{"id":51986,"date":"2020-10-14T10:13:40","date_gmt":"2020-10-14T10:13:40","guid":{"rendered":"https:\/\/ingeoexpert.com\/en\/?p=51986"},"modified":"2021-02-10T14:32:31","modified_gmt":"2021-02-10T14:32:31","slug":"reflection-seismic-acquisition-methods","status":"publish","type":"post","link":"https:\/\/ingeoexpert.com\/en\/2020\/10\/14\/reflection-seismic-acquisition-methods\/","title":{"rendered":"Reflection seismic acquisition methods"},"content":{"rendered":"
\n

Here you can read a fragment of one of the “Seismic interpretation on 2D and 3D
\n” course units, that delves into reflection seismic acquisition methods<\/b><\/p>\n<\/blockquote>\n

<\/p>\n

Reflection seismic acquisition methods<\/h2>\n

In this chapter we will explore the theory behind reflection seismic acquisition, some of the most important
\nconcepts and how data acquisition is done using modern methods. We will cover 3 main topics:<\/p>\n

– Rock properties
\n-Wave propagation, reflection, refraction, reflectivity, impedance
\n– Data acquisition \u2010 onshore and offshore<\/p>\n

Seismic interpretation on 2D and 3D<\/a><\/p>\n

Rocks properties<\/h3>\n

Seismic waves travel through Earth\u2019s interior as a result of earthquakes, volcanic eruptions, magma movement,
\nlandslides or man\u2010made explosions.<\/p>\n

The propagation of seismic waves will depend on the density and elasticity of the medium they travel through.
\nIn general, the velocity of propagation increases with depth, both in the crust and in the Earth’s mantle.
\nEach medium (layer, geological body, etc.), depending on its composition, density, porosity, etc., will have
\ndifferent physical properties, namely the velocity of wave propagation.<\/p>\n

\"\"<\/a>
Figure 1 \u2010 Variation in the velocity of seismic waves, P and S with depth. Wikipedia.<\/figcaption><\/figure>\n

 <\/p>\n

Types of acoustic waves<\/h3>\n

The primary waves, or P, are elastic or pressure waves, since the materials they travel through undergo
\ncompression and rarefaction. They travel through all types of materials, solid, liquid or gaseous. Typical values for
\nthe velocity of P waves in solid media are in the range of 5 to 8 km \/ s, faster than any other wave.
\nThey are the main source of information in seismic acquisition campaigns.<\/p>\n

\"\"<\/a>
Figure 2 \u2010 Propagation of P seismic waves in a 3D block, causing compression and rarefaction of the materials they travel through. Wikipedia<\/figcaption><\/figure>\n

 <\/p>\n

Secondary waves, or S, are shear waves, since the materials they travel through are displaced perpendicular to
\nthe direction of propagation. They only travel through solid materials. Typical values for the velocity of S waves
\nare ca. 60% slower than the P waves. They are not very useful in seismic acquisition campaigns, causing some
\nproblems in land campaigns, but they can be used for some advanced geophysical calculations.<\/p>\n

\"\"<\/a>
Figure 3 \u2010 Propagation of S seismic waves in a 3D block, causing movements perpendicular to the propagation of the materials they cross. Wikipedia<\/figcaption><\/figure>\n

 <\/p>\n

Surface waves propagate across the Earth’s surface from the epicenter of an earthquake. There are two types of
\nsurface waves: Rayleigh waves and Love waves. They are not generally used as acquisition data.<\/p>\n

Properties of acoustic waves and their propagation<\/h3>\n

Acoustic waves, like other types of waves, can be characterized using several descriptive and measurable terms:<\/p>\n

\u03bb \u2010 length is the size of a wave, the distance between two valleys or two ridges.
\n\u03b3 \u2010 The amplitude of a wave is a measure of the magnitude of a disturbance in a medium during a wave cycle. For
\nexample, waves on a rope have their amplitude expressed as a distance (meters).
\nT \u2010 The period is the time of a complete cycle of an oscillation of a wave.
\nf \u2010 Frequency is a period divided by a unit of time (example: one second), and is expressed in hertz:
\nf = 1 \/ T
\nv \u2010 The speed of a wave is described by the following equation: v = \u03bb.f<\/p>\n

