The screenshot http://df6nm.bplaced.net/VLF/spherics/dk7fc_VLF_150718_1326.jpg shows a number of narrow tweek-mode resonances at multiples of 1.72 kHz. These are obviously spherics from nearby lightnings, bouncing multiple times vertically between the ionosphere and ground (much the same as clapping your hands between two parallel brick walls). The resonances are rather sharp indicating a high Q-number (ie. around 100 bounces until decay). They are visible up to about 20 kHz, showing unusually small damping of vertical incidence reflections at these ferequencies. There is a small variation of resonance frequency over time, reflecting the variable height of the ionospheric ceiling. The fundamental resonance at 1.7 kHz is probably not visible due to the frequency response of the loop and receiver.