Prospectors locate oil in a number of different ways. Scientists can send sound waves through rock. The waves travel at different speeds through different types of rock. Scientists listen to the sound waves with geophones and measure the speed at which the wave travels. Using this measurement they can tell what kind of rock it is and where oil might be located. To generate the sound in the first place, scientists use vibrator trucks, explode small charges, or, at sea, shoot compressed air guns. Another method prospectors use is to employ a sniffer. This funny sounding device detects gaseous hydrocarbons (part of oil) escaping through the ground. Scientists may also measure how an electric current moves through the rock, or will even look at rock cores, or samples. These cores will be viewed under a microscope where scientists can see tiny oil droplets.
But, with all this equipment, it still takes a lot of good luck. An exploratory well drilled a mile or more from a proven well has a 1 in 10 chance of striking oil. A well drilled in an unproven area has a chance of 1 in 40.