LECTURE MATERIALSINTRODUCTION (1X) DefinitionGeophysical Methods and their main applicationsLevel of Petroleum Investigation

REFLECTION SEISMIC (8X) Fundamental of Seismic Reflection MethodAcquisitionProcessingStructural InterpretationStratigraphic InterpretationExercise Field Trip (if possible)

GRAVITY (3X) Introduction and general application of gravity dataGravity data analysis for Oil and Gas ExplorationParadigm Shift in Gravity data utilizationGravity data analysis for Oil and Gas Reservoir Monitoring (Time lapse)

MAGNETIC (1X) General Application of Magnetic Data

Seismic Methods: Refraction and Reflection Seismic methods, as typically applied in exploration seismology, are considered active geophysical methods. In seismic surveying, ground movement caused by some source* is measured at a variety of distances from the source. The type of seismic experiment differs depending on what aspect of the recorded ground motion is used in the subsequent analysis.

One of the first active seismic experiments was conducted in 1845 by Robert Mallet, considered by many to be the father of instrumental seismology. Mallet measured the time of transmission of seismic waves, probably surface waves, generated by an explosion. To make this measurement, Mallet placed small containers of mercury at various distances from the source of the explosion and noted the time it took for the surface of the mercury to ripple after the explosion. In 1909, Andrija Mohorovicic used travel-times from earthquake sources to perform a seismic refraction experiment and discovered the existence of the crust-mantle boundary now called the Moho.

The earliest uses of seismic observations for the exploration of oil and mineral resources date back to the 1920s. The seismic refraction technique, was used extensively in Iran to delineate structures that contained oil. The seismic reflection method, now the most commonly used seismic method in the oil industry, was first demonstrated in Oklahoma in 1921. History

Elastic WavesWhen the is Earth rapidly displaced or distorted at some point, the energy imparted into the Earth by the source of the distortion can be transmitted in the form of elastic waves. A wave is a disturbance that propagates through, or on the surface of, a medium. Elastic waves satisfy this condition and also propagate through the medium without causing permanent deformation of any point in the medium. Elastic waves are fairly common. For example, sound propagates through the air as elastic waves and water waves propagate across the surface of a pond as elastic waves.

The wave is characterized by :

Amplitude is the peak to trough height of the wave divided by two.

Wavelength is the distance over which the wave goes through one complete cycle (e.g., from one peak to the next, or from one trough to the next).

Period is wavelength measured in time

Frequency is number of cycle in 1 second

Velocity is the speed of wave propagation

SEISMIC WAVES ARE BODY WAVES: Elastic waves that propagate through the Earth's interior. In reflection and refraction method, body waves are the source of information used to image the Earth's interior. Like the ripples on the surface of the pond, body waves propagate away from the source in all directions. If the speed at which body waves propagate through the Earth's interior is constant, then at any time, these waves form a sphere around the source whose radius is dependent on the time elapsed since the source generated the waves. Shown above is a cross section through the earth with body waves radiated from a source (red circle) shown at several different times.

Seismic Body Waves

Wave Type (and names)Particle MotionOther CharacteristicsP(Compressional), Primary, LongitudinalAlternating compressions (pushes) and dilations (pulls) which are directed in the same direction as the wave is propagating (along the raypath); and therefore, perpendicular to the wavefront.P motion travels fastest in materials, so the P-wave is the first-arriving energy on a seismogram. Generally smaller and higher frequency than the S and Surface-waves. P waves in a liquid or gas are pressure waves, including sound waves.S (Shear), Secondary, TransverseAlternating transverse motions (perpendicular to the direction of propagation, and the raypath); commonly approximately polarized such that particle motion is in vertical or horizontal planes.S-waves do not travel through fluids, so do not exist in Earths outer core (inferred to be primarily liquid iron) or in air or water or molten rock (magma). S waves travel slower than P waves in a solid and, therefore, arrive after the P wave.

Compressional Wave (P-Wave)Deformation propagates. Particle motion consists of alternating compression and dilation. Particle motion is parallel to the direction of propagation (longitudinal). Material returns to its original shape after wave passes.

