Seismic Processing

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Seismic Processing - Table of Contents

02.01 Chapter 2 - Seismic Acquisition 02.02 The ideal seismic source 02.03 Onshore seismic sources 02.04 Other Land sources 01.01 Chapter 1 - In the beginning ... 01.02 An introduction

01.03 The geological time scale 01.04 Rock types

01.05 Sedimentary basins 01.06 Oil and Gas formation 01.07 Oil and Gas Traps

01.08 Approximately Mathematical 01.09 Chapter 1 - Questions

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02.05 Recording the data - Onshore 02.06 Offshore seismic sources 02.07 Recording the data - Offshore 02.08 Chapter 2 - Questions

03.01 Chapter 3 - Waves and Raypaths 03.02 Waves

03.03 P and S Waves 03.04 Wavefronts 03.05 Ray Paths 03.06 More Ray Paths 03.07 Ghost Reflections 03.08 Multiple Reflections 03.09 Diffractions 03.10 Chapter 3 - Questions 04.01 Chapter 4 - Geometry 04.02 Marine 2D Geometry 04.03 Multi-Fold Geometry 04.04 Calculating the fold 04.05 Land Geometry

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04.06 Elevations and shot depths 04.07 Field Statics

04.08 Crooked Lines

04.09 The problems with 2D 04.10 3D Geometry

04.11 3D binning

04.12 Chapter 4 - Questions

05.01 Chapter 5 - Recording the data 05.02 Number systems 05.03 Binary numbers 05.04 Binary arithmetic 05.05 Digitisation 05.06 Aliasing 05.07 Multiplexed data 05.08 Recording filters 05.09 Tape Formats 05.10 Field Tape Formats 05.11 SEG-D

05.12 SEG-Y

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06.01 Chapter 6 - Frequencies 06.02 Complex Numbers 06.03 Frequencies

06.04 Combining different frequencies 06.05 The Fourier Transform

06.06 The Forward Transform 06.07 The Inverse Transform 06.08 Example FFT's

06.09 Digital Filtering 06.10 Convolution

06.11 The frequency content of the seismic trace 06.12 Phase

06.13 Chapter 6 - Questions

07.01 Chapter 7 - Processing Flows (1) 07.02 A typical processing flow 07.03 Transcription

07.04 Vibroseis data & correlation 07.05 Shot summing

5 07.07 Signature Deconvolution (1) 07.08 Signature Deconvolution (2) 07.09 Signature Deconvolution (3) 07.10 Signature Deconvolution (4) 07.11 Gain Recovery 07.12 Trace editing 07.13 Chapter 7 - Questions

08.01 Chapter 8 - Processing Flows (2) 08.02 Resample

08.03 Spatial Resampling 08.04 Spatial Frequencies 08.05 Spatial filtering 08.06 FK Filtering

08.07 Linear Tau-P filtering 08.08 FX Deconvolution

08.09 Spatial Filtering Examples (1) 08.10 Spatial Filtering Examples (2) 08.11 Spatial interpolation

08.12 Sorting & Gathering 08.13 Chapter 8 - Questions

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09.01 Chapter 9 - Processing Flows (3) 09.02 Dynamic corrections

09.03 NMO and Velocities 09.04 Seismic Velocities 09.05 Picking Velocities (1) 09.06 Picking Velocities (2) 09.07 Velocity problems 09.08 Velocity QC

09.09 Other velocity analyses and QC 09.10 DMO (1)

09.11 DMO (2) 09.12 DMO (3)

09.13 Chapter 9 - Questions

10.01 Chapter 10 - Processing Flows (4) 10.02 Multiples - Again!

10.03 Deconvolution (1) 10.04 Deconvolution (2) 10.05 Deconvolution (3) 10.06 Deconvolution (4)

7 10.07 De-Multiple (1) 10.08 De-Multiple (2) 10.09 Residual Statics (1) 10.10 Residual Statics (2) 10.11 Other Statics (1) 10.12 Other Statics (2) 10.13 Chapter 10 - Questions

11.01 Chapter 11 - Processing Flows (5) 11.02 Trace muting

11.03 CDP Stack 11.04 Filtering (1) 11.05 Filtering (2)

11.06 Equalisation & AGC 11.07 Migration (1) 11.08 Migration (2) 11.09 Migration (3)

11.10 Other Post-Stack Processes / Display 11.11 Display (2)

11.12 What happens next? 11.13 Chapter 11 - Questions

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12.01 Chapter 12 - Advanced Processing 12.02 Complex Trace Analysis 12.03 Well Log Processing 12.04 Data Matching 12.05 Zero Phasing 12.06 Seismic Inversion 12.07 AVO (1)

12.08 AVO (2)

12.09 Scanning & Reprocessing 12.10 QC - Trials and Tribulations 12.11 Yesterday, Today and Tomorrow 12.12 Chapter 12 - Questions

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Chapter 1 - In the beginning ...

