Body waves p-waves and s-waves Body waves = Primary compressional and Secondary shear waves Buried dynamite charges are widely used as the surface energy source for VSP because of their
Trang 11 Two-way time (TWT) or two-way travel time is?
The time required for a wave to travel from source to reflector and back to
receiver
Th i gian song truy n t b m t đ n m t phân cách gi a 2 môi trền từ bề mặt đến mặt phân cách giữa 2 môi trường và phản ừ bề mặt đến mặt phân cách giữa 2 môi trường và phản ền từ bề mặt đến mặt phân cách giữa 2 môi trường và phản ặt đến mặt phân cách giữa 2 môi trường và phản ến mặt phân cách giữa 2 môi trường và phản ặt đến mặt phân cách giữa 2 môi trường và phản ữa 2 môi trường và phản ư ng và ph n ản
x ngạ ngược lại đến điểm nhận ược lại đến điểm nhận ạ ngược lại đến điểm nhận ến mặt phân cách giữa 2 môi trường và phản c l i đ n đi m nh n.ểm nhận ận
2 It may be difficult or impossible to identify a thin layer if it is?
Less than 1/4 wavelength thick
B dày l p h <lamda/4 (đ phân gi i kém).ền từ bề mặt đến mặt phân cách giữa 2 môi trường và phản ớp h <lamda/4 (độ phân giải kém) ộ phân giải kém) ản
3 Faulted layers effects on seismic?
Produce diffraction hyperbola in the unmigrated sections
Xu t hi n nh ng đất hiện những đường nhiễu xạ Hypebole trên các mặt cắt cố định ện những đường nhiễu xạ Hypebole trên các mặt cắt cố định ữa 2 môi trường và phản ư ng nhi u x Hypebole trên các m t c t c đ nh.ễu xạ Hypebole trên các mặt cắt cố định ạ ngược lại đến điểm nhận ặt đến mặt phân cách giữa 2 môi trường và phản ắt cố định ố định ịnh
4 What is migration a seismic section?
is a way to correct for the displacement of position and the shape of a
non-horizontal reflector
Migration is a process that moves data from apparent subsurface positions to true positions
5 "Multiples" occur in a seismic section when
Waves reflect off interfaces multiple times before finally arriving at the receiver
6 True or False: A section produced by seismic reflection can be
interpreted in a very straightforward way as a geological section
False
7 In reflection seismology, we infer Earth sturcture by using the time of
travel of refracted and reflected seismic waves created by earthquakes The core does not allow shear waves to pass through it, while the speed of travel (seismic velocity) is different in other layers
8 How does source frequency affect resolution and penetration depth in seismic reflection and GPR?
Higher frequency = low depth, higher resolution.
Trang 2Lower frequency = high depth, low resolution.
T n s càng cao, đ h p th môi trố định ộ phân giải kém) ất hiện những đường nhiễu xạ Hypebole trên các mặt cắt cố định ụ môi trường càng lớn, sóng ko thể đi sâu ư ng càng l n, sóng ko th đi sâu.ớp h <lamda/4 (độ phân giải kém) ểm nhận
9 What is acoustic impedance? The product of density and seismic velocity It
is the impedance contrast between layers that causes the reflections that are
recorded along a surface profile
10 Discuss the factors that control penetration depth in a seismic reflection survey
Higher frequency = low depth, higher resolution
Lower frequency = high depth, low resolution
11 Estimate the vertical resolution in a reflection seismology or GPR
survey What does it depend on? Lamđa/4.
12 What constitutes a reflector in reflection seismology? Be able to
calculate (given no formula) the reflection and transmission coefficients.
- A change in the density or velocity of on an interface boundary
Reflection: p2v2 - p1v1 / p2v2 + p1v1
Transmission: 2p1v1 / p2v2 + p1v1
13 What is stacking in reflection seismology, and why do we do it?
Average of multiple seismographs together to increase signal/noise ratio
14 What is normal moveout, and how do we use it to determine velocity in
a seismic reflection survey?
the extra travel time reflected rays take via not taking a vertical path on a
horizontal reflector
15 P-wave velocity
alpha=sqrt((lambda + 2 mu)/rho))
What is the ground motion produced by P-waves and the relative velocity?
