Seismic reflection Flashcards
(17 cards)
What does the angle of incidence =?
The angle of relfection
How will antiforms appear in seismic reflection?
appear much broader
Rays converge downwards and diverge upwards
How will different depths of synforms be represented in a seismic relfection?
SHallow - appears mjuch tighter then actually is (less lateral extent, steeper sides)
Intermediate (focus on 1 reciever) - get strong reflection on 1 or 2 traces so dont pick up structure
Deep - focus inbetwen structure and recivers so waves cross shwoing antiform instead
What are diffractors and what do they do?
Small spherical or 2d bodies energy spread over large area giving a broad structure
Can be generated from the edges of layers or sheets
What is seismic migration?
Remeber rays are waves so actual point plotted where waves converge get addition of amplitude and strong reflection the rest is cancelled out
What can be used to remeber the visual difference between a migrated and unmigrated section?
Unmigrated will be unhappy with lots of antiforms
Migrated will be happy with smiles especially at the base of section
Why is 3d migration done?
Done in hemisphere of shots around not just line can allow better differentation and identification of structures
What is the issue related to salt having low porosity?
it doesnt compact with burial this means as other rocks around become denser due to more overlying weight the salt (halite) tries to rise bouyanlty first flwoing as a pillow (nostructure broken) then exploiting faults to rise diapirically (layers above broken)
What effect if produced by halite layers?
Sound (seismic p-waves) travels faster through salt than through most other sedimentary rocks. As a result, significant pull-up effects develop beneath salt pillows and diapirs
What is the process of halokinesis?
Salt migration - pre-kinematic where no movement
Early pillow structure/ early syn-kinematic
Salt migration to pillow allows subsidence on periphery of pillow called PPS (primary peripheral sink)
Formation of secondary rim sink as salt withdraws from pillow into diapir
How can you calculate twt for angled rays?
Right angled triangle and pythagorous
Have equation 2L/V = t
(L+L)^2 = (x)^2 + (2h)^2
t^2 = x^2/V^2 + 4h^2/V^2 (everything divded by V^2 to fit twt equation)
a^2 + b^2 = c^2
L = hypotenous
H = depth
What does x from this equation t^2 = x^2/V^2 + 4h^2/V^2 mean for coincidental recivers?
source-receiver (x=0), t0 = 2h/v t02 = 4h2 / V2
=> t2 = x2 / V2 + t02
What is NMO?
At any receiver position, calculate the time correction required to bring the arrival time back to t0
This is the normal moveout
Can get a strong wiggle by stacking into 1 signal
What is needed to imporve the accuracy of NMO?
CMP - Common MIdpoint GAther
SO only shots from a certain set shot number used
Common reflection point
What are multiples in siesmic reflection?
Multiple bounces or reverberations especially with strong reflecting boundaires present i,e., sea surface =-1 and sea floor = 0.2 for soft, 0.5 for hard
The multiple comes in zero offset and 2x, 3x 4x etc traveltime of sea floor
The polarity will be flipped for each multiple so if seafloor was postive 2x multiple will be negative due to -1 reflection coefficnt at sea surface
What are the different types of multiples?
Water bottom multiple: two or more bounces between sea-surface and seafloor
Water column multiple; one or more extra bounces in water column (sea-surface-seafloor) as well as one on other reflector
Interbed multiples: multiple bounces between certain strata
Peg-leg; one multiple leg shorter than the other
How does stacking supress multiples?
ignores curves with different curvature to primary refection due to different avg velocity down to reflector