 <\/p>\n

\"\"<\/a>
Figure 4 \u2010 Acoustic wave properties and the terms used to describe them. Adapted from Wikipedia<\/figcaption><\/figure>\n

The velocity of wave propagation is one of the main variables for advanced geophysical interpretation.
\nThe velocity will depend on the properties of the materials (of the rocks) it travels through as follows:<\/p>\n

 <\/p>\n

\"\"<\/a><\/p>\n

Where:<\/p>\n

Vp P Wave velocity
\nK incompressibility (bulk) module
\n\u00b5 modulus of rigidity (for liquid materials = 0)
\n\u03b4 density of the material to be traversed<\/p>\n

Wave propagation<\/h3>\n

Acoustic impedance can be defined as the set of physical properties of a medium that determine the propagation
\nof a wave through that medium. The impedance difference determines the amount of energy that is reflected or
\nrefracted.<\/p>\n

 <\/p>\n

\"\"<\/a>
Figure 5 Propagation of acoustic waves through a layer contact, resulting in a reflected and a refracted wave.<\/figcaption><\/figure>\n

 <\/p>\n

For P waves, in rock mediums, its value is determined by the density of the rock and the velocity of propagation
\nof P waves in that rock.<\/p>\n

\"\"<\/a>
Figure 6 \u2010 Chart of P and S wave propagation velocities and densities in various types of rocks. From https:\/\/openei.org\/wiki\/Seismic_Techniques<\/figcaption><\/figure>\n

 <\/p>\n

The greater the differences between the properties of the 2 layers, the greater the reflected energy \u2010 and the
\ngreater the reflectivity of the surface that divides them. In a seismic profile, the greater the reflectivity, the
\ngreater the amplitude of that surface.<\/p>\n

Each layer will have different density and Vp.
\nThe contrast between layers will determine the \u201cvisibility\u201d of the reflective surfaces, i.e. their amplitudes in a
\nseismic profile.<\/p>\n

\"\"<\/a>
Figure 7 \u2010 Simplified scheme of seismic wave propagation during an offshore acquisition showing several layers with different densities (\u03c1) and P wave propagation velocity (Vp). Wikipedia.<\/figcaption><\/figure>\n

 <\/p>\n

In a seismic reflection acquisition, in each layer, each point will be \u201canalyzed\u201d by several waves from the source\u2010
\nhydro \/ geophone set. In this case S1\u2010D1, S2\u2010D2, S3\u2010D3, etc. Each point is called Common Depth Point (CDP).
\nWhen strata are horizontal, the CDP is equivalent to a Common Mid Point (CMP).<\/p>\n

As the time that a wave takes to travel from S2 source to D2 receiver is greater than S1 to D1, the S2\u2010D2 trace
\nwill be offset. The same for the S3\u2010D3, etc. This wave travel time between the source and the receiver (reflected
\nby the reflection surface) is called Two\u2010Way Time (TWT).<\/p>\n

\"\"<\/a>
Figure 8 \u2010 Travel time (TWT) between the source and the receiver for several pairs. https:\/\/gpg.geosci.xyz\/<\/figcaption><\/figure>\n

 <\/p>\n

This trace offset between the several pairs of source\u2010receivers is called Normal Move Out and is corrected during
\nthe first stages of processing.<\/p>\n

The data that arrives first is called near offsets (or \u201cnears\u201d) and the later ones are called far offsets (or \u201cfars\u201d). All
\nare used for the final seismic product, but can be used separately for quantitative interpretation.<\/p>\n

\"\"<\/a>
Figure 9 Record of a seismic signal (gather) from the same point (CMP) from several pairs of source\u2010receiver. a) without correction. b) with correction<\/figcaption><\/figure>\n

 <\/p>\n

Seismic wave properties and propagation – some references<\/h3>\n