Shear Wave (S-Wave) Deformation propagates. Particle motion consists of alternating transverse motion. Particle motion is perpendicular to the direction of propagation (transverse). Transverse particle motion shown here is vertical but can be in any direction. However, Earths layers tend to cause mostly vertical (SV; in the vertical plane) or horizontal (SH) shear motions. Material returns to its original shape after wave passes.

Seismic Surface Waves

Wave Type (and names)Particle MotionOther CharacteristicsL, Love, Surface waves, Long wavesTransverse horizontal motion, perpendicular to the direction of propagation and generally parallel to the Earths surface.Love waves exist because of the Earths surface. They are largest at the surface and decrease in amplitude with depth. Love waves are dispersive, that is, the wave velocity is dependent on frequency, generally with low frequencies propagating at higher velocity. Depth of penetration of the Love waves is also dependent on frequency, with lower frequencies penetrating to greater depth.R, Rayleigh, Surface waves, Long waves, Ground rollMotion is both in the direction of propagation and perpendicular (in a vertical plane), and phased so that the motion is generally elliptical either prograde or retrograde.Rayleigh waves are also dispersive and the amplitudes generally decrease with depth in the Earth. Appearance and particle motion are similar to water waves. Depth of penetration of the Rayleigh waves is also dependent on frequency, with lower frequencies penetrating to greater depth. Generally, Rayleigh waves travel slightly slower than Love waves.

Rayleigh Wave (R-Wave) Deformation propagates. Particle motion consists of elliptical motions (generally retrograde elliptical) in the vertical plane and parallel to the direction of propagation. Amplitude decreases with depth. Material returns to its original shape after wave passes.

Love Wave (L-Wave) Deformation propagates. Particle motion consists of alternating transverse motions. Particle motion is horizontal and perpendicular to the direction of propagation (transverse). To aid in seeing that the particle motion is purely horizontal, focus on the Y axis (red line) as the wave propagates through it. Amplitude decreases with depth. Material returns to its original shape after wave passes.

1. What seismic wave type is shown here?

2. What seismic wave type is shown here?

3. What seismic wave type is shown here?

4. What seismic wave type is shown here?

Wavefronts and RaypathsRaypaths - Raypaths are nothing more than lines that show the direction that the seismic wave is propagating. For any given wave, there are an infinite set of raypaths that could be used. In the example shown above, for instance, a valid raypath could be any radial line drawn from the source. We have shown only a few of the possible raypaths.

Wavefront - Wavefronts connect positions of the seismic wave that are doing the same thing at the same time. In the example shown above, the wavefronts are spherical in shape. One such wavefront would be the sphere drawn through the middle of the dark blue area. This surface would connect all portions of the wave that have the largest possible negative amplitude at some particular time.

In principle and in practice, raypaths are equivalent to the directions of current flow, and wavefronts are equivalent to the equipotential lines. They are also equivalent to field direction and strength in magnetism.

Wave Interaction with Boundaries

Snell's Law These raypaths are simply drawn to be perpendicular to the direction of propagation of the wavefield at all times. As they interact with the boundary, these raypaths obey Snell's Law. Snell's Law can be derived in a number of different ways, but the way it is usually described is that the raypath that follows Snell's Law is the path by which the wave would take the least amount of time to propagate between two fixed points.

Consider the refracted raypaths shown above. In our particular case, v2, the velocity of the halfspace, is less than v1, the velocity of the layer. Snell's Law states that in this case, i2, the angle between a perpendicular to the boundary and the direction of the refracted raypath, should be smaller than i1, the angle between a perpendicular to the boundary and the direction of the direct raypath. This is exactly the situation predicted by the wavefronts shown in the figure above.

