Comparison of 2D frequency analysis result: (a–d) results from data using a 20 Hz source wavelet, (e–h) results from data using a 40 Hz source wavelet, and (i–l) results from data using a 60 Hz source wavelet. (a, e, i) Raw data, (b, f, j) high-pass filtered data, (c, g, k) predicted data, and (d, h, l) the difference between raw and predicted data.