seismic basics Flashcards
(20 cards)
What is the velocity and density of water?
1500 m/s
1000 kg/m^3
what is the equation for twt?
t= 2h/V
h = distnace
v = velocity
What is the reflective amplitude at normal incidence given by?
R = reflection coefficient
what are the equations for reflection coeffiecnt?
R = (DbVb – DaVa ) / (DbVb + DaVa )
or
R = (Zb-Za)/(Zb+Za)
a = above
b = below
Z = acoustic impedance
What is the refleciton coefficient?
the amplitude of the reflected wave
What is a seg polarity wavelet?
left is positve
right is negative
colour in right side
Why might mean Zb is bigger than Za?
when density or velocity is greater above then below
WHat happens to relfections from depth?
get weaker and weaker
What is attenuation?
loss of amplitude due to friction
when frequency higher wave bouncing more thus losing more energy
How is the dominant period defined and how can yu get frequecny from this?
T - distance between adjacent peaks
Frequency is inverse of T
T = 1/f
WHat is the limit of vertical resolution?
Limit of this “vertical resolution” is λ/4 (quarter wavelength) λ/4 = V / (4f)
Cannot seismically separate layers closer than this
What happens to frequency and velocity with depth?
Frequency decreases
Velocity increases
WHat will layers be shown as for siesmic reflection in sedimentary rocks?
Instead of facie chnages it will be bedding
What are the types of unconformity?
Upper boundary =Erosional truncation, Toplap, Concordance
Lower boundary = onlap, downlap, concordance
What will faults and strata look like in relfections?
Bedding planes but because of the limits of resolution we do not see all. Instead we primarily see the overall pattern of bedding: time-lines
Faults – can see where the strata are terminated against a sharp boundary
How does pore fill affect relfections?
increasing seismic velocity and density from air and gas to solid hydrates
What is BSR?
Bottom simulaintg reflector = Seismic reflection event that mimics the seafloor topography and is primarily used to indicate the presence of gas hydrates beneath the seabed
What will the polarity be when going from a denser material with higher velocity to less dense and lower velocity water?
Positive
What will the polarity of this be BSR reflection from boundary between hydrate filled sediment (high velocity and density) and gas/water filled sediment (lower velocity and density)?
Negative
