Increasing SNR during the stacking period. Amplitude spectra of E_x at Rx6 obtained by stacking for 3 (left), 20 (centre), and 192 h (right); (a) and (b) show different frequency ranges of 0.1–0.4 and 46.0–46.4 Hz, respectively. Blue squares indicate signals at 0.2 and 46.1 Hz from the NS dipole; red squares show signals at 0.3 and 46.3 Hz from the EW dipole. Results are obtained using the weighted stacking method.