Processing flow

We processed the raw data records with the following steps:

1.

Notch-filtering to remove spectral spikes (e.g. 60 Hz noise).

2.

Sort into (x,y,t) cube.

3.

Zero pad x and y-axes.

4.

3-D autocorrelation or spectral factorization Rickett and Claerbout (2000).

5.

Stack over record axis.

6.

Stack over radius (distance from zero spatial lag).

7.

Low-pass filter to 30 Hz.

8.

Offset dependent gain correction [$d'(h_i)=h_i \, d(h_i)$] for display purposes.