Study of the earth mantle structure by using compressional waves from underground nuclear explosions

Abstract

First arrival P wave travel time data from the epicen-tral distance range 300 to 900, from underground nuclear explosions at Eastern Kazakh, Novaya Zemlya and Nevada have been used in this study to find three profiles of P wave velocity distribution with depth. The travel time data have been used to obtain (p,) values which are then smoothed by using summary values. These summary values are then fitted with a number of straight line segments covering the whole range of epicentral distance considered here. Each line segment is then assumed to represent (p-) profile for the corresponding epicentral distance range. Herglotz-wiechert method is used to invert (p-) values to get P-wave velocity depth profile for each line segment. Method of stripping is applied to find velocity for different layers. Since no travel time data is used from 00 to 300 epicentral distance (from the surface to about 750 km depth) this layer was stripped off by taking average P wave velocity for this layer from Herrins velocity distribution model. All the three velocity-depth profiles show increase of velocity with depth from about 10.70 km/sec to 13.70 km/sec. There are superposition of high and low velocity anomalies in each profile. Most of these occur in different depths in different profiles. For eastern Kazakh profile the velocity peaks occur at 800, 975, 1150, 1225, 1450, 1725, 2150 km and velocity jumps at 1225 and 2425 km depths. For Novaya Zemlya profile, peaks occur at 875, 1025, 1300, 1475, 1775 km and a jump at 1125 km depths. For Nevada profile peaks occur at 925, 1325, 1625, 2075 km and a velocity jump at 1225 km depths. Velocity deeps (sharp fall in velocity) occur just after the peaks. Velocity steadily increases between peaks and deeps. The profiles also show relatively low velocity regions at 1100-1250 km (low), 2200-2450 km (low) for Eastern Kazakh, at 900-1050 km (high), 1100-1350 km (high) and 1500-1800 km (low) for Noyaya Zemlya, and at 1200-1350 (high) and 2100-2375 km (low) for Nevada profile. The intrinsic...