Svadhishthana chakra, the second of the seven chakra beginning from the Root(from below) which stands for Creation.
When I initiate the meditation phase of each yoga class, we usually chant OM 3 times, and then sings the Patanjali invocation. Normally I sit on 2 wooden blocks so my knees are below the bellybutton's level for sure. Just chanting OM all in itself soothes me enough. I feel like this on picture most of the time.