Seismic Images of the Earth

Seismic images can reveal remarkable pictures of the earth's subsurface geology. Such pictures can be used to understand and identify faults at deep depths, and therefore help us identify the location of oil and gas deposits. Figure 1.2 shows some seismic pictures of faults taken by optical and seismic cameras. The explorationist is keenly interested in such faults because the lighter-than-water oil+gas will flow along a layer towards a topographic high until it is stopped by some permeability barrier such as a fault. There it accumulates in large reservoirs, waiting to be found by the explorationist with the most capable seismic camera.

  

Figure 1.2: Geologic faults revealed by a 100 m x 100 m road cut on left and 8 km x 4 km seismic section from a marine seismic survey on right. Seismic section (courtesy of Dave Lumley's homepage) in the X-T plane shows faults denoted by dark solid lines.

A fault is not a sufficient condition for a hydrocarbon reservoir, so one must look for additional hydrocarbon clues in the seismic image. Such clues can sometimes be found in the brightening of the reflection amplitude. Gassy marine sandstones will typically reflect more energy than non-gas sands, so a gas can reveal its presence by a large amplitude bright spot in the seismic section. A bright spot example from the Gulf of Mexico is shown in Figure 3 where the gas-sand reflection is much "brighter" than the surrounding reflections. This brite spot technology represented one of the technological breakthroughs in the 1970's that led to a significant increase in the hydrocarbon discovery rate in the Gulf of Mexico and other marine environments.

  

Figure 1.3: Brite spot, or large seismic amplitude in the seismic section indicating a large reservoir of gas. Vertical axis is in seconds, and 1 second indicates a depth of about 5,000 feet (courtesy of Conoco).

Another breakthrough in seismic exploration, 3-D reflection experiments, came about with the advent of fast computers in the mid-1980's. Instead of just placing geophones and sources along the same line, 3-D seismic experiments distribute geophones and sources on a large 2-D patch of ground. A large 2-D grid of geophones is activated for each source location so that 3-D seismic scattering from out-of-the XZ plane could be properly accounted for to reveal geologic complexity in unprecedented detail. An example is given Figure 1.4, where a 3-D reflectivity cube of data is sliced horizontally to reveal a meandering river channel at a depth of more than 16,000 feet!. River channels can make oil geologists gush with excitement because they contain highly porous sands that can quickly sop up vast pools of migrating hydrocarbons.

  

Figure 1.4: Seismic 3-D image cube XYZ space showing a horizontal XY slice of a meandering river channel at a depth of more than 4 miles. The color variations correspond to variations in reflectivity. This picture might look similar to an airplane view of the ground if you flew over the Mississippi River at a 20,000' elevation. The ability to "seismically see" within the earth to depths of many miles is one of the great technological achievements of the 20th century (modified from Dave Lumley's homepage)!