seismic measurement-systems and data acquisition landmarine
TRANSCRIPT
Seismic sources and receivers
On land On water
Sources:Impact
Sledge hammerDrop-weightAccelerated weight
Impulsive DynamiteDetonating cordAirgunShotgunBorehole sparker
Airgun Pinger Gas gun Boomer Sleeve gun SparkerWater gun Steam gun
Vibrator VibroseisVibrator plateRayleigh wave generator
MultipulseGeochirp
Receivers: Geophone, accelerometer
Hydrophones (streamers)
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
Earliest known seismoscope
Used to indicate the occurrence of an earthquake in the year 138(Reynolds)
Characteristic of a geophone
Damping factor: hCritical damping (h=1) is minimumamount required which will stop anyoscillation 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)
Acquisition Techniques
Ocean Bottom Cable
Streamer
Vertical Cable Dragged Array
Anchored Cable
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:fN=1/(2 ), TN= 2 : two samples per period is the minimum!
sampling
12 samples per period
Aliasing
(Kearey and Brooks, 1991)
Nyquist Frequency:
fNy12---=
Typical sampling intervals:0.25, 0.5 ms:
High resolution seismic1 ms, 2 ms Oil exploration4 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
min
maxlog20
A
A
Dynamic range is expressed in dB:
dB481mV
256mVlog20
dB144V1
16Vlog20
Examples:
mV
124816
32
64
128
Analog Signal 53
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: 96Sampling rate: 2 msTime window: 0.8 sFormat: 4 Bytes per value
(800 / 2) Values x 96 channels x 4 Bytes = 0.146 MBytes
Saving requirements