Seismic Reflection Surveys

Seismic reflection surveys provide a relatively efficient survey tool to investigate the structure inside the earth, including under the seafloor.  They are active systems that transmit a sound pulse, and measure the reflections that return when there is a change in the rock layers.   The amplitude of the reflection is based on the acoustic impedance Z (density times sound velocity).

R = (z2-z1) / (z1 + z2)

The largest reflection will come from the water sediment interface, but unconformities and faults and major changes in the sediment composition or compaction will also create reflections.

Subbottom profilers record relatively shallow subsurface features.  The initial instruments used a single frequency in the 1-20 kHz range.  New systems use a swept frequency, or chirp pulse, which can achieve higher resolution and greater penetration by using digital signal processing techniques.  A single transducer both transmits the pulse and records the reflected pulses.

 

Impulse systems (air guns, water guns, sparkers, or boomers) generally generate the sound and receive the returns with separate hydrophones.  Using an array of hydrophones allows each feature to be imaged multiple times by successive hydrophones, and the results can be "stacked" to increase the signal to noise ratio.


Wikipedia

 

Wikipedia (from USGS)

Seismic interpretation

References:


Last revision 12/3/2014