Explosion-generated seismic signals have been of interest for many years, especially in connection with geophysical studies for scientific and industrial purposes. Increasing the yield of an explosion in a given type of rock does not necessarily mean a proportional increase in the seismic-signal amplitude. The geological environment in which a nuclear explosive is emplaced has a major influence on the strength of seismic signals.
Estimating the yield of underground nuclear explosions from far-field seismic waves is a difficult problem, since accurate estimates require a detailed accounting for source-region and travel-path differences between known and unknown yield events. For short-period P- waves, the first problem is to obtain an unbiased measure of the far-field energy. Conventional m_b seems unsatisfactory because of the large scatter in station values. The dominant cause seems to be wave focusing/defocusing by complex  earth  structure. For  yield  estimation, it is important to characterize each  event  by an  unbiased  measure  of the
P-wave energy leaving the source region. It is clearly very difficult, and perhaps impossible, to substantially reduce the scatter in conventional m_b data. The suggestion with the best theoretical support is to use the “b phase” , since it is least contaminated by pP and other later arriving energy.
The data used in this study were taken from 25 Semipalatinsk test site explosions recorded by SRO located at Mashhad. Reasons for selecting 25 records from 50 total records was based on signal to noise ratio and availablity of announced yields. For each event the amplitude of short-period  vertical component of the records were read. Also magnification was removed from all records. With attention to epicenteral distance (about 19 degrees) the first arrival is the direct P-wave passed through the mantle. Since with one station we cannot remove focus/defocus effect, it is better to use the amplitude instead of the magnitude in order to estimate the yield. However, the yield-magnitude relationship was calculated. We could obtain better results  when using “b phase” rather than the other phases. It is concluded that “b phase” is a good tool for decreasing scattering of the data. The derived relationship is:  log(A) = 0.807 log(Y) + 1.049, R² = 0.8014. So, this formula is recommended for calculating the yield of Semipalatinsk explosions recorded in SRO station, at Mashhad.