Seismic Measurement-Systems

Land Marine

Seismic System

Source Receiver

ComputerSeismic waves

Seismic sources

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

Seismic/Acoustic spectrum

Seismic sources and receivers

MultipulseGeochirp

VibroseisVibrator plateRayleigh wave generator

Vibrator

Hydrophones (streamers)Geophone, accelerometerReceivers:

Airgun PingerGas gun Boomer Sleeve gun SparkerWater gun Steam gun

DynamiteDetonating cordAirgunShotgunBorehole sparker

Impulsive

Sledge hammerDrop-weightAccelerated weight

Sources:Impact

On waterOn land

Hammer

Shear wave hammer

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

Betsy gun

Shot fired into a protected chamber

Vibrator:Vibroseis truck

Reynolds, 1997

Earliest known seismoscope

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

Principle of a geophone

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)

Array of geophones:“destructive and constructive interference”

α

Apparent velocity:sinα

vvapp =

fvapp

app =λApparent wavelength:

∞=appv∞=appλ

v=appv

fv

=appλ

( )β sin)(nβsin

=RResponse function:

where:

app

xλπβ ∆

=

And n is the number of geophones in a group

x∆ x∆ x∆ x∆

Array response function

apparent

Geophone array

Clustered geophones(no filter effect)

Geophone arrays

Marine seismicMarine seismicdata acquisitiondata acquisition

PGS J.W. Schoolmeesters

Airgun

Principle of an Airgun

(Bolt-Systems)

Inside of an airgun

(Bolt-Systems)

Response from an airgun

Air gun array

One air gun Array of air guns

Sonobuoy

Chirp:MarineVibrator5-100 HzSweep of 6 seconds

Boomer

(Huntec-systems)

Hydrophone

Principle of piezoelectric effect

Voltage proportional to the variation of the pressure

Acquisition Techniques

Ocean Bottom Cable

Streamer

Vertical Cable Dragged Array Anchored Cable

PGS J.W. Schoolmeesters

Marine streamer

PGS J.W. Schoolmeesters

Tail Buoy

Processing room

Multi-channel seismic recording system

(Kearey and Brooks, 1991)

Registration of the measured data at certain time intervals

Sampling interval τsampling rate (sampling frequency) 1/ τSampling will preserve all frequencies up to the Nyquist frequency:fN=1/(2 τ), TN= 2 τ : two samples per period is the minimum!

sampling

12 samples per period

Aliasing

(Kearey and Brooks, 1991)

Nyquist Frequency:

fNy12---τ⋅=

Typical sampling intervals:0.25, 0.5 ms:

High resolution seismic1 ms, 2 ms

Oil exploration4 ms or larger

Crust seismic

τ

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 µV - 16 V

Dynamic range

⎟⎠⎞

⎜⎝⎛

min

maxlog20AA

Dynamic range is expressed in dB:

dB481mV

256mVlog20 =⎟⎠⎞

⎜⎝⎛

dB144V1

16Vlog20 =⎟⎟⎠

⎞⎜⎜⎝

⎛µ

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

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?fN=1/(2τ)=125 Hz

125-(200-125)=50Hz