P-waves = Compressional motion (horizontal x direction), and fastest
Trang 3P-waves propagate in solids, liquids and gasses P-waves always have higher propagation velocities than S-waves, in the same medium
16 S-wave velocity
beta = sqrt (mu/rho)
What is the ground motion produced by S-waves and the relative velocity?
S-waves = particle motion perpendicular to wave propagation either vertical or mostly horizontal y direction, slower then P-waves
S-waves can be used to obtain more detailed or special information about the subsurface propagates only in solids
17 Body waves
p-waves and s-waves
Body waves = Primary (compressional) and Secondary (shear) waves
Buried dynamite charges are widely used as the surface energy source for VSP because of their effectiveness in producing seismic body waves
18 In a continuous velocity gradient (increasing downward), waves turn
upwards
19 In a stepped velocity gradient (increasing downward), waves turn the angle of incidence increases until the ray is horizontal
20 When do triplications occur? when the ray experiences a rapid velocity
increase
21 How are low velocity zones identified? As gaps in the record: shadow
zones on plots of x (distance or depth) vs t
22 What happens to waves originating in a shadow zone? The zone acts as a
wave guide sending waves along the zone to large distances
Where is a P-wave shadow and why does it happen?
Trang 4between 105 and 142 degrees Happens because of refraction via different
density layers
Where is the S-wave shadow and what does it imply?
103 degrees and beyond to 180 degrees This implies the Earth has a liquid outer core; shear (rigidity) modulus is 0 for liquids because water has no resistance, so S-waves cannot travel through this liquid
23 What is a passive seismic source? One over which we have no control Ex:
earthquakes Need location & time info for inversion
24 What is an active seismic source One which we control We control the
number of sources, the locations, amplitude, and the timing
25 Reflection angle angle of incidence
26 How do we calculate transmission angles? snell's law
27 Seismic boundary conditions: Continuity of displacement across a
boundary Continuity of traction (stress or force) across a boundary
28 Phase conversion, P-waves generate, S-waves generate: non-vertical
p-waves hitting a boundary generate 4 different scattered p-waves: reflected and
transmitted P and S waves
A shear wave will generate reflected and transmitted shear waves.???
29 What is the origin of surface waves? Consist of energy brought to the
surface by body waves
30 Describe Love waves : "surface shear" waves, related to S-waves.
Origin: constructive interference of higher order S-wave multiples
Require velocity increase with depth or spherical geometry to exist
High amplitude, low frequency Amplitude decreases with depth Amplitude decays
as 1/sqrt(r) = can travel far from the event
What is the ground motion produced by Love waves and the relative velocity? Love wave = horizontal perpendicular (y direction), slower than P-waves & S-waves
Trang 531 Describe Rayleigh waves "ground roll"
Origin: coupled P and SV waves
Particle motions are retrograde at surface and prograde at depth
Amplitude decays as 1/sqrt(r)
Dispersive in velocity gradients: lower frequencies go faster
What is the ground motion produced by Reyleigh waves and the relative
velocity?
Reyleigh wave = elliptical motion in mostly vertical z plane, slower than P-waves
& S-waves
32 What is a forecast? A non-absolute, probabilistic statement about an event
within a given time frame
Components include: likelihood of the event, time window, location (depth), and magnitude
33 Strike-slip behavior: what does strain accumulation / failure threshold depend on? velocity of fault movement, coefficient of static friction.
