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Transcript

GROUP 1

RAY

  • A ray is an arrow perpendicular to the wave front.
  • Indicating the direction of travel at that point on the wavefront.
  • There are an infinite number of rays on a wave front.

THEORY

MUHAMMAD SALIHIN BIN JAAFAR A141372

FAUZIAH BINTI YAMAY A140013

FATIMATUL NAIMAH BINTI HANAFI A141201

NOR NAZATUL SYIMA BINTI MUHAMAD A142033

SEISMIC WAVEFRONT

FIVE IMPORTANT CONCEPTS

Seismic Wavefront

  • Ray
  • Huygen’s Principle
  • Snell’s Law
  • Reciprocity

SEISMIC REFRACTION

  • The surface surrounding the advancing wave is called a wave-front.
  • The speed at which a wavefront travels is the seismic velocity of the material, and depends on the material’s elastic properties.

In order for the seismic refraction method to accurately estimate velocity layer depths, certain natural conditions should exist:

a.) Layers should increase in velocity and in thickness with depth. A typical example would be ten feet of unsaturated soil at 1,500 fps overlying 50 feet of saturated soil at 5,000 fps that overlies bedrock at 16,000 fps.

b.) There should be a sufficient velocity contrast between different layers. Ideally, each velocity layer would be 2 to 3 times faster than the overlying layer.

c.) The velocity within a layer should be relatively constant throughout that layer (lateral homogeneity). In addition to these conditions, it is also important that there be a low level of background noise at the site. Nearby road traffic noise or heavy equipment operations may delay the field survey. Ground truth data, if available, will be used to compare and calibrate the seismic information. Under favorable conditions seismic refraction results can be fairly precise, within +/- 10 percent.

3) Determining the physical properties of the bed rock. (design foundation)

Application Of Seismic Refraction

4) Detecting buried structures of small dimensions. (location of structure)

2) Mapping of sedimentary basins. (sediment transport)

1) Determining lateral extensions of layers. (percentage of total lateral flow contribute by each soil layer)

5) Determines rippability parameters (properties of rock)

6) Locating a water table (design foundation)

VIDEO

SEISMIC REFRACTION

2. Geophones and cable

  • Transform the P-waves energy into a voltage that can be recorded by the seismographs.
  • The natural frequency of the geophones varies from 4Hz to 14Hz (have flat frequency response between 4 Hz to 14 Hz)
  • Signals (geophones to seismographs) brought through geophones cable.

  • Refraction surveys use the process of critical refraction to determine interface depths and layer velocities.

  • Critical refraction requires an increase in velocity with depth. If not, then there is no critical refraction

  • Hidden layer problem will be faced.

  • 1.3 signal enhancement
  • Improves the signal to noise ratio is available in most seismographs.
  • Accomplish by adding the refracted seismic signals for a number of impacts.

EQUIPMENT

SET - UP

Interpretation of Refraction Travel Time Curves

5. Drill shot holes and bury explosive shots.

6. Monitor ambient noise and conduct seismic shots along survey segment.

7. Inspect integrity of shot data - repeat if necessary.

8. Move equipment to next survey segment and repeat steps 3-7 above.

9. Process and interpret seismic data.

10. Prepare profiles and report.

1. Seismographs

1.1 single channel seismographs

  • Simplest seismic instruments used with single geophones.
  • Geophones usually placed at a fixed location and grounds is struck with hammer at increasing distance from the geophones.
  • Wave arrival time are identified on the instrument.

1.2 multiple channel seismographs

  • Use 6,12,24,48 or more geophones.
  • Wave forms are recorded simultaneously for all geophones.

THREE HORIZONTAL LAYERS CASE

1. Follow the sampling pattern outlined in the Work Plan

2. Locate, mark and clear survey lines.

3. Layout seismic cables and set geophones.

4. Setup seismograph and computer and test geophone responses.

Crossover Distance (Xc)

The offset at which critical refraction becomes first arrival

Determining the Depth of each layer

3. hydrophones

  • Detector which is sensitive to variation is pressure.
  • Sensing elements (piezoelectric ceramic materials)
  • Eg: barium titanite, lead zirconate, lead metaniobate
  • High impedance device and signal from hydrophones
  • Hydrophones array maybe passed through pre-amplifiers or impedance

4. Energy sources

  • Selection depend on the depth of investigation and geologic condition.
  • 4 types energy sources commonly used

- Sledge hammer

- Mechanical weight drop or impact devices

- Projectile (gun) sources

explosive

Thank You!

Note: why we take at critical angle

Data

  • Because the wave reflected at the critical angle simply propagates along the refractor about which we would like to obtain information. The waves produced in this way are called HEAD WAVES.

  • Although a Head Wave must travel along a longer path than the direct arrival before it could be recorded at the surface, it travels along the bottom of the layer at a faster speed than the direct arrival.

  • Therefore, Head Waves can be recorded prior to the time of arrival of the direct wave at certain distances
  • Data will be processed and interpreted using the RIMRock Geophysics SIPT-2 (formerly U.S.G.S. SIPT-2) seismic interpretation program.
  • This program calculates seismic velocities by regression and by the Hobson-Overton method, and solves for layer thickness using the delay-time method and iterative ray tracing modeling.
  • Reporting Profiles of each seismic refraction line will be produced that show the interpreted results of the survey.
  • Typically, the profiles will show three interpreted layers: unsaturated overburden, saturated overburden, and bedrock.
  • Abrupt lateral changes in the bedrock seismic velocity can be identified that may indicate fractures or rock type changes. Any such features will also be shown on the seismic interpretation profiles.

Snell’s Law

BASIC THEORETICAL PRINCIPLES OF SEISMIC METHODS

Huygens' Principle

Every point on a wave front can be thought of as a new point source for waves generated in the direction the wave is traveling or being propagated.

the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media

Critical refraction concept:

  • When the velocity in the upper layer is lower than in the underlying layer, there is a particular angle of incidence, for which the angle of refraction is 90°.
  • Seismic refraction makes use of critically refracted, first-arrival energy only. The rest of the wave form is ignored

If V2>V1, then as i increases, r increases faster

ECG

62

bpm

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