Seismic Reflection Flashcards
What 2 properties determine the propagation velocity of seismic pulses?
The elastic moduli and density of the materials through which they pass
How do P-waves move?
Through the compressional and dilational uniaxial strains in the direction of travel
How do S-waves move?
Through pure shear strain in a direction perpendicular to wave travel
Which type of body wave travels faster?
P-waves
What is the form of a wavefront of a pulse travelling through homogeneous rock?
A perfect sphere
How are bulk rock properties described?
As the average of the properties of matrix minerals and any pore fluid, weighted according to porosity
How does P-wave velocity change with confining pressure?
It increases
What is acoustic impedance?
The product of the density and speed of sound in a rock
How does acoustic impedance control the proportion of energy transmitted through an rock interface?
The smaller the contrast in acoustic impedance, the greater the proportion of energy transmitted through
What does a negative R value signify about a reflected ray?
180° phase change
What does T (the transmission coefficient) represent?
The ratio of the amplitude of the transmitted ray to the amplitude of the incident ray
What does I (energy intensity) represent?
The amount of energy flowing through a unit area normal to the directional wave propagation in unit time
How do air guns work?
A chamber is charged with very high pressure compressed air, which is released through vents into the water as a high pressure bubble
What is bubble pulse reverberation?
The lengthening effect of an overall pulse caused by a train of bubbles following the primary pulse of an air gun
How do geophones work?
A cylindrical coil is suspended from a spring within the field of a permanent magnet. When the instrument moves, the relative motion between the suspended coil and fixed magnet generates a voltage which can be converted into velocity
How do hydrophones work?
Ceramic piezoelectric elements produce an output voltage proportional to the pressure variations exerted on them with the passage of a P-wave through the water
Which 2 multiple reflection types are reflected at intervals of high enough R to almost match primary reflections in amplitude?
Ghost reflections caused when rays from a buried explosion are reflected from the ground surface and water layer reverberations caused when rays are repeatedly reflected at the sea bed and sea surface
What is a common mid-point?
A collection of all traces sharing a point of reflection
How is midpoint used to calculate subsurface velocity?
Traces are compared, as variation of travel time with offset depends only on the velocity of subsurface layers
What are the 2 most common shot-detector configurations in multichannel reflection profiling surveys?
Split spread (detectors distributed on either side of the shot point) and single spread (the shot point is located at 1 end of the detector spread)
What does vertical resolution measure and how is it determined?
The ability of a setup to recognise individual, vertically closely spaced reflectors, determined by pulse length on the recorded seismic section
What is the Fresnel Zone and how does it limit horizontal resolution?
The part of the reflector from which energy is returned within half a wavelength of the initial reflected arrival, with reflectors separated by a smaller distance indistinguishable
How does the Fresnel Zone limit geophone spacing?
Geophones should be fixed at no more than 1/4 the width of the Fresnel Zone for target horizons
What do static corrections do?
Remove the effect of low velocity surface layers from individual traces and reduce travel times to a common ‘height datum’
What do dynamic corrections do?
Remove the effect of normal moveout (distance between source and receiver impacting reflection time). This is the final step before velocity stacking
What does the migration correction do?
Reconstructs seismic sections so that reflection events are repositioned under their correct surface location and at a corrected vertical time when a reflector is not horizontal
How is reflection point displaced in the presence of a component dip along the survey line and across the survey line?
It is displaced in the up-dip direction and out of the plane of the section
What form does structural distortion in non-migrated and migrated sections take
Bowties (curved crosses) and smiles (undulation)
How is 3D seismic data usually measured on land?
The cross-array method, with shots and detectors distributed along orthogonal sets of lines to establish a grid
What is structural analysis and how does it work?
The study of reflector geometry on the basis of reflection times, tracing reflection events back to established geology
What is stratigraphical analysis and how does it work?
The analysis of reflection sequences as seismic expression of lithologically distinct depositional sequences, with stratigraphical units marked by unconformities
What is seismic modelling and how does it work?
The process of constructing theoretical seismograms for layered models to derive insight into the physical significance of reflection events
How is seismic reflection data processed?
Analog outputs are digitised as shot point gathers, with noise edited out. Gathers are sorted into CMPs and static and dynamic corrections are applied to individual trace velocities, which are then stacked. Stacked traces are grouped on a seismogram, then migration repositions reflection events. Seismic modelling can be used to analyse changes in shape, polarity, and amplitude. Structural or stratigraphical analysis is performed based on the context
