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Seismic Measurement-Systems And Data Acquisition LandMarine

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Seismic Measurement-Systems LandMarine

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Seismic System SourceReceiver Computer Seismic waves

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Seismic sources Important properties: Energy Waveform Repeatability Cost and use in the field

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Seismic/Acoustic spectrum

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Seismic sources and receivers On landOn water Sources: Impact Sledge hammer Drop-weight Accelerated weight ImpulsiveDynamite Detonating cord Airgun Shotgun Borehole sparker Airgun Pinger Gas gun Boomer Sleeve gun Sparker Water gun Steam gun VibratorVibroseis Vibrator plate Rayleigh wave generator Multipulse Geochirp Receivers:Geophone, accelerometer Hydrophones (streamers)

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Hammer

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Shear wave hammer

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Dynamite (explosive impulsive source): 40% of the seismic measurements Not really repeatable Exact time of detonation is difficult to obtain Detonators are sometimes used for shallow applications High energy For each application the amount of dynamite can be adjusted dynamite Trigger cable

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Betsy gun Shot fired into a protected chamber

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Vibrator:Vibroseis truck

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Reynolds, 1997

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Earliest known seismoscope Used to indicate the occurrence of an earthquake in the year 138 (Reynolds)

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Principle of a geophone

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Characteristic of a geophone Damping factor: h Critical damping (h=1) is minimum amount required which will stop any oscillation of the system from occurring. Most geophones are slightly underdamped, typically around h= Damping can be changed by changing the shunt resistor (Reynolds)

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Geophone array Clustered geophones (no filter effect) Geophone arrays

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Marine seismic data acquisition PGS J.W. Schoolmeesters

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Inside of an airgun (Bolt-Systems)

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Response from an airgun

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Air gun array One air gun Array of air guns

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Sonobuoy

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Chirp: Marine Vibrator Hz Sweep of 6 seconds

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Hydrophone Principle of piezoelectric effect Voltage proportional to the variation of the pressure

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Acquisition Techniques Ocean Bottom Cable Streamer Vertical Cable Dragged ArrayAnchored Cable PGS J.W. Schoolmeesters

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Marine streamer

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Multi-channel seismic recording system

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(Kearey and Brooks, 1991) Registration of the measured data at certain time intervals Sampling interval sampling rate (sampling frequency) 1/ Sampling will preserve all frequencies up to the Nyquist frequency: f N =1/(2 ), T N = 2 : two samples per period is the minimum! sampling 12 samples per period

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Aliasing (Kearey and Brooks, 1991) Nyquist Frequency: f Ny = Typical sampling intervals: 0.25, 0.5 ms: High resolution seismic 1 ms, 2 ms Oil exploration 4 ms or larger Crust seismic 8 samples per period 6 samples per 5 periods two samples per period are needed to avoid aliasing

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Range which can be measured using different number of bits: 8-bit :1 mV mV 24-bit: 1 V - 16 V Dynamic range Dynamic range is expressed in dB: Examples:

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Working of an AD-converter =

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Saving requirements depend on: Number of channels Number of values per channel (Sampling rate,Time window of sampling) Number of bytes per sampled value Example Channels:96 Sampling rate:2 ms Time window:0.8 s Format:4 Bytes per value (800 / 2) Values x 96 channels x 4 Bytes = MBytes Saving requirements

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Question Sampling at 4ms intervals What is the Nyquist frequency In absence of anti-alias filtering, at what frequency would noise at 200 Hz be aliased back into the Nyquistinterval? f N =1/(2 )=125 Hz 125-( )=50Hz

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