OFFSHORE OIL EXPLORATION BY SATELLITE?

The signal strength received by a dish antenna is proportional to its area. Every square inch of it, so to speak, listens and passes its message on to the receiver.

But if the message changes only very slowly, one can receive by an antenna covering only a narrow strip, move the antenna over the area (or "aperture") and by a certain method of averaging figure out afterward what the signal was over the entire area. This will give a signal as good as from the entire area at the cost of only the strip. The method was first used in radio astronomy to listen to the slowly changing message of distant radio stars.

Quite similarly, one can use a "synthetic aperture radar" (SAR) with a wide, thin beam to scan an area (like a moving spray wagon wetting a long street with a thin spray), and a computer can then evaluate the signal as well as if it had been received by an enormous antenna.

A satellite with this type of radar examining the ocean surface stunned scientists when it sent back fairly clear pictures of the ocean floor up to 300 ft below the surface, although radar waves of a few centimeters, if they know their physics, cannot penetrate salt water.

What seems to be happening is that the sea floor modifies the surface; for example, currents will flow faster where the water is shallower, and this will cause hydrodynamic stresses producing certain kinds of short water waves. They will bounce back the radar waves in a distinctive way that appears only when the signals are processed, providing an image of the sea floor that caused them.

This way of surveying the sea floor in shallow places (up to 300 ft) by airborne or satellite radar is much quicker and cheaper than by acoustic soundings from a ship. It will doubtlessly have beneficial indirect effects on offshore oil exploration, though it cannot detect oil by itself.

[More: A picture and an article appears in Popular Science, Sept. 1982; our source: interview with scientists from the Office of Naval Research.]