Land seismic sources

Transcription

1 Seismic Sources HOW TO GENERATE SEISMIC WAVES? Exploration seismology mostly artificial sources à active technique Natural sources can also be used (e.g. earthquakes) usually for tectonic studies (passive seismic exploration)! What is a good source? - economical, efficient, convenient - safe and environmentally acceptable - sufficient energy over suitable frequency range - repeatable

2 Land seismic sources Explosives: - usually detonated in boreholes or buried PROS - sharp, impulsive, high amplitude (mostly P-wave) - reasonably cheap CONS - signal is not repeatable - slow (borehole drilling) - can be destructive

3 An impulsive signal can also be generated by: - hammer on a metal plate (Lab 3) - weight dropped from truck or helicopter - firearms - shotguns, rifles - building implosions - nuclear explosion tests ( ships/nyt2001.html) ( Monitoring/News/nkorea_2006.html)

4 Peaceful Nuclear and chemical bomb tests were conducted by the former Soviet Union ( ),some profiles have produced prolific information on the crust and upper mantle structures. Ryberg et al.,

5 Vibroseis Method ( Thumper trucks ) ayer-1heavy truck that transmits low amplitude vibrations - sweep over a range of frequencies (10-100Hz) for s us.jetpcb.com ayer 2 2 (

6 Vibroseis Method ( Thumper trucks 10+ MPa) PROS - well-defined, repeatable - little disturbance - can also create S-waves! CONS - need hard surface - need accessible areas - need to buy trucks ($$$) (

7 Processing Vibroseis Data - signal is spread out over time therefore reflections of interest will also be spread out - can isolate reflections using cross-correlation - works well even with lots of noise 2 Vibroseis signal

8 Marine seismic sources Explosives - generally not used Air gun - most common source - bubble of compressed air PROS: - reliable, repeatable, inexpensive CONS: - bubble pulse oscillates - only P-waves are created (why?) - may be harmful to marine animals 1 air gun 7 air guns (Kearey et al. 2002)

9 Air gun arrays: 1. minimize reverberation 2. Recover all frequencies (10s of MPa pressure).

10 Sparkers, Pingers, Boomers - high frequency, electrically produced source - order of increasing strength (pingers, sparkers, boomers, airguns).! PROS: - well-defined, repeatable, inexpensive! CONS: sample squid sparker ACD Technology - high frequency (10-100,000 Hz) à rapid attenuation! - good for shallow seafloor mapping / surveying

11 woodshole.er.usgs.gov The sparker is a relatively higher powered sound source, dependant on an electrical arc which momentarily vaporizes water between positive and negative leads. The collapsing bubbles produce a broad band (50Hz - 4 khz) omni directional pulse.

12 Seismic sources cover a wide frequency range (Kearey et al., 2002)

13 Seismic detectors (chapter 3) - convert ground motion into an electrical signal (recorded) - displacement, velocity, acceleration - 3 components - for seismic exploration - source is usually a P-wave that arrives near-vertical - in the past, record only vertical motion (fast changing) - recording system timing must be accurate (< 1 ms), gps is being used for 3-component seismometers.

14 Moving-Coil Geophone (most common) - coil inside magnet - relative motion of coil and magnet due to ground motion produces electric voltage - voltage proportional to velocity Seismometers - similar to geophones, more sensitive to lower frequencies - Usually 3 component - earthquake/microseismic studies

15 Deployment of our group: (CRANE seismic array) Trillium 240

16 Temporary Seismometer Setup Gu et al., 2011

17 Marine detectors Hydrophones -detect a change in water pressure -convert to electric signal - only record P-waves - usually towed behind a ship on a neutrally buoyant streamer - pressure sensors, compasses, tail buoys - adjust depth with fins - often multiple streamers are used to get 3D coverage!

18 Ocean Bottom Seismometers (OBS) - seismometer/geophone in a metal casing - dropped overboard and sink to seafloor - recovered by releasing weights - can contain 3 component seismometers - hard to position accurately - limited life (battery, disk space) - deep-sea fishing issues - rotation induced direction uncertainty ( operations/obs/) (

19 Sample OBS system diagram Release transducer hydrophone battery digitizer disk Upper sphere sensor Lower sphere hook anchor

20 Seismic Reflection We will focus P-waves in this class but the same ideas apply to S-waves.

21 4.1 Reflection at Normal Incidence (section 2.3.3) Reflection coefficient: R = A r /A i! Transmission coefficient: (refracted wave)! T = A t /A i Note that these coefficients give the relative amplitudes of all the waves - usually assume that incident wave amplitude is 1

22 Reflection at Normal Incidence Partitioning of energy into reflection and transmission depends on: the angle of incidence the acoustic impedance of each layer:! Z 1 = ρ 1 v 1 Z 2 = ρ 2 v 2

23 Zoeppritz Equations For normal incidence, can show: Z Z R = 2 1 = 12 Z + Z 2 1 2Z T = 1 = 12 Z + Z 2 1 A A r i A A t i Note that: R 12 + T 12 = 1

24 Z Z R = 2 1 = 12 Z + Z 2 1 A A r i 2Z T = 1 = 12 Z + Z 2 1 A A t i Z 1 =Z 2 à no reflection (does NOT mean v 1 = v 2 ) ρ 1 =ρ 2 & R: -1 to +1 generally ±0.2 or less sign = polarity negative when Z 1 >Z 2 T is positive, can be >1

25 CASE 1: Velocity increase with depth Assume (unless otherwise stated): initial amplitude is 1 velocity and density are constant in each layer no geometrical spreading, attenuation, scattering What are A r and A t? What time does the reflected wave arrive back at the surface?