- looks at applications of seismic methods in geological mapping
Main objective
- to enable the eventual integration of seismic data with other geological data by giving insight into the basis for seismic velocity differences between rocks, the detection of those differences, but principally by understanding the operations behind the production of seismic sections.

• Describe physical basis of seismic wave propagation in rocks
• Discuss simple theory associated with refraction and reflection measurement methods
• Show responses for simple models
• Case histories

• Response of material to deforming stresses is described by its Elastic Properties:
  • Bulk Modulus (response to compression)
  • Shear Modulus (response to twisting)
  • Poisson's Ratio
    • Specifically, for small deformations in short times

These are examples only!

• These properties apply to specific samples of competent rock mass
• Actual ranges are very much wider
• Property contrasts are important
• Elastic properties are of more interest to Engineers

  What do geologists do?

• Elastic Properties affect
  • the speed of seismic (sound) waves
  • the amount of seismic (sound) energy transmitted through interfaces
• The speed and amplitude of the waves can be measured at the Earth's surface.

• Two kinds of elastic waves exist in most materials
  • Compressional Waves (Push waves)
  • Shear Waves (Twisting waves)
    
    
• P waves (compressional) are faster, so generally easier to observe, and will be considered solely here
• S waves are not transmitted by fluids

• Chiefly measure propagation of compressional (P) waves
• P velocity depends on elastic constants
  
  
• 
• 
  
  
  • E is Young's Modulus
  • sigma is Poisson's Ratio
  • rho is density

Vp for selected rock materials

• At interfaces, where seismic speed changes, sound (seismic) waves might be
  • Reflected
  • Refracted
  • but usually both!
• That is, some energy usually passes through the interface.

Passing through interfaces, where the seismic speed changes, in general waves are refracted.

Snell's Law applies:

Passing through interfaces, where the seismic speed changes, in general waves are reflected and transmitted.

Energy split depends on both speed and density contrast (and angle).

Seismic Overview

• Widely used in Engineering Geology
• Reports near-surface P-velocity distribution
• Source of energy is mechanical impulse (hammer, explosives)
• Geophones (detectors) are "microphones"

• Waves from source critically refracted twice at interface, then detected by geophones
• Plot of wave traveltime vs distance from shot gives seismic velocities, depths of interfaces
• Closer analysis also gives variation in these

This is a very early (c 1930) record -

several channels, some distance from shot

Sudden collapse under house near old coal workings in Seattle.

• Where is the unsafe ground?

• Seismic energy can travel from source to detector
  • directly
  • via refracted paths, following Snell's Law.
• This allows detection of, and measurement of properties of, layers near Earth's surface.

Earth Division into Core, Mantle, Crust
Subdivision of these
Location of Earthquakes

• All are mainly based on measurement and use of seismic waves through refraction-style models

• Seismic experiments involve waves with frequencies of 1-100 Hz
• The speeds are a few km per second
• The wavelengths therefore are of the order of 10-100 m (and more)
• "Interfaces" are property-changes over distances less than this.

Seismic reflection reminder

• Seismic energy is reflected (echoed) from interfaces as well as transmitted.
• Strength of reflection depends on
  • Seismic speed contrast across interface
  • Density contrast across interface
• Transmitted energy can be partially reflected by still-deeper interfaces

• Travel-time, strength of "echoes" from layers below source measured
  • Travel-times give relative depth
  • Strength gives property contrast
• Repeated at close spacings along profile (compare echo-sounder)
• Result is seismic section, which mimics layer distribution in subsurface

• Interface (perhaps many) with depth variation small compared with average depth
• Seismic "experiment" repeated at frequent intervals

• The "model" so far resembles an echo-sounder.
  • The vertical scale is time
• We need to know seismic wave speeds, to get geometry.
• We do this, and reduce noise, by acquiring more data...

• Each geophone (channel) produces a graphical output or trace
• To correlate from trace to trace, half of the trace is often filled in.