As the beginning seems an appropriate place to start, Chapter 1 provides a tiny bit of geological and mathematical background to Seismic Processing. Please make sure that you have read Navigating through this course before going any further!

Page 01.02 - An introduction

We start with a brief introduction to the whole process of Seismic Exploration, a mention of its history, and an explanation of the types of seismic trace displays used in this course.

Page 01.03 - The geological time scale

As a prelude to a four page instant geological background, nothing less than the history of the world is used to familiarise you with the formation names used by geologists. Although not strictly related to Seismic Processing, some background to the overall geological setting of seismic data is useful.

Page 01.04 - Rock types

Specific rock types, and their formation, are discussed, with particular emphasis on the sedimentary rocks that are the foundation of the world's Oil & Gas production.

Page 01.05 - Sedimentary basins

Since all of the world's Oil & Gas reserves are found in sedimentary basins, this page illustrates the formation of a typical basin. The structures produced during basin

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formation provide the setting for both the generation and subsequent "trapping" of Oil & Gas deposits.

Page 01.06 - Oil and Gas formation

The chemical and physical processes necessary for the production of Oil & Gas are discussed, together with an example of an actual sedimentary basin cross-section.

Page 01.07 - Oil and Gas Traps

Once the Oil & Gas has been produced, the conditions must exist for the "trapping" of the Oil & Gas in a suitable structure. Examples of the major types of trap are shown.

Page 01.08 - Approximately Mathematical

To finish this introduction, something non-geological. A look at least-squares regression, one of the mathematical algorithms used extensively in the Seismic Processing route.

Page 01.09 - Questions

A set of questions to test your knowledge of the topics covered in Chapter 1.

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An introduction

Although this course will concentrate on the processing of reflection seismic data for hydrocarbon exploration, it's probably worth reviewing the historical context and the other uses of seismic techniques.

The Chinese had a device as early as 100 AD. for detecting earth tremors - probably the first seismic receiver! The first use of explosives to delineate structures under the earth was in the 1920's and 30's in the Southern US and South America. The techniques used developed fairly slowly over the next twenty or so years until the advent of tape recording in the 1950's, and digital computer processing in the 1960's. Since then the technology has increased exponentially, with the ups and downs of the world oil price controlling the overall research effort.

This course deals exclusively with seismic reflection data processing. The recording of refracted seismic waves is used for shallow investigations and will be mentioned in passing (we can't avoid recording refractions with the reflections). Reflection techniques can also be used for very shallow site investigations (either for placing an oil rig, or for engineering work), or very deep penetration into the earth for examination of the limits of the earth's crust. Other geophysical techniques (gravity, magnetics etc.) will not be discussed. Like most real-world activities, seismic exploration has not featured well in the movies. The 1953 film "Thunder Bay" has James Stewart throwing sticks of dynamite from a boat in a Louisiana bayou, and the 1976 re-make of "King Kong" used the excuse of seismic exploration as a reason for visiting the appropriate island. Neither of these is

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"Jurrasic Park" producing a 3D image, though it would be interesting to develop such a technique!)

In simplistic terms, seismic exploration can be thought of as the sonic equivalent of radar. An energy source produces sound waves that are directed into the ground. These waves pass through the earth and are partially reflected at every boundary between rocks of different types. The response to this reflection sequence is received by instruments on or near the surface, and recorded on magnetic tape for computer processing. The process is repeated many times along a seismic "line" (generally a straight line on the surface), and the resultant

processed data provides a structural picture of the sub-surface. Sophisticated processing techniques can be used (usually in conjunction with calibration data recorded down a well) to turn the resultant seismic section into a direct indicator of rock types and (possibly) detect the presence of hydrocarbons (oil & gas) within the earth.

The data recorded from one "shot" (one detonation of an explosive or implosive energy source) at one receiver position is referred to as a seismic trace, and is recorded as a function of time (the time since the shot was fired). As this time represents the time taken for the energy to travel into the earth, reflect, and then return to the surface, it is more correctly called "two-way time" and the vertical scale is generally measured in milliseconds (one thousandth of a second - 0.001 seconds). During the processing sequence these traces are combined together in various ways, and modified by some fairly complex mathematical operations, but are always referred to as "traces". The display of many traces side-by-side in their correct spatial positions produces the final "seismic section" or "seismic profile" that provides the geologist with a structural picture of the sub-surface.

