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USGS CMG "Seismic" Definition

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  • This documents InfoBank's usage of "seismic"

  • This is the National Archive of Marine Seismic Surveys (NAMSS).

  • This site is independent of InfoBank.

  • This site uses InfoBank to provide data.

  • "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
    Small Airgun
    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
    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
    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
    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

  • 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?
    Illustration of multiple raypaths
    • 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).

  • On maps:
    • "shots" can be plotted for single channel or multichannel data
    • "CDPs" locations can be used only from multichannel data.

  • Seismic information occurs in the InfoBank as:
    scattered < Activity-ID >.4## files containing time sequential information along ship tracks.
    metadata information about seismic analog paper rolls, inventory of gear, operation of gear, magnetic tapes, and paper printer listings

  • InfoBank seismic data file names:
    <Activity-ID>.40# Raw seismic shot points
    <Activity-ID>.42# Edited seismic shot points

  • The <Activity-ID>.4## files do not include navigation.
  • Navigation must be obtained from <Activity-ID>.0## or <Activity-ID>.6## files.
  • Navigation should therefore also be extracted when you retrieve these files.
  • A "time" key is associated with each record for correlation with other scattered information datasets.

  • "Don't find fault, find a remedy." -- Henry Ford

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