Close-up view of the LSPs in the low-frequency range. The upper panels show the LSPs of the four data sets in that region. The red horizontal line indicates our adopted detection threshold of 5.2 times the local median noise level. The black dots mark the position of signals with S/N larger than 5.2. Black circles are possible signals just below the threshold. The lower panels show time-frequency maps built from the LSPs of each sector in the same frequency range. Amplitudes are colour-encoded according to the scale shown in the colour bar on the right-hand side.