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For reasons of efficiency and data redundancy, the results from each shot are actually recorded at many different receiver positions. Receivers are placed at regular intervals around or to one side of the shot position typically extending over some 3 kilometres or more of the surface. The resultant collection of traces from one shot is generally recorded together and referred to as a "Field Record". Each shot position is numbered, and the position of each shot (normally the "shotpoint") is accurately mapped. This "shotpoint map" shows the position of the seismic cross-section on the surface. For conventional so-called two-dimensional (2D) seismic data (we'll talk about 3D later...) this cross-section is assumed to lie directly below the surface "line".

Since the early 1960's seismic data has been recorded and processed digitally. What we visualise as a seismic trace is actually nothing more than a string of numbers, where each number represents the amplitude (or height) of the seismic trace at a particular time. Traces are typically recorded with a sample period (the time interval between numbers) of 1-2 milliseconds (0.001 - 0.002 seconds), and recorded to a total two-way time of (typically) 5 or 6 seconds. Each trace thus consists of some 2,500 - 6,000 numbers recorded on tape. As each of these numbers can take up to 4 bytes of computer storage, and as one field record can consist of (for example) 240 seismic traces recorded every 25 metres on the surface, the data storage problems are, even now, considerable. Six seconds of data recorded at a 2 millisecond sample period, with one field record of 240 traces shot every 25 metres = 115,200,000 bytes per kilometre (recorded every 5-6 minutes offshore, or 1.3 gigabytes per hour!)

The seismic trace

There are many different ways of displaying a seismic trace, most of which are used in this course. When we are dealing with a short piece of trace, and wish to examine the numbers that make up the digital

representation of the trace we will probably use one of the examples shown here.

1) Shows the trace as a series of "spikes" representing the numerical value of the trace at each sample. 2) Shows the same series with the implied continuous waveform drawn in. If we wish to be more precise, we will use 3) which shows the

continuous waveform in a smooth fashion (such a curve can be reconstructed, subject to certain conditions, from the numerical

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In all of these cases we will assume that values above the zero line represent positive numbers, whilst those below are negative numbers.

When we are showing longer traces, or collections of traces, we will resort to more conventional displays.

1) Shows a "Wiggle-Trace" display of a whole trace, 2) a "Variable-Area/Wiggle-Trace" display - used for seismic sections as it enhances continuity, and 3) and 4) show types of "variable density" or colour displays which we will use when lots of traces are involved.

For all of these displays, the positive values will normally be plotted towards the right, or as black or red "peaks" on the variable density displays. Variable-Area/Wiggle-Trace displays normally have the peaks filled-in as black.

The geological time scale

ERA PERIOD EPOCH YEARS PAST LIFE FORMS

CENOZOIC

Quaternary { Recent 10,000

Humanoids

{ Pleistocene 2,500,000

Tertiary { Pliocene 12,000,000 Grazing & Carnivorous Mammals

{ Minocene 26,000,000

{ Oligocene 38,000,000

{ Eocene 54,000,000

{ Paleocene 65,000,000

MESOZOIC Cretaceous 135,000,000 Primates & Flowering

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Jurassic 195,000,000 Birds

Triassic 225,000,000 Dinosaurs & Mammals

PALEOZOIC Permian 280,000,000 Reptiles & Fern Forests

Carboniferous { Pennsylvanian 320,000,000

{ Mississippian 345,000,000

Devonian 395,000,000 Amphibians & Insects

Silurian 430,000,000 Land Plants

Ordovician 500,000,000 Fish

Cambrian 570,000,000 Shellfish

PRECAMBRIAN 700,000,000 Algae

700,000,000 Single-cell organisms 3,500,000,000

4,650,000,000+ Formation of the Earth

The age of the Earth is approximately 4½ thousand million years (4,650,000,000). Life first appeared some 600 million years ago, with humanoids appearing about 3 million years ago. The Earth's age is conveniently divided into two major time divisions marking the

boundary when life first appeared: the Cryptozoic (hidden life), or Precambrian; and the Phanerozoic (obvious life), or Cambrian.

Until comparatively recently, with the advent of radio-carbon dating and similar processes, the only way to establish the relative ageing of rocks was by means of the fossilised remains within those rocks. This led to difficulties in dating rocks earlier than the earliest solid remains (greater than 600 million years old, or Precambrian).

The time period over the remaining geologic time scale has been sub-divided into three eras (Palaeozoic, Mesozoic and Cainozoic), each sub-divided into named periods and ages, as well as further sub-divisions referred to by the name of a characteristic fossil.

Most, but not all, of the major oil discoveries are in rocks formed during the last 200 million years (Triassic and younger).