Simple geometry: simple outcome

Measure times at known distances:

• find speed to reflector
• find depth to reflector

• Knowing the velocity (distribution)
  • Remove the "Normal Moveout" (the timelag on reflections)
  • Add the results from each shot
  • Increase the signal-to-noise ratio
• This is really necessary in most cases, because the reflected energies are low.

• "Stacking" (as in previous screen) results in simulated coincident shot/receiver outcome.
• Displaying results of many "shots" as shown earlier generates a seismic section.
• Sections will still usually have time as "depth" coordinate.

Offshore Foundation Study

• Shallow seismic reflection widely used for marine surveys prior to dredging or raising offshore structures (such as drilling platforms!)
• Some drilling used to control interpretation of seismic data
• significant cost savings over grid drilling

(from Geotechnical and Environmental Geophysics)

Shallow seismic section

• This is still single-receiver data, but shows construction of section.
• Source impulses (shots) at 50 m intervals along profile.
  • Note continuity of reflections.
  • Note "character" of reflections.

Exploration in deeper basins (example: Bass Strait, Otway Basin) may involve 1000+ detectors at 50 m intervals for each source point, source points at 50 m spacing, a new shot every 30 seconds, each shot producing ~10 Mb of data.

• Data processing is important!

• Widely used in hydrocarbon search, because reflection surfaces usually relate to sedimentary layering
• Data acquisition, processing produces accurate images of reflector geometry
• How is this interpreted?

Reflection seismic

Structural interpretation

• Viewing seismic section as (scaled) image of subsurface gives significant structural interpretation capability.
• Tying to a well usually necessary to produce quantitative identification.
  • Reflections rarely identify themselves!

Next screen shows

• Single-channel data
• Section split to allow
  • Insertion of lithological log from borehole
• Correlation with reflectors

possible groundwater sources

• Process of identifying reflectors
• tracing where continuous
• relocating where discontinuous (for example, at faults)
  
  
• Preliminary stage in constructing geological interpretation.

• Attribute, usually related to amplitude
• Determined by nature of geological contrast
• Example
  • Shale beds usually continuous, hard, so
  • High-amplitude, continuous reflections

Overhead-transparency demonstration of picking/correlation/character based on examples from Coffeen (Interpreting Seismic Data Workbook).

• Major tool in oil,gas exploration
• Hydrocarbons trapped in deformed beds (for example, folds, domes)
• Relative travel-time of reflections shows relative depths
• Structural interpretation possible up to several km below surface

Seismic stratigraphy

• Further information gleaned from seismic sections, by
• explaining geometry of seismic reflectors in terms of geological events which led to them.
• What are seismic reflectors?

• Each reflecting surface was a chronostratigraphic unit, since
• Each reflector was either
  • a surface on which deposition was occurring (stratal surface), or
  • a surface on which erosion was occurring (unconformity)
• "everywhere" at the same time.
• Note-not necessarily same process everywhere

• Inspection of sections shows
• Existence/occurrence of "packages" of reflections with geometries consistent with recognisable (sedimentary) processes ("sequences").

• Note the external shape of the sequence
• Note the internal geometry of the reflections

• A seismic sequence is
  • a stratigraphic interval bounded by
    • unconformities or
    • their correlative conformities
• Identification uses more information from section than continuous reflectors

Upper reflections are horizontal
Deeper reflections dip to the right

• What has happened?

• Erosional truncation
• followed by onlapping deposition

Note change in reflector geometry:

• What kind of sedimentary environment is this?

Prograding deposition

• Where are coarser sediments likely?

This will be offscreen, to allow detail presentation

Wells, logs, ties, and truth.

• Geophysics aims to show third-dimensional information.
• Generally, obvious test of predictions lies in drilling.
• "Well logs" are results of experiments run in well (geological and geophysical)

• Drilling should not be seen as the final stage, but
• as further step in investigation of geology.
• Drillhole information (samples, further geophysics) should be iterated into interpretation process.

• Drilling (whether on seismic prognosis or other geophysics) should be directed to test model (hypothesis)
• If drilling stops without confirmingor refuting model
  • Drillhole design/execution is faulty (but seldom wasted!)

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