Seismic Wave Speeds and Rock Properties It can be shown that in homogeneous, isotropic media the velocities of P and S waves through the media are given by the expressions shown to the right. Where Vp and Vs are the P and S wave velocities of the medium, r is the density of the medium, and m and k are referred to as the shear and bulk moduli of the media. Taken together, m and k are also known as elastic parameters. The elastic parameters quantitatively describe the following physical characteristics of the medium. Bulk Modulus - Is also known as the incompressibility of the medium. Imagine we have a small cube of the material making up the medium and that we subject this cube to pressure by squeezing it on all sides. If the material is not very stiff, we can image that it would be possible to squeeze the material in this cube into a smaller cube. The bulk modulus describes the ratio of the pressure applied to the cube to the amount of volume change that the cube undergoes. If k is very large, then the material is very stiff, meaning that it doesn't compress very much even under large pressures. If k is small, then a small pressure can compress the material by large amounts. For example, gases have very small incompressibilities. Solids and liquids have large incompressibilities.

Seismic Wave Speeds and Rock Properties

Seismic Velocities of Earth Materials The P and S wave velocities of various earth materials are shown below.

Material P wave Velocity (m/s) S wave Velocity (m/s) Air 332 Water 1400-1500 Petroleum 1300-1400 Steel 6100 3500 Concrete 3600 2000 Granite 5500-5900 2800-3000 Basalt 6400 3200 Sandstone 1400-4300 700-2800 Limestone 5900-6100 2800-3000 Sand (Unsaturated) 200-1000 80-400 Sand (Saturated) 800-2200 320-880 Clay 1000-2500 400-1000 Glacial Till (Saturated) 1500-2500 600-1000

Simple Earth Model: Low-Velocity Layer Over a Half space Shown below are a few snapshots of the seismic waves as they propagate away from the source at times of 65, 80, and 110 ms.

Simple Earth Model: Low-Velocity Layer Over a Half spaceAs the refracted arrival propagates through the half space, because it travels faster than the direct arrival in the layer, it begins to move across the layer boundary before the direct arrival. The refracted arrival is propagating horizontally at the speed of the half space, and the direct and the reflected arrivals propagate horizontally at the speed of the layer.

LESSONS LEARNEDSeismic method uses body waves to carry earth subsurface information to the surfaceAs seismic wave hit the elastic boundary it will get reflected, refracted, and transmittedThe type of seismic methods differs depending on what aspect of the recorded reflected or refracted is used in the analysisSeismic Method: Refraction and Reflection

SEISMIC REFRACTION

SEISMIC REFRACTION

Near OffsetFar Offset1st shot2nd shotDistance between shot pointsnnt Receiver1st ReceiverSpread Length (RL)Distance between Receiver points1st shot2nd shot3rd shot4th shot1122334444444444332211Fold Coverage For four times shots Full Fold Coverage Common Shot Points Common Mid Points Common Receiver Receiver PointsSource PointsMid PointsMultiple Coverage Seismic Reflection Survey

Hyperbolic Move OutReflected waves recognized by its hyperbolic shape in seismic recordNormal move out correction is a time shift apply to seismic reflection record to get zero offset response

SEISMIC REFLECTION RECORDS

ttttttQSourceReceiverRaw dataseismic SectiongeologyReflection Seismic

SECTION OF GEOLOGY MODELSECTION OF ZERO OFFSET SEISMIC ZERO OFFSET SECTION

*Its up to us. I will with this quote from Dr Rob Millar Professor of Civil Engineering, University of British Columbia.Peak Oilers are typically characterized as a pessimistic bunch or prophets of doom. While there may be some truth to this, I believe that there is much opportunity and reason for optimism. Rather than focusing only on what I see as futile and costly attempts to continue to grow the supply of liquid fuels, efforts must be redirected to the demand side: efficiency (doing more with less); conservation (just doing less); designing compact, walkable urban communities; emphasizing public transit including electric light rail; switching to biofuels and other renewable energy sources; relocalising organic food production, and so on. These are all very desirable actions that will be necessary not only to mitigate the effects of Peak Oil, but also to reduce green-house gas emissions and to shrink our ecological footprint, while developing a more livable and sustainable society. It is heartening to see just such efforts being made here at UBC as outlined in the recent Sustainability Issue of UBC Reports. In fact, if Peak Oil helps us to accelerate these efforts, then it could turn out to be just about the best thing that ever happened to us.