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Rock types

Rocks can be classified into three main types, depending on the chemistry of their formation.

17 Igneous rocks

Igneous rocks were formed by the cooling and subsequent solidification of a molten mass of rock material, known as magma. These rocks are still being produced by active volcanoes.

Plutonic (coarse-grained) rocks, such as granite and syenite, were formed from a magma buried deep within the crust of the Earth which cooled very slowly during the initial stages of the Earth's solidification, thus permitting large crystals of individual minerals to form. Volcanic rocks, typified by basalt and rhyolite, were formed when the molten magma rose from a depth and filled cracks close to the surface, or when the magma was extruded upon the surface of the Earth through a volcano. The subsequent cooling and solidification of the magma were very rapid, resulting in the formation of fine-grain minerals or glasslike rocks.

As igneous rocks are composed almost entirely of silicate minerals, they are often classified by their silica content. The major categories are referred to as acid (granite and rhyolite) and basic (gabbro and basalt).

Metamorphic rocks

Metamorphic rocks are those whose composition and texture has been altered by heat and pressure deep within the Earth's crust. Dynamothermal, or regional, metamorphism refers to those rocks where both heat and pressure have caused changes. Where the changes have been produced by the heat of an intrusion of igneous rock, the metamorphism is termed thermal, or contact.

Metamorphism occurs to both igneous or sedimentary rocks, where the resultant rock depends on the amount of heat and pressure they have been subjected to. Shale is

metamorphosed to slate in a low-temperature environment, but if heated to temperatures high enough for its clay minerals to re-crystallise as mica flakes, shale becomes

metamorphosed into a phyllite. At even higher temperatures and pressures, shale and siltstone completely re-crystallise, forming schist or gneiss, rocks in which the alignment of mica flakes produces a laminated texture called foliation.

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Among the non-foliated metamorphic rocks, quartzite and marble are the most common. Quartzite is typically a tough, hard, light-coloured rock in which all the sand grains of a sandstone or siltstone have been re-crystallised into a fabric of interlocking quartz grains. Marble is a softer, more brittle, varicoloured rock in which the dolomite or calcite of the original sedimentary material has been entirely re-crystallised.

Sedimentary rocks

Sedimentary rocks are the weathered debris derived by the slow processes of erosion of upland regions containing other rock types.

These weathering products are ultimately transported, by water, wind or ice to the seas, lakes or lowland areas where they settle out and accumulate to form clastic (fragmented) sediments. As the sediments are transported, the individual grains are battered and tend to become more rounded and sorted according to size by the varying strengths of current and wave action.

By the time it reaches the sea, and is distributed over the sea-floor, the sediment is

commonly well-enough sorted to be distinguishable as shingle, sand, silt or mud. Although mixtures are also present, these are the deposits that later become hardened to form the sedimentary rocks, conglomerate, sandstone, siltstone and mudstone or shale respectively. During this process, trapped organic material gives rise to the oil and gas we are trying to find!

Two other types of sediment are additionally deposited in the marine environment in which they are created. An abundance of calcium carbonate secreting organisms, including

certain algae, corals and animals with shells, can give rise to limestone, a rock composed almost entirely of the mineral calcite. An important variety is dolomite, where the basic mineral constituent is a calcium-magnesium carbonate and which most commonly results from changes to the rock due to percolating waters after deposition.

The intense evaporation of restricted or isolated sea areas can give rise to the accumulation of sea salts on the sea floor. They may reach considerable thickness and include anhydrite

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(calcium sulphate), calcium carbonate, rock salt (sodium chloride) and, eventually, potassium salts. They are collectively referred to as evaporites.

At the edges of the continental land masses, this eroded and precipitated material,

deposited in the shallows of ancient seas, caused the sea bottom to subside and the gradual build-up of sedimentary basins.

Sedimentary basins

Sedimentary basins were formed over hundreds of millions of years by the action of the deposition of eroded material and the precipitation of chemicals and organic matter in the sea water. External geological forces then distort and modify the layered strata. The following pictures show (vertically exaggerated) the formation of a typical basin.

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Sediment collects on the sea-bed, the weight causing subsidence. Different materials collected at different times, so producing the regular "layering" of strata in the basin.

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Volcanic action, or the movement of land masses, causes faults to appear in the basin. These same forces cause rotation of the overall basin forming a new mountain range.

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Erosion of the highlands, and additional subsidence forms yet another area of low-lying land that is filled with water forming another ancient sea.

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Finally, land mass movement causes folding and distortion of the basin.

In reality the above processes may have occurred again and again and in any order during the formation of a sedimentary basin, giving rise to thick, complex structures in the

sediments.

