Studies of the uppermost part of the mantle have largely been undertaken with man-made sources. Long-range profiles using conventional explosive sources rarely achieve data coverage beyond 1000 km. In consequence, much of our knowledge of the structures below 100 km depth in the deeper mantle lithosphere and mantle transition zone come from studies using earthquake sources, with composite results from many events. Nuclear explosions can provide sufficient seismic energy to yield records spanning the complex returns from the upper mantle discontinuities. Nuclear tests in the United States of America formed the basis of the work by Helmberger & Wiggins (1971), Wiggins & Helmberger (1973) which used synthetic seismograms to control the nature of the wavespeed profile in the upper mantle. In the former Soviet Union an extensive campaign of ultra-long-range profiles was undertaken using peaceful nuclear explosions, often specifically detonated for the experiments. Seismometers were deployed out to 3000 km from the source with multiple coverage along a number of profiles (see Section 22.2.1).

The uppermost mantle

The times of arrivals for stations from 200 to 1000 km away from the source show little variation in apparent slowness for P waves. A typical phase slowness for Pn is around 0.124 s/km, corresponding to a phase velocity of 8.15 km/s. However, once the opportunity arose to examine the nature of the seismograms in detail, it became apparent that this relatively uniform slowness for the first arrivals did not correspond to a single phase branch but rather to a sequence of en echelon P contributions. Such behaviour has emerged in all cases where there is sufficient data density to follow the details of the arrivals returned from the uppermost mantle.

For example, in the 1971 long-range profile across France (Hirn et al., 1973), a Pd arrival was correlated from 150 to 250 km and is then superseded by a set of further phases PI from 300 to 500 km and PII from 450 to 600 km; a further PIII phase was correlated by Kind (1974). On the basis of the amplitude behaviour (Hirn et al., 1973; Kind, 1974) and systematic travel time inversions (Kennett, 1976), the retrograde