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
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=0.6-0.66. Damping can be changed by changing the shunt resistor (Reynolds)
Geophone array Clustered geophones (no filter effect) Geophone arrays
Marine seismic data acquisition PGS J.W. Schoolmeesters
(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
Aliasing (Kearey and Brooks, 1991) Nyquist Frequency: f Ny 1 2 --- = 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
Range which can be measured using different number of bits: 8-bit :1 mV - 256 mV 24-bit: 1 V - 16 V Dynamic range Dynamic range is expressed in dB: Examples:
Working of an AD-converter 1 0 1 0 1 1 0 0 = 1+..+4+..+16+32+..+..
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 = 0.146 MBytes Saving requirements
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-(200-125)=50Hz
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