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Oil and Gas formation

Sedimentary basins exist around the world at the edges of ancient continental shelves. These basins can be of complex structure, with many different layers within the sediments deposited in a particular geological age.

The temperature increases with depth within the Earth's crust, so that sediments, and the organic material they contain, heat up as they become buried under younger sediments.

As the heat and pressure increase, the natural fats and oils present in buried algae, bacteria and other material link and form kerogen, an hydrocarbon that is the precursor of

petroleum.

As this source rock becomes hotter, chains of hydrogen and carbon atoms break away and form heavy oil. At higher temperatures the chains become shorter and light oil or gas is formed. Gas may also be directly formed from the decomposition of kerogen produced from the woody parts of plants. This woody material also generates coal seams within the strata.

If the temperature and pressure gets too high, the kerogen becomes carbonised and does not produce hydrocarbons.

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The oil and gas produced by these processes may be in any combination and are almost always mixed with water. The minute particles of hydrocarbon are produced within the pores of permeable rocks (for example sandstone) and, being lighter than the surrounding material, move up through the rock until prevented from doing so by an impermeable rock.

Although the initial source rock may only contain minute amounts of hydrocarbon, as the particles of oil, gas and water move, or migrate*, through the pore space within younger permeable rocks, they coalesce into larger volumes.

By the time this movement is stopped by the presence of a cap of impermeable rock (or when they reach the surface) the total hydrocarbon volume may be large enough to be a produce an oil or gas field that will be profitable to develop. The ultimate profitability of such a field depends, of course, on external economic forces and world demand as much as on ease of extraction.

As seismic exploration is concerned with the imaging of sub-surface structures, it is those structures that may indicate a potential hydrocarbon trap that are of most interest to the explorationist.

* Don't confuse the migration of hydrocarbons with the use of the term migration in seismic processing - we'll be talking about that later!

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Oil and Gas Traps

There are many types of traps associated with hydrocarbon accumulations, here are a few examples.

A structural trap formed by the folding of the strata into an anticline. The

hydrocarbons permeate up through the porous rock below until trapped by the denser rock above. One of the first types of traps to be identified, as they are sometimes associated with anticlines on the surface.

This type of structure is readily visible on a seismic section, but, as seismic sections are normally "time-sections", the actual structure may be obscured (or even artificially produced) by velocity changes above the anticline.

A structural trap formed around a fault. Many combinations are possible, requiring the impermeable cap rock to be present on the other side of the fault.

Under certain conditions the hydrocarbons will actually migrate along the fault plane, and so move several kilometres from the source rock.

May be difficult to identify on the seismic data due to raypath problems around the fault.

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Some of the possible traps associated with a salt intrusion. The actual trap may form above the salt plug, in which case it may be easier to image on the seismic data than those on the sides of the plug.

Another early discovery; the first seismic reflection surveys were used to identify salt intrusions in and around the Gulf of

Mexico and southern USA.

A more complex structural trap, where the reservoir rock was first folded and eroded, then sealed by an impermeable rock which was deposited later over the eroded

structure.

Although this type of structure may be visible on the seismic data, it may not be obvious that this is a trap.

A stratigraphic trap formed by lateral changes within one (apparent) rock layer. In this case, the reservoir rock may, for example, have come from river sands which were deposited amongst clays from

surrounding swamps. The clays have solidified into an impermeable seal, so trapping the hydrocarbons within the sandstone.

The most difficult type of trap to find on seismic data. May require additional processing (tied to information from wells) to identify.

There are obviously many more types of trap possible (some still to be found) but you should now have a basic idea of the kinds of structures that may be "interesting" on a seismic section. We'll now move on to some very basic mathematics!

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Approximately Mathematical

Although seismic processing utilises some of the most sophisticated numerical algorithms known, the processing geophysicist can probably get by with simply understanding what the processes do; not how they do it!

It is useful, however, to understand some of the basic principles involved. A basic

knowledge of arithmetic, geometry and trigonometry is almost essential. Although we rely on the computer to do most of the work, we need to supply parameters for each processing stage that are mathematically (as well as geologically) sensible, and to be able to check the computer's results.

The single most common mathematical thread that runs through seismic processing is that of "Least-Squares", the process of minimising the errors involved in making some

approximation to our data. This technique is used again and again within the seismic processing sequence and is worth spending a few moments on. If you are quite happy with this technique, or simply bored or terrified at the prospect then please feel free to click the "Next Page" button above.

