For synthetic data generation, a test configuration (Fig. ) includes 49 receivers on the surface and 16 sources at 50km depth, uniformly-distributed in 2-D grids . The receiver grid is 7 × 7 at 15 km , and the source grid is 4 x 4 at 40 km spacing in both directions.
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The cell size of 1km x 1km x 1km was used for simulation.The simulated domains vary from 200x200x100 to 300x300x150. For both domain, the low-velocity sphere (Vs=1.25 km/s) centered at 25km depth, with 15km radius is embedded to a homogeneous model of stiffer material (Vs=2.5 km/s).
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The waveform data are generated using a Gaussian source at 1Hz central frequency. A uniform filter is applied to the simulated wavefields including the seismograms at the station locations and the strain-wavefield. For the first inversion run, the filter has a high-cut frequency fmax=0.05Hz. For this frequency band, the phase difference between observed data (according to the true model) and simulated data (according to the initial model). The reversed-in-time displacement residuals between the observed and simulated data for all 3 components (in x, y and z directions) are used as the adjoint sources for backward simulations from the receiver locations.
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Seismograms filtered at 0.05Hz, according to the true and initial models
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