GEOPHYSICS (SEISMOLOGY) Flashcards
Seismlogy caused by NATURAL SEISMIC WAVE of EQUAKES and derives infor on physical properties, composition and the gross internal structure of Earth
Equake Seismology
Waves are caused by artificial balsts such as detonating explosives on land and non explosive vibroseis or compressed air (Marine) at selected sites to infer info about regional/local structures
Explosion Seismology
Main purpose
?Interpret the interfaces of rock boundaries
?Layered sed Sequence
?Location of Watertable
?Oil and Gas Exploration
Main premise of the survey
Waves are propagated to travel in rock media and which the subsurface is assumed to be homogenous and isotropic to simplify wave propagatig resulting in interpretato of the measured effect at the plane of discontnuity
Strain that causes change in shape
Shear or distortional strain
Strain that changes the volume without change in shape
Dilataion or Contraction
The ratio between longitudinal stress and longitudinal strain
E Young’s Modulus
The ratio of Uniform compressive stress to fractional change in Volume
K Bulk Modulus
the measure of the stress/strain in the case of simple tangential shear
u Rigiity stress
measure of the geometrical change in the shape of a clastic body or the compressibility of a material
Poissons Ratio
Fastest of all seismic waves
longitudinal
Travels to all media (S,L,G)
Propagate by compressional and dilational uniaxial strain (Forward backward) or alterate compression and dilation)
Pwaves, compressonal
Transverse waves which VIBRATE about their fixed mean position in a plane PERPENDICULAR to the direction of wave propagatio (Up down)
Slower than P waves
Cannot travel thrugh the outer core because they cannot exist in fluids (Air, Water, and Molten Rock)
S waves, shear
Formed when particle motion is a combination of both longitudinal and transverse vibration thus giving an ELLIPTICAL RETROGRADE MOTION (CCW) in the vertical plane along the direction of travel
Rayleigh Waves
Major type of surface wave having a horizontal motion that is shear or transverse/Perpendicular to the direction of the propagation
Love Waves
Velocity or progpagation is equal to?
Elastic Moduli and densities
Velocity of P wave
Square root (4/3 Shear Modulus + Bulk Modulus)/Density)
Velocity of S wave
Square root of shear modulus / density
What is being measured of waves as they return after rfraction or reflection
Travel times
Variation of velocity with density
The higher the density the higher the velocity
Which will travel faster P or S?
P
Rocks with best linera relationship between velocity and densiy
Sed Rocks (Isipin mo na lang na lagi syang ginagamit sa petroleum kaya sed rocks
Rocks with velocities that increase with depth of burial and with age
Sandstone and shale (The more the burial, the denser the rocks, the higher the velocity)
Porosity and weathering effect on seismic velocity
The higher the porosity and weathering, the lower the velocity since density decreases
Materials in increasing Vp (Basic-Acidic Ign, Coarse to Fine grained Met, Sed Rocks)
Gabbro
Iron
Granite
Gneiss
Slate
Sandstone limestone
Water
In reflection, angle of incidence is equal to
Angle of refraction
In refraction, angle of incidence
Snells law
Sin I / Sin R = V1/V2
A particular angle of incidence when velocity is higher in the underlying layer for which angle of refraction is at 90 deg and which gives rise to critically refracted ray that travales along the interface at higher velocity
Critical Angle
Th perturbation caused by the passage of the critically refracted ray along the top of the lower layer which travels at velocity which is higher than that of the upper layer
Headwave
P wave velocity as a function of time
P = 1.47 [Depth (km) T (Age in Ma)]^1/6
Detonated in shallow holes of 6-30m and has a depth of over 10m
Explosives
Non explsisve source that uses COMPRESSED AIR which is release in the form of HIGH PRESSURE BUBBLES
Air Gun
Truck mounted Vribrator which punds 50k-60K output
Vibroseis
Other Non explosive seismic sources
Weight Drop
Hammer
Actual Earthquake
Shotgun, Rifles
Bufallo Gun
A marine seismic source which uses explosive mixture of gases instead of High pressure ubbles
Water Guns
Devices that converts electrical energy into acoustic energy and which the pulse is generated by the discharge of a large capacitor bank directly into the sea waters
Sparkers
A rigid aluminum plate attached below heavy duty electrical coil by a spring loaded mounting typically towed behind survey vessel in a catamaran mounting
Boomers
Small ceramic piezoelectric transducers mounted in a towing fish w/c when activated by electrical impulse will emit a very short, high frequencey acoustic pulses of low energy
Pingers
Electro mechanical transducers that produce an extended repeatable source wave form w/c allows greater energy outpur
Chirps
Converts ground motion to electrical signal
Transducers
Devices used on land to detect seismic ground motions
Geophones or Seismometers
Most common seismometer
Moving Coil Geophone
Used to detect passage of compreesonal seismic waves marked by transcent pressire canges in sea waeter
Hydrophones
Siesmic Wave Recorders
Seismographs
Any vibration which is not part of the signal
Noise
A noise generated by the shot itself like swaves, surface waves and reflections from near surface irregularities
Coherent Noise
Noise not generate by the shot itself such as traffic, livestock, pople that can be controlled
Random Noise
Most widely sed technique and well known geophysical technique that aborb more than 90% money spent worldwide on applie geophysics
Carried out in areas with SHALLOW DIPPING SED SEQUENCES
Reflection Surveys
What is the vertical scale of a seismic reflection profile?
Time not depth
Limitations
Costly
Computer intensvive w/c requires super computers
Data processing is tedious
Seismic survey that utilizies seismic energy that returns to the surface after traveling through a ground along refracted ray paths
Siesmic Refraction Survey
if Vlayer 1>V layer 2
Use snells Law
if Vlayer 2>V layer 1
Sin Critical Angle = V1/V2
Ray that travels along a straight line through the top of the upper layer from source to detector at velocity V1
Direct Ray
Obliquely incident to the interface and is reflected back through the top layer to the detector travalling along its entire path to top with layer velocy V1
Reflected Ray
Travels obliquiley down to the interface at velocity V1 then along the segment of the interface at V2 and back up through the upper layer at V1
Refracted Ray
The distance at which the refracte ray overtakes the direct ray
Cross over distance
The distance at which the travel tmes of the reflected and refracte rays coincid because they follo the same path
Critical distance
First arrivals at the critical distance
Direct Rays
First arrivals at the cross voer distance
Refracted Ray
Minimum line length of refraction
10x of depth of horizon of interest
Reflection line length
Same order as reflctor depth of interest (Equal lang)
Differentiate Spread and Array
Spread pertains to a line of geophones laid out for a refraction survey while array is reserved for geophone feeing a single recording channel
Mining and Econ Geol
Hydrocarbons (offshore and onshore)
Structural
High Res studies of Shallow Geology (Quat Sed Sequence, Bedrock Mapping)
Crustal and lithospheric studies
Structural
Regional Investiogation of internal structure and thickess of crust
Hydrogeol
Exploration for Underground water supplies in sed sequences w/ ER
Environmental and Engg
Foundation studies to map bedrock surfaces
Estimation of bulk and shear moduli from P and S Waves
