Seismic Imaging Using Cross- and Auto-correlation of High-frequency (2-20 Hz) Ambient Noise and Event-coda Data

Passive seismic methods, including auto- and cross-correlation of noise or coda waves, have gained the attention of industry due to their low cost and low environmental impact. To examine the feasibility of the methods for imaging deposit-scale anomalies, a dense array is deployed near Marathon, Canada, by the PACIFIC Consortium, with a part of the array covering a Cu-PGE sulfide deposit, which had previously been surveyed in detail.

The array, consisting of about 1000 sensors, covered an area of 3´6 km and recorded continuously for about 1 month. Lake Superior to the southwest of the array and trains running along the lakeshore serve as the main local noise sources.

During the operation period, a M 2.1 local blast occurred, with long coda wave excitation. Autocorrelation of the coda wave indicated two clear reflection signals within the first 1.0 s from the surface.

In an alternative approach, we use surface waves to locate the interfaces. We applied ambient-noise cross-correlation to a 1-hour dataset in order to retrieve surface wave signals within the 0.5 - 20 Hz frequency band. We used station pairs with shorter distances to get high-frequency surface waves and ones with longer distances to get low-frequency surface waves. To address possible spatial heterogeneity, we divided the whole array into 6 regions. We searched for the simplest layered model fits the average dispersion curves in each region, with a method to simultaneously invert for velocity and layer thickness.

The robust phase-velocity curves we have obtained span 2 - 20 Hz, which enables us to constrain velocity structures within the top 1000 m of the crust. Results from all the six regions show that a 3-layer model is adequate for explaining the data closely. The models generally have high velocities, in the 2.5-3.6 km/s range, consistent with the occurrence of exposed igneous and metamorphic rocks. Bootstrapping analysis indicates that a relatively low-velocity thin layer with shear wave velocity of 2.5-3.0 km/s is commonly required just below the surface, bounded at the bottom by a velocity jump of 0.4-0.8 km/s. The second velocity discontinuity is at about 300-400 meters depths. Our results demonstrate the applicability of the phase-velocity measurement and interface-detection methods at high frequencies of up to 20 Hz and illustrate their potential utility in mineral exploration.