Engineers spent hours on a seismic survey, creating a map of the rock under the sea floor to spot potential dangers, like gas pockets. It also provides a baseline to compare with later surveys during and after the test to see if the pressure on the well is causing underground problems.
An unstable area around the wellbore could create bigger problems if the leak continued elsewhere in the well after the cap valves were shut, experts said.
It was unclear whether there was something in the results of the mapping that prompted officials to delay. Earlier, BP Vice President Kent Wells said he hadn't heard what the results were, but he felt "comfortable that they were good."