A method of searching for radial velocity anomalies in the lower mantle using interfering signals

Abstract

Radial velocity anomalies in the lower mantle that give rise to triplications in the travel-time curve for short-periodP waves will produce arrivals havingdT/dΔ values that differe by roughly 0.2–0.5 s/deg. The first two arrivals associated with such triplications will be separated by less than one second over a distance range of 4°–10° they may not, therefore, be separable visually on single seismograms, so that their presence can only be inferred from some measurable property that depends on their mutual interference. If there are lateral variations in the regions of anomalous velocity gradients, the interfering signals will also have different azimuths of arrival. Using two synthetic wavelets we have investigated the effect of interference on bothdT/dΔ and azimuth measurements at the Yellowknife Array. We found that if the interfering pulses have a dominant frequencyv, there is a range of time separations (0.30/v→0.55/v) over which the measureddT/dΔ and azimuth values may fluctuate by much more than the differences indT/dΔ and azimuth between the interfering signals. We have evaluated the following empirically defined functions for three different primary signals, and for three different relative amplitudes of the interfering signals:f τ(t), the drift function, which expresses how the measured slownesses,p, and azimuths, ϕ, differ from the slownesses and azimuths of the primary wavelets; Δf(τ), the range function, which describes the behaviour of the upper and lower bounds ofp and ϕ as a function of the difference in arrival times of the signals, τ andf ′ τ , studied the properties of these functions, and have outlined how these properties provide criteria based on the numerical and statistical characteristics of the arrival vectors, and on the waveform of the signal that will enable small radial velocity anomalies to be more clearly delineated.