In order to explain the principle of Least-Squares, here's a set of numbers representing the time of my journey to work over several days:-

Day 1 - 25 minutes Day 2 - 37 minutes Day 3 - 28 minutes Day 4 - 35 minutes

I would like to establish the "best-estimate" for my journey time, without having any concept of the meaning of "average", but with a knowledge of some basic maths.

I'll start by making a guess at the best time - say 30 minutes. In order to see how good a guess this is, I'll simply subtract it from each actual value:-

Day 1 - 25: 25 - 30 = -5 Day 2 - 37: 37 - 30 = 7 Day 3 - 28: 28 - 30 = -2 Day 4 - 35: 35 - 30 = 5

The total error could be described as -5 + 7 + -2 + 5 = 5, but this does not take into account the fact that some of the numbers are positive, and others negative. I'm really only

interested in the magnitude of the error, not it's sign, so I'll do the simplest thing

(mathematically) I can do and square the numbers before adding them. This gives me (for a guess of 30) a total error squared of 25 + 49 + 4 + 25 = 103. My best guess will be where this number is a minimum.

Here's a whole series of guesses, with the associated "error-squared":

Guess 20 - error squared = 603 Guess 30 - error squared = 103 Guess 22 - error squared = 439 Guess 32 - error squared = 99 Guess 24 - error squared = 307 Guess 34 - error squared = 127

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Guess 26 - error squared = 207 Guess 36 - error squared = 187 and so on ...

Here's a plot of the error squared against guess for a whole range of numbers:-

It's fairly obvious that the Total Error-Squared reaches a minimum value at the base of the curve, and this corresponds to the position of the "best-fit". In order to calculate its value, we need to express our calculations in general terms.

If we call our initial data values X1, X2 etc., then any given value's error-squared is:-

where G is the guess. The total error squared (E) is:-

where N is the number of values. We can expand this to:-

or (by simplifying):-

Now, the only (slightly) heavy bit. In order to minimise this total error-squared, we need to differentiate the right-hand side of the above with respect to G (the thing we're trying to find). This will give us the slope of the resultant curve, and the "turn-over" point where

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The differential of E is then:-

which we set equal to zero to find the minimum error position for G:-

and rearrange (and divide by 2):-

and solve for G:-

This appears to tell us that the best guess for G is the sum of all of the individual values (25+37+28+35=125), divided by the number of values (125/4=31.25). A very long-winded way of proving that the best-fit of a constant value to a set of numbers is the AVERAGE! Of course, this process can be extended to "best-fit" any mathematical function to any set of numbers of any dimension; the solution in each case having the minimum

error-squared. Here's our data points once again with a whole set of higher-order curves fitted:-

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The red line is the average we just computed, the green is a straight line, the blue quadratic and the brown cubic. The brown (cubic) curve fits four points exactly, but is hardly the most meaningful "fit" in this case. (It implies that, next week, it'll take me a month to get to work, I suppose the traffic's going to be really bad!).

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Chapter 2 - Seismic Acquisition

This Chapter presents a very brief look at Seismic Acquisition, with particular emphasis on those topics relevant to the seismic processor. If you've spent the last 15 years on a land crew in the Amazon, or an equivalent length of time in the middle of the Arctic Ocean collecting marine data (or are doing either of these as you read this), you might want to skip this bit!

Page 02.02 - The ideal seismic source

What goes into the ground comes back out again! The characteristics of the ideal seismic source are discussed.

Page 02.03 - Onshore seismic sources

The use of explosives onshore - still the most common source for "Land" recording.

Page 02.04 - Other Land sources

Other land sources are mentioned, and vibratory sources (the second most common source) are examined in some detail.

Page 02.05 - Recording the data - Onshore

The logistics and mechanics of recording data onshore can be enormous. A brief explanation of the standard recording device is followed by a wider review of recording techniques.

Page 02.06 - Offshore seismic sources

From land to sea! The differences in the marine environment, and details of Air-Guns, the most common offshore seismic source.

Page 02.07 - Recording the data - Offshore

Highlights the differences between onshore and offshore recording and addresses the general problems associated with Marine Acquisition.

Page 02.08 - Questions

A set of questions to test your knowledge of this very brief introduction to Seismic Acquisition.

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The ideal seismic source

Changes in the speed (velocity) of sound and the density within particular rocks causes reflection and refraction of the sound waves produced by a seismic source. Specifically, variation of these parameters at an interface between two different rock types causes a reflection of some of the seismic energy back towards the surface. It is the record of these reflections against time that produce our seismic section.

A seismic reflector can only reflect back to the surface an image of the energy pulse it receives. If we send a complex pulse into the ground, that pulse will be superimposed on every reflector we record. For this reason we wish to make the actual seismic source as close as possible to a single pulse of energy - a spike.

