Which technique was used to paint the picture?
I believe the background was created by the use of randomly layering acrylic paints from darkest to lightest on the background canvas using something like sea sponges, waiting for each paint layer to dry, then adding the next lighter color. Turn the sponge frequently &amp; use different sponges as well so that you get more random patterns all over. Given that this is acrylics, you may not have to wait for the paint to dry; it won't mess up the layer below like watercolor would. 

The "lines" definitely look like slightly thinned paint dropped on and the canvas tilted to allow the paint to run in a line.  A spatula knife may have been used to pull the paint into lines as well - use it to put a small dot of slightly thinned paint on the canvas, tilt the canvas if not already tilted, and pull the spatula knife towards you and/or in a downwards direction in a relatively straight line.