GPH chapter 5

Geophysics Chapter 5: Seismic Reflection: Acquisition, Processing, and Waveform Analysis

1. Overview

• Focus on reflected waves with short offset (X).

  • For small X, reflected wave arrives after the direct wave, making it easier to detect.

2. Source-Receiver Geometry

• The source-receiver geometry varies between refraction and reflection surveys.

  • Reflection Survey:

  • Firing a shot (explosion) at each receiver position to capture near-normal incidence reflections.

  • Example: Fig. 5.2 (shows geometry in a reflection survey).

  • Refraction Survey:

  • Firing a shot and observing the difference in arrival times at distant receivers.

  • Example: Fig. 4.1 (shows geometry in a refraction survey).

3. Seismic Reflection Experiment Data

• Example data obtained at sea illustrating two-way travel time (seconds):

  • 0.1, 0.2, 0.3, 0.4 seconds.

• Receivers and shots closely spaced to provide a cross-sectional image of rock layers.

  • Enhanced visualization of subsurface structures.

4. Comparison of Seismic Methods

• Seismic Reflection vs. Seismic Refraction:

  • Reflection:

  • Offers detailed geometrical insights into subsurface structures, particularly shallow sedimentary layers.

  • Refraction:

  • Commonly used to obtain velocities and layering geometry of deeper layers, such as igneous basement rocks.

5. Types of Seismic Surveys

• Single-channel Seismic Reflection Survey:

  • Involves one pair of source-receiver.

• Multi-channel Seismic Reflection Surveying:

  • One source with multiple receivers.

  • Example: Offshore experiments conducted by Oregon State University involving airguns as sources and hydrophones as receivers.

6. Seismic Data Acquisition Offshore and On Land

• For offshore data acquisition:

  • Water transmits P-waves only.

  • Important to understand the environment of seismic data collection.

• On land:

  • Example of land seismic survey using geophones (measure vertical ground movement).

7. Multi-channel Surveying Challenges

• In a reflection survey, source-receiver distance is usually shorter, complicating the observation of direct wave arrivals.

• The experimental data may not resemble a geological cross-section due to most seismic rays not being normal-incident.

8. Reflected Wave Equation

• The reflected wave relationship: $V = X \cdot \frac{T}{t<em>{f}} + V</em>0$

  • Where:

  • V = Velocity

  • X = Offset

  • T = Total travel time

  • $t_{f}$ = Time to reflect and return.

9. Normal Moveout Correction (NMO)

• CMP Gather:

  • Used to improve seismic data by aligning reflections from different shots.

  • Stacking enhances reflection arrival signals, an advantage of multi-channel seismic surveying.

10. CMP Stacking Process

• CMP stacking performed using a computer to convert seismograms into numerical data.

  • Each number indicates the amplitude at a given time.

• Example of amplitude data: -4, 0, 4, 7, 6, 3, 0, -1.

11. Velocity Analysis

• Various definitions of velocities:

  • Combined thickness of layers and one-way travel time.

  • Root Mean Square (RMS) velocity is used to obtain an aggregate velocity of layers.

• Define average velocity of layers: V{avg} < V{RMS}

  • Because seismic rays spend longer in faster layers.

12. Estimating Depths to Layer Boundaries

• Two-way travel times indicating reflection from various boundaries (e.g., sea bottom, layer boundaries):

  • Sea bottom reflection: 0.12 sec.

  • First rock boundary reflection: 0.18 sec.

  • Second rock boundary reflection: 0.27 sec.

• Requires knowledge of thickness and seismic wave speeds in each layer.

• Depth calculation to layer boundaries using:

  • $Depth = \text{speed of the wave} \times \text{one-way travel time}$

13. Examples of Depth Calculation

• Depth to sea bottom:
  $Depth = \frac{1.5\text{ km/s} \times 0.12\text{ sec}}{2} = 0.09\text{ km} = 90\text{ m}$

• Thickness of rock layers:

  • First layer:
    $h_1 = \frac{2.0\text{ km/s} \times 0.06\text{ sec}}{2} = 0.06\text{ km} = 60\text{ m}$

  • Second layer:
    $h_2 = \frac{2.2\text{ km/s} \times 0.09\text{ sec}}{2} = 0.099\text{ km} = 99\text{ m}$

14. Singling Out Reflections

• Not all reflections correspond to rock boundaries.

• Multiples may confuse interpretations (reflective signals bouncing multiple times).

15. Primary vs. Multiple Reflections

• Primary reflection:

  • Ray reflects once.

• Multiple reflection:

  • Ray reflects multiple times.

• CMP stacking can help mitigate seeing these multiples, improving clarity in the data.

16. Noise Sources in Seismic Data

• Noise sources include wind, animals, human activities on land; at sea: wind, waves, and engine noise.

17. Benefits of CMP Stacking

• Enhances reflection arrival signals, reduces noise, estimates velocities of seismic waves, eliminates certain multiples.

• Stacking allows preprocessing simplicity assuming flat source-receiver arrangements.

18. Seismic Wave Forms

• P-waves and S-waves, both are analyzed for their properties.

• Properties include period, frequency, and wavelength, critical for understanding wave propagation:

  • Period: time for a complete cycle (e.g., period = 0.2 seconds; frequency = 5 Hz).

  • Frequency calculation:
    $ext{frequency} = \frac{1}{\text{period}}$

  • Wavelength is defined as the distance between neighboring peaks or troughs.

19. Sine Wave Properties

• Seismic waves modeled with sine waves, where:

  • Amplitude (A) relates to phase (φ) as:
    $A = \sin(φ)$

• For example, a seismic wave with velocity $Vp = 3 \text{ km/s}$ and frequency $f = 30 \text{ Hz}$ yields a wavelength ($\lambda$): $\lambda = V</em>p / f = \frac{3\text{ km/s}}{30 \text{ Hz}} = 0.1\text{ km} = 100 \text{ m}$

20. Reflection Coefficient

• Determined by the density and velocity of the layers, which influences the amplitude and polarity of reflected waves:

  • Formula for reflection coefficient (RC):
    $RC = \frac{\rho<em>2 V</em>2 - \rho<em>1 V</em>1}{\rho<em>2 V</em>2 + \rho<em>1 V</em>1}$

• Strong reflections occur when density and velocities vary significantly.

21. Waveform Mixing Issues

• If the input wave pulse is too long, different reflected pulses may overlap, complicating the data interpretation.

22. Exercises and Additional Reference Materials

• Exercises focusing on waveforms and reflection coefficients.

• References to skip (pages 126-133) and specific exercises indicated (e.g., Rayleigh wave).