Crustal and upper mantle reflectors from noise autocorrelations

The depth of reflector layers in the Earth is usually estimated using controlled sources or earthquake signals. Ambient noise, however, can also be used for this purpose. It has been theoretically demonstrated that "the reflection seismogram from a surface source and a surface receiver is one side of the autocorrelation of the seismogram from a source at depth and the same receiver" (Claerbout, 1968). This conjecture has theoretically been shown to apply to the recording of ambient noise by co-located source and receiver. To verify this theory, we investigated stacks of continuous record autocorrelations at broadband USArray EarthScope Transportable Array (TA) stations in the western Great Basin and the Sierra Nevada, in a region with complex crustal and upper mantle structure. We interpret our results as the first direct evidence, using short-period (~ 1 sec) data, of crustal and upper mantle reflectors obtained from ambient noise autocorrelations: notably, one reflector at the crust-mantle boundary (Moho discontinuity) and a second sharp reflector due to a velocity gradient, ~ 100 km above the 410 km upper mantle discontinuity. Other reflectors are only weakly revealed. Our results compare well with controlled source and earthquake, and with tomography, findings in the region. Our method could be applied at any desired sensor spacing to estimate Earth reflector depth beneath surface-located sensors, providing unprecedented resolution.