Imaging with seismic noise: improving extraction of body wave phases from the deep Earth through selective stacking based on H/V ratios

Summary Generating high-resolution images of the deep Earth remains a challenge. Body waves extracted from noise correlations hold high promise to complement earthquake-based studies, but data processing and interpretation are still under development. We develop a methodology to improve signal to noise ratio of P410P and P660P, waves reflected at the top and bottom of the mantle transition zone, using data from the greater Alpine area and focussing on the second microseismic peak (2.5 s—10 s period). Rather than stacking all available data, we only stack correlations for days with a low ratio of amplitudes between the horizontal plane and vertical direction (H/V). Due to an improved signal to noise ratio we can stack over fewer correlation pairs, with the result that horizontal resolution is significantly improved. We propose a systematic approach to determine at each study point the optimal combination of station pairs and the H/V threshold. We observe that the optimal choice of parameters is location dependent and that it is generally different for P410P and P660P. Additionally, we show that in our study area the maximum interstation distance needs to be reduced to ∼150 km for P410P to avoid that this arrival is contaminated by surface waves. Applied to the greater Alpine area we demonstrate a significant improvement of signal extraction: while P410P and P660P were only sporadically identified in standard stacks, with the new processing scheme these arrivals are clearly identified with coherent phases across large distances. We also show that amplitudes of P660P decrease drastically around longitude ∼11-∼12° E, indicating that the lower discontinuity of the transition zone in that area is too broad to have a significant reflexion coefficient for P-waves in the second microseismic peak.