Geophysical Applications Seismic Reflection Surveying

Transcription

1 Seismic sources and receivers Basic requirements for a seismic source Typical sources on land and on water Basic impact assessment environmental and social concerns EPS435-Potential-08-01

2 Basic requirements Technical: (a) source must have sufficient energy to generate a measurable signal with a good signal-to-noise ratio (b) Short-duration pulse (with high enough frequency content) for the required resolution (c) A source wavelet of known shape (d) Minimal source-generated noise Operational: (a) Efficient and safe to use with minimum maintenance (b) Repeatable source wavelet (c) Cheap (reasonably priced, low operational costs) EPS435-Potential-08-02

3 Seismic energy sources On land On water Impact-type hammer dropped weight Impulsive dynamite airgun, sleeve gun, water gun shotgun sparker, boomer, pinger Vibrator Vibroseis chirp-sonar EPS435-Potential-08-03

4 Sledge-hammer seismic Used in engineering-type investigations; Shallow refraction surveys, detection of ground-water, top of bedrock etc. Hammer is hit on metal-plate, accelerometer triggers recording unit (yellow-box in image on the left). EPS435-Potential-08-04

5 Impact-sources Sparker (single/multiple electrodes), Medium-frequency range up to 1 khz Source not very repeatable Secondary bubble pulse EPS435-Potential-08-05

6 EPS435-Potential-08-06

7 Oscillating bubbles generate secondary pulses Not wanted as part of signal need to be eliminated in field or by processing EPS435-Potential-08-07

8 Airguns (sleeve-gun, water-gun, GI-gun) waveshape-kits Simple device of two chambers of compressed air; upper pressure shuts shuttle and seals off lower chamber; when pressure is released in upper chamber, the shuttle opens and compressed air from the lower chamber leaves in an instant generating seismic impulse; Problem of rising secondary pulses can be reduced by using waveshape-kits, using half of total volume to be released a few ms after initial puls, thus de-flating the primary bubble. Same concept is used in GI gun. Airgun.ppt EPS435-Potential-08-08

9 Airgun-cluster Typical airgun signature EPS435-Potential-08-09

10 Seismic vibrators on land: Small engineering devices (adapted from construction tools) Low impact on environment (compared to dynamite), but low penetration, high absorption, and complicated processing required. EPS435-Potential-08-10

11 Large-scale vibrator trucks deep exploration EPS435-Potential-08-11

12 Often used in convoy logistical problem of synchronization Versatile, terrain-adaptable Low environmental impact Complicated pilot-sweep requires careful post-processing EPS435-Potential-08-12

13 Concept of crosscorrelation of pilot sweep with field recording to get sharp reflections EPS435-Potential-08-13

14 Environmental impact On land: obvious destruction of flora and fauna; damage to buildings, roads etc. dynamite can cause cavities and can leave chemical residue In water: marine mammals are of highest concern In Canada: no source larger than 275 1m can be used without full impact assessment (CEA) impact on mammals can be direct loss of hearing, but also behaviour changes (unable to find food-source, communication to pod-members) regular monitoring during surveys by observers need to be certified by DFO or work with certified observer No full understanding of impact up to date lack of data Each species behaves differently makes assessment complicated EPS435-Potential-08-14

15 Seismic receivers On land: In water: On ocean floor: Geophones (3 components) Hydrophones (single or bundled in streamer) Ocean-bottom-seismometers or Ocean-bottom-cables (multi-components) EPS435-Potential-08-15

16 Geophone: Simple device that measure vibrations of ground. Coupling in ground by spike; requires vertical orientation in deployment, or use gimballed geophones (right picture) Uses principle of magnetic induction to transform motion of a core inside a magnetic field to generate secondary voltage; Groups of geophones are used to amplify signal but cause frequency distortion (array-response) EPS435-Potential-08-16

17 Array-response Geophones or hydrophones are often used in groups or arrays to increase signal-to-noise ratio. However, the fact that the phones are not all at the same physical location distorts the receiver function and it is the art of the survey design to plant the phones in such a way, that the frequency spectrum of the source is maintained as much ass possible over the useful bandwidth. EPS435-Potential-08-17

18 A series of geophones (N=4, d=10 cm) is planted in the ground and all energy received from these phones is stacked (summed) together to represent one trace in a seismogram. The attenuation is apparent only for waves approaching the array at non-vertical orientation: In case (A), all phones receive energy from the impinging wave at the same time, whereas in case (B), the phone on the left receives energy first while all other phones are still not receiving anything. This time-delay and associated interference can be expressed in an attenuation function (called array-effect or directivity pattern): Attenuation (db) = 20 log [sin (π d k N / N sin (π d k)], (EQ 2.31) Where k is wave-number: k = 1 / λ = f / V (f=frequency, V=velocity). Certain frequencies will be completely attenuated (notches). EPS435-Potential-08-18

19 Hydrophones: Use piezo-effect of crystals (quartz) to generate voltage induced by pressure changes. EPS435-Potential-08-19

20 EPS 435 Hydrophones are typically bundled in a streamer that is towed behind the ship. Streamers can be oil-filled or solid, analog or digital. EPS435-Potential-08-20

21 Industry seismic vessels are specifically designed to tow multiple streamers and gun-arrays; requires special positioning equipment to locate streamers. EPS435-Potential-08-21

22 Common problem in marine surveying is feathering drifting of streamers by sidecurrents. Depths-changes also can occur along a 10-km long streamer. Controlled by birds (active/passive), compasses and depth-sensors plus tail radar-buoy. EPS435-Potential-08-22

23 Loss of energy Spherical spreading (divergence) Non-elastic attenuation (Q, α) Scattering at inhomogeneities Spherical spreading Seismic energy spreads spherically away from the source and decreases in amplitude with distance from the source. The total energy emitted at the source is spread over the entire surface of the sphere, thus the larger the sphere, the smaller the energy per unit area with increasing radius (R). The energy is reduced in proportion 1/R 2, whereas amplitude (proportional to the square-root of energy) diminishes in proportion to 1/R. EPS435-Potential-08-23

24 In-elastic effects EPS 435 Some seismic energy is lost due to in-elastic behaviour of the sub-surface rocks. This intrinsic (or visco-elastic) attenuation is described with the attenuation factor α or the quality factor Q. The attenuation coefficient α is related to the seismic velocity (V) and seismic frequency (f) as follows: α = π f / Q V (EQ 2.32) where Q (quality-factor) is defined as: Q -1 = 2 α λ (EQ 2.33) with λ as wavelength. A different definition of attenuation is given by the loss of energy (ΔE) relative to the total energy E per unit wavelength: 2π Q -1 = ΔE / E (EQ 2.34) EPS435-Potential-08-24

25 Summary-cartoon of different types of energy loss From Reynolds, 1997 EPS435-Potential-08-25