A spike of energy sent into the earth produces a set of clear reflectio ns. A more complex energy pulse produces confused reflectio ns

In practice and ideal spike is impossible to achieve. As we will see later, a spike implies that an infinitely wide range of frequencies need to be present in the source, all released over an infinitesimally small time range.

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The earliest seismic surveys used explosives as a seismic source with, for offshore exploration, up to 50 pounds (23 kg) of dynamite being exploded just below the surface of the water.

This is a very effective source, still used for onshore surveys, but is environmentally obviously no longer a desirable source for offshore acquisition.

Many other designs of source have been used over the years, and we will now go on to discuss the most common.

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Onshore seismic sources

There are enormous logistical problems associated with Onshore Seismic Exploration.

The seismic "line" must first be accurately marked out by surveyors.

This may mean painting marks on roads through residential areas ...

... or cutting through dense jungle to mark shot and receiver positions.

In either case modern GPS equipment has simplified the positioning, but accurate elevation (height above sea level) measurements must also be made using conventional surveying techniques.

Once the line position is marked, the shooting and recording equipment can be transported onto the line. Oil & Gas deposits tend to be in some of the more inhospitable regions of the Earth, so the actual terrain conditions may limit the available shooting / recording

positions as well as define the costs of the acquisition.

Dynamite and other explosives are still used for roughly half of all onshore seismic

exploration. 1 kg of seismic explosive releases about 5MJ of energy almost instantaneously, enough energy to keep your electric fire burning for almost 1½ hours. The part of this energy which can usefully be converted into a compressional wave of seismic energy depends on the depth of the shot in the ground, and the local ground conditions.

Seismic shots are normally placed below the near-surface highly weathered layers of the earth. This improves the coupling of the source to the ground, and avoids problems with the vary variable (slow) acoustic velocities in the weathering layer. Tests may be necessary in a new exploration area to determine the optimum shot depth and charge size.

It may be necessary to fire an array of shots for each shotpoint. The number and position of each shot designed to improve the downward-going energy but to attenuate the energy going in other directions.

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In areas where the near-surface

conditions demand it, large drilling rigs may be necessary to drill the shot-holes for burial of the explosive charge.

As mentioned above, the position and height of each shot-hole are carefully recorded, and the depth of the shot in the hole is measured by simply noting the length of the tamping rod used to place the shot in the hole.

These parameters are necessary for the correct processing of the data.

Charges can vary from a few grams to several tens of kilograms, depending on the depth of the target reflectors.

The explosive used is very stable, almost impossible to detonate without the correct electrical detonator, and often comes in small "cans" which can be combined together to make larger charges.

Where better surface conditions exist, or access is difficult, a portable form of drilling rig may be used.

Water & mud pumps, compressed air, emulsion and foam have all been used to improve the circulation of the drill bit in different conditions. The types of drill used extends from hand-held augers to large truck-mounted hammer drills. Production rates for "conventional" (dynamite) exploration depend almost entirely on the rate at which holes can be drilled.

If we assume that the shooting medium is consistent in all directions, we can make some generalisations about the effect of an explosive charge on the surrounding rock.

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In the immediate vicinity of the charge extremely high pressures will exist very briefly at the shot instant, pulverising the neighbouring earth materials and displacing these to form a physical cavity within the shot hole. Beyond this actual cavity there will exist a region where movement is so great that materials are stressed beyond their elastic limits. This part of the sub-surface will be permanently altered.

This region is expected to be a spherical volume, and is commonly referred to as the equivalent cavity. Beyond the radius of this notional equivalent cavity, displacements are such that we can simply consider the compressional wave produced by the shot as a conventional "seismic" wave, and assume that any pressure changes will only cause transient changes in the rocks - they will return to their normal state after the wave has passed.

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Almost anything that could possibly be used as a source of acoustic energy has been tried onshore at one time or another.

Metal plates hit with hammers (still used for very shallow refraction surveys), weights dropped from various devices, explosive charges and offshore seismic sources in water containers, even shotguns fired into the ground.

Modified hardcore-tamping equipment has been used for shallow surveys, and ground penetrating radar is used by archaeologists (and some police forces) for detecting near-surface disturbances.

Out of all of these, by far the majority of other onshore surveys are conducted using a vibratory source on the surface. The most common of these being the vibroseis method developed by Conoco.

Vibroseis

In a Vibroseis survey, specially designed vehicles lift their weight onto a large plate, in contact with the ground, which is then vibrated over a period of time (typically 8-16 seconds), with a sweep of frequencies.