34 Strike-slip behavior: What does the magnitude / amount of slip depend on? coefficient of dynamic friction.
35 What is resistivity? What are its units?
- Resistivity is the fundamental property of the material on which resistance depends
- The units of resistivity are ohm meters
36 What is conductance? Conductivity?
- Conductance is the inverse of resistance
- Conductivity is the inverse of resistivity
37 What is the resistivity of air? Air has infinite resistivity.
38 How does current flow in relation to equipotential surfaces? Current
flows perpendicular to equipotential surfaces
39 How do we determine the locations of equipotential surfaces?
Trang 6- If we calculate the potential at many points, we can draw contours through points of equipotential
- We can then draw current flow lines perpendicular to these surfaces
40 For a homogeneous, isotropic medium, at what depth is 50% of the current confined? 50% of the current is confined above a horizontal plane with a
depth = 1/2 the current electrode spacing
41 In the resistivity method, what three main actions occur? In the
resistivity method, current is entered into the ground, potential difference is
measured, and resistivity is determined
42 Current flow tends to avoid a poor conductor in favor of a good
conductor Does this mean that more or less current will flow above an
interface with a poor conductor? A greater percentage of current will flow above
the interface with a poor conductor compared to the homogeneous case
43 As current electrode spacing is increased, the current flow at depth is
increased
44 The percentage of current penetrating below an interface is controlled
by what two factors? The percentage of current penetrating below an interface is
controlled by the relative magnitude of the resistivities of the two layers and the electrode spacing
45 If the resistivity of the second layer is greater, the flow lines bend
(toward/ away) the normal and as a consequence are (more/less) widely spaced
46 Variations in current density near the surface will result in variations in
apparent resistivity
47 For the measured apparent resistivity values to approach the resistivity
of the second layer, how much of the subsurface must be sampled?
- A great volume of the subsurface must be sampled in order for the measured apparent resistivity values to approach the resistivity of the second layer
Trang 7- This seriously limits the resistivity method because it requires sufficient space in which to place electrodes
48 What are the axis of an apparent resistivity curve? x-axis: electrode
spacing; y- axis: apparent resistivity
49 The effect of the shallow interface is felt even at small electrode spacing.
50 What resistivity curves are so similar that they may be impossible to differentiate with field data
p1< p2 and p1 < p2 < p3 or p1 > p2 and p1 > p2 > p3
51 Which three layer resistivity curves are easiest to identify?
Those due to the presence of a low resistivity layer between two higher resistivity layers and those due to the presence of a high resistivity layer between two lower resistivity layers
52 An electrical resistivity traverse oriented parallel and close to a vertical contact appears remarkably similar to what?
A traverse oriented parallel and close to a vertical contact appears remarkably similar to a two layer curve
53 In the shallow subsurface, current is conducted almost entirely by what?
In the shallow subsurface, current is conducted almost entirely by the fluid present.
54 What controls the resistivity of sediments and rocks in the shallow subsurface?
The resistivities of sediments and rocks in the shallow subsurface are controlled by the amount of water present and its salinity
55 Increasing silt or clay contents in poorly sorted rocks or sediments will (reduce / increase) resistivities
56 In saturated materials, increasing porosity will (reduce / increase) resistivity
Trang 857 Well sorted materials have (poorer/greater) porosities than poorly sorted materials.
58 In general, finer-grained sediments will have (lower/higher) porosities, and (lower / higher) resistivities than coarse-grained sediments.
59 Bedrock almost always has (higher / lower) resistivities than overlying sediments.
60 Generally, unsaturated sediments above the water table will have
(higher/lower) resistivities than saturated sediments below the water table.
BONUS:
1 What is the source of seismic survey in offshore? A combination of air or
water guns and other acoustic sources
2 What is the receiver of seismic survey in offshore? A streamer consists of
long cables with geophones spaced evenly along it with a tail buoy at the end
3 What is the Zero-Offset? Zero-Offset, where offset is the distance between
the source and receiver
4 What is CDP, CMP? Common-Depth-Point, Common-Mid-Point
5. Loss of Amplitude due to the associated dispersion, and the loss of
resolution over time This makes interpretation more difficult