Seismic Reflection

Question 1

What 2 properties determine the propagation velocity of seismic pulses?

Answer 1

The elastic moduli and density of the materials through which they pass

Question 2

How do P-waves move?

Answer 2

Through the compressional and dilational uniaxial strains in the direction of travel

Question 3

How do S-waves move?

Answer 3

Through pure shear strain in a direction perpendicular to wave travel

Question 4

Which type of body wave travels faster?

Answer 4

P-waves

Question 5

What is the form of a wavefront of a pulse travelling through homogeneous rock?

Answer 5

A perfect sphere

Question 6

How are bulk rock properties described?

Answer 6

As the average of the properties of matrix minerals and any pore fluid, weighted according to porosity

Question 7

How does P-wave velocity change with confining pressure?

Answer 7

It increases

Question 8

What is acoustic impedance?

Answer 8

The product of the density and speed of sound in a rock

Question 9

How does acoustic impedance control the proportion of energy transmitted through an rock interface?

Answer 9

The smaller the contrast in acoustic impedance, the greater the proportion of energy transmitted through

Question 10

What does a negative R value signify about a reflected ray?

Answer 10

180° phase change

Question 11

What does T (the transmission coefficient) represent?

Answer 11

The ratio of the amplitude of the transmitted ray to the amplitude of the incident ray

Question 12

What does I (energy intensity) represent?

Answer 12

The amount of energy flowing through a unit area normal to the directional wave propagation in unit time

Question 13

How do air guns work?

Answer 13

A chamber is charged with very high pressure compressed air, which is released through vents into the water as a high pressure bubble

Question 14

What is bubble pulse reverberation?

Answer 14

The lengthening effect of an overall pulse caused by a train of bubbles following the primary pulse of an air gun

Question 15

How do geophones work?

Answer 15

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

Question 16

How do hydrophones work?

Answer 16

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

Question 17

Which 2 multiple reflection types are reflected at intervals of high enough R to almost match primary reflections in amplitude?

Answer 17

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

Question 18

What is a common mid-point?

Answer 18

A collection of all traces sharing a point of reflection

Question 19

How is midpoint used to calculate subsurface velocity?

Answer 19

Traces are compared, as variation of travel time with offset depends only on the velocity of subsurface layers

Question 20

What are the 2 most common shot-detector configurations in multichannel reflection profiling surveys?

Answer 20

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)

Question 21

What does vertical resolution measure and how is it determined?

Answer 21

The ability of a setup to recognise individual, vertically closely spaced reflectors, determined by pulse length on the recorded seismic section

Question 22

What is the Fresnel Zone and how does it limit horizontal resolution?

Answer 22

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

Question 23

How does the Fresnel Zone limit geophone spacing?

Answer 23

Geophones should be fixed at no more than 1/4 the width of the Fresnel Zone for target horizons

Question 24

What do static corrections do?

Answer 24

Remove the effect of low velocity surface layers from individual traces and reduce travel times to a common ‘height datum’

Question 25

What do dynamic corrections do?

Answer 25

Remove the effect of normal moveout (distance between source and receiver impacting reflection time). This is the final step before velocity stacking

Question 26

What does the migration correction do?

Answer 26

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

Question 27

How is reflection point displaced in the presence of a component dip along the survey line and across the survey line?

Answer 27

It is displaced in the up-dip direction and out of the plane of the section

Question 28

What form does structural distortion in non-migrated and migrated sections take

Answer 28

Bowties (curved crosses) and smiles (undulation)

Question 29

How is 3D seismic data usually measured on land?

Answer 29

The cross-array method, with shots and detectors distributed along orthogonal sets of lines to establish a grid

Question 30

What is structural analysis and how does it work?

Answer 30

The study of reflector geometry on the basis of reflection times, tracing reflection events back to established geology

Question 31

What is stratigraphical analysis and how does it work?

Answer 31

The analysis of reflection sequences as seismic expression of lithologically distinct depositional sequences, with stratigraphical units marked by unconformities

Question 32

What is seismic modelling and how does it work?

Answer 32

The process of constructing theoretical seismograms for layered models to derive insight into the physical significance of reflection events

Question 33

How is seismic reflection data processed?

Answer 33

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