frequencies of all regular ionospheric layers vary diurnally, seasonally, with geographic latitude and longitude, and with solar activity so that, for any location and any season
where f0 is the critical frequency, G and H are functions of (t), the time of day, and S is the sunspot‐number. For locations where such ionospheric trends are well established, observations of critical frequency may be used to determine an ionospheric “sunspot‐number”. If values of F2‐layer critical frequency for hours near local noon are used, since these generally have the most pronounced variation with solar activity, the ionospheric “sunspot‐number” obtained is considerably closer to its running‐average value than are ordinary sunspot‐numbers to their running averages. In addition, this measure is practically independent of varying atmospheric conditions and personal variation among observers, and therefore probably presents a more precise index of solar activity.
Discontinuity enhancement attributes are commonly used to facilitate the interpretation process by enhancing edges in seismic images and providing a quantitative measure of the significance of discontinuous features. These attributes require careful preprocessing to maintain geologic features and suppress acquisition and processing artifacts, which may be artificially detected as a geologic edge. We have developed the plane-wave Sobel attribute, a modification of the classic Sobel filter, by orienting the filter along seismic structures using plane-wave destruction and plane-wave shaping. The plane-wave Sobel attribute can be applied directly to a seismic image to efficiently and effectively enhance discontinuous features or to a coherence image to create a sharper and more detailed image. Two field benchmark data examples with many faults and channel features from offshore New Zealand and offshore Nova Scotia demonstrate the effectiveness of this method compared with conventional coherence attributes. The results are reproducible using the Madagascar software package.
Comparison was made between the correlation of monthly‐average ƒ°F2 at several ionospheric stations with monthly‐average Wolf relative sunspot‐number as compiled at Zurich, and with a new index of solar activity which has been provisionally called the “ionospheric sunspot‐number.” This quantity is derived from the ƒ°F2 observed at Washington, D.C.; Huancayo (Peru) and Watheroo (Western Australia)—stations having the longest time series of such data—for the hours 10 through 14 local time, using the well‐established trends of ƒ°F2 with sunspot‐number for these times and places. The average of correlation‐coefficients between smoothed annual sunspot‐number and monthly average of ƒ°F2 at noon for these stations for June, September, and December is 0.94.
The stations for which comparative correlations were made were not those whose data were used in deriving this so‐called “ionospheric sunspot‐number”, but others, having a series of ƒ°F2‐data extending over nearly as long a period of time. The correlation‐coefficients for these are given in Table 1.
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