USGS CMG "Seismic" Definition

• "Seismic" information (as used within the CMG InfoBank ) covers a wide range of information including:

  General Type	Description	Equipment
  conventional multichannel	seismic reflection profile data typically acquired with a large multiple airgun array; 10 to 100 Hz spectral content (2 ms sample) and 8 to 20 sec record length; CDP interval typically 25 to 100 meters; 12 to 48 fold.	24 Channel 48 Channel 96 Channel
  high-resolution multichannel	seismic reflection profile data typically acquired with a single small airgun, GI gun or watergun; 50 to 500 Hz spectral content (0.5 ms sample) and 1 to 2 sec record length; CDP interval typically 3 to 5 meters; 6 to 24 fold.	GI Gun Minisparker Small Airgun Watergun
  conventional single channel	seismic reflection profile data typically acquired with a large (10 to 20 kJ large airgun or multiple airgun array; 10 to 100 Hz spectral content and 8 to 10 sec record length; shot interval typically 25 to 50 m; no fold; prior to 1990, rarely digitally recorded.	Airgun Sparker
  high resolution single channel	seismic reflection profile data typically acquired with a small sparker (2 kJ), small airgun, GI gun, or watergun; often includes two-channel data, which are independent receivers; spectral content 100 to 1000 Hz (0.25 to 0.5 ms sample); shot interval 3 to 10 meters.	Minisparker Watergun
  very high resolution single channel	seismic reflection profile data typically acquired with any of a wide variety of electromechanical transducers, broadly classed as wide-band (boomer, geopulse, seistec) or narrow-band (chirp); spectral content 1000 to 10000 Hz (sample rate 8 to 32 kHz); shot (or ping) frequency 0.125 to 0.5 sec (0.2 to 1 meter).	3.5 kHz 12 kHz Datasonics CAP600 Datasonics SIS1000 Chirp Delnorte Geopulse Huntec Boomer Bubblepulser Seistec Uniboom
  wide angle reflection/refraction	seismic reflection/refraction profile data typically acquired with a large airgun source to offsets of 20 to 100 km.	Ocean Bottom Hydrophone Ocean Bottom Seismometer Sonobuoy

• Seismic data are collected by "shooting" a source at either:
  • a regular time interval, or
  • a regular distance interval.
• Each "shot" is an actual sound burst in the water.
• Time associated with shots and with navigation are used for plotting on a map.
• Shots can have other names like:
  • FFID (Field File ID), or
  • PING (usually a shot from a high-frequency source like huntec, chirp, or geopulse).
• These systems use devices such as: airguns, waterguns, sparkers, sonar, and sonobuoys to produce signal pulses.

• "CDPs" (Common Depth Point), sometimes called CMP (Common Mid-Point)
  • CDP's are actual points on the ocean bottom that the ship passes over.
  • CDP's are used with multichannel reflection data for processing.
  • CDP's are not necessary with single channel data.
  • Because a multichannel streamer collects many channels, and each channel is located over a different spot on the earth, the channels must be binned into a common location for processing.
  • When this is done, the channels can then be summed together -- this is called "stacking".
  • Once the data are stacked, the resulting data now contains one summed channel at each CDP location.

• But, how is the latitude and longitude of each CDP location, when the navigation is for the ship at a certain time and no unique time is associated with a CDP?

  

  • A commonly used, rough approximation is to assign a time to the nearest channel (sensor closest to the ship), the time of the shot it sensed. Then use the navigation of the ship at that time as the CDP location.
  • Another approximation is to:
    • use the speed of the ship and the offset of the nearest channel from the ship, to determine the length of time required for the ship to cross a point and then the channel to cross the same point.
    • Then using the ship's location at the (ping_time - offset_time) as the CDP location.
  • A more accurate computation of the CDP location additionally includes adjusting for horizontal and vertical offsets between:
    • the ship's navigation antenae and the sound source equipment on the ship,
    • the ship's navigation antenae and the streamer equipment on the ship,
    • the sound source equipment on the ship and the sound source in the water,
    • the streamer equipment on the ship and the nearest streamer channel to the ship.
  • CDP locations can be calculated using these offset geometries, the course of the ship, and assuming the equipment is being towed behind the ship (not drifting or turning).