In upsweeping, the frequency begins low and increases with time and in downsweeping the sweep begins high and decreases in frequency with time. Up Swee p Down Swee p

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Regardless of the sweep direction, vibroseis trucks normally operate in synchronised groups, so increasing the total amount of vibratory energy input into the ground.

With less chance of damage to surrounding property than explosives, vibrators are ideally suited for used in urban areas and have been used successfully in the centre of major cities and along the shoulder of major highways.

Of necessity, vibrators are a surface source.

On the first page of this Chapter, it was pointed out that the ideal seismic source is a spike, or as close to it as possible. Explosives meet this criteria very well, but vibroseis is obviously very different - it's akin to the "chirp" used by radar systems - very long in duration but carefully controlled and very repeatable.

Because the vibroseis sweep is so carefully controlled (and directly recorded for each shot), it's effects can be readily removed during the data processing. The technique, involving correlation of the recorded data with the recorded sweep, reduces the apparent source to a symmetrical wavelet containing the same frequencies as the sweep. The vibroseis correlation is now often performed directly in the field and will be discussed when we come to look at correlation.

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Onshore seismic data is recorded using a simple (normally electro-magnetic) device known as a geophone.

A moving-coil geophone consists of a coil of wire, suspended from a spring, surrounded by a "W" shaped magnet.

The metal spike is pushed into the ground, and any upward-travelling energy from the seismic shot is recorded as the electric current generated by the movement of the coil relative to the magnet.

The amount of energy recorded by the geophone is, of course, minute. Where conditions allow (for example on relatively flat ground), several geophones may be grouped together, or place in strings around the central (nominal) receiver position. This not only improves the total signal output from the group, but also "tunes" the geophones so that energy from below is enhanced whilst that from the side (ground-roll) is attenuated. We'll see more on this when we discuss marine seismic sources.

In some cases, where the surface elevation varies rapidly, or there is very loose near-surface material, it may be necessary to drill holes and bury the geophones below ground level. In other cases (for example along a shopping street in the middle of a town) the geophones may be mounted on metal cones and just placed in contact with the ground. Both of these techniques have their own problems which must be addressed in the processing of the data.

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Once the shot positions have been marked, the geophones are laid out (in groups if necessary) and connected either by wire or radio (a telemetric system) to the recording truck.

Although this may seem a trivial task, the planting of one geophone every metre over maybe 5 kilometres is non-trivial, and an enormous amount of planning (and labour) must be used to "lay" the line of geophones.

Remember once again the possible environmental problems - from desert to frozen tundra!

The geophone groups may be laid out for several kilometres prior to the start of shooting.

As the shot position advances down the line, different sections of the recording groups are made "live" by the recording instruments to maintain a similar range of offsets (distances from shot to receiver) for each shot.

At some stage the groups of geophones must be physically moved in order to maintain the "live" section.

Even in a rural setting, geophone placing can be arduous, and noise can be

generated by traffic or power lines (typically 50-60 Hz).

In more remote locations, transportation problems can add to the already

considerable cost of Land Acquisition.

With these problems a conventional (explosive) seismic crew may only record about 80-100 shots per day. A Vibroseis crew (in ideal conditions) may achieve as many as 800.

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Offshore seismic sources

not have the logistical problems associated with Onshore exploration, but the

operational difficulties more than compensate for this!

All of the necessary shooting and recording equipment must be deployed from the rear of the vessel, and towed continuously.

Of necessity, offshore seismic exploration vessels have become larger over the years to cope with the increasing workload, and are expected to operate in all but the very worst weather conditions.

In many areas of the world 24-hour operations are conducted, requiring sufficient personnel to operate both the vessel and the seismic equipment round-the-clock. Multiple seismic sources and receiver cables are used to provide the high densities of data now required. Even in the simplest 2D work, such a vessel can easily obtain 10 times the amount of data as the best (Vibroseis) land crew in any given time.

Although explosives were once used as an energy source for offshore exploration, the environmental repercussions, and the need for rapid firing and repeatability have brought about the design and construction of new sources.

The most common offshore (or Marine) source in use today is a variety of Air-Gun, first produced in the 1960's. These guns use compressed air (at typically 2,000 to 5,000 psi) to produce an explosive blast of air into the water surrounding the gun. The latest of these, with the movable shuttle that releases the air on the outside, is the Sleeve-Gun.

Air enters through the pipe (A) and is fed into the main chamber (D) and the air-spring return chamber (C).

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Once the Solenoid valve (B) opens, air is allowed into the firing chamber (E), and the pressure differential forces the outside sleeve to the left with great force, releasing the air from the main chamber. The resultant air-bubble produces a shock-wave in the

surrounding water.

A single air-gun produces a pulse of energy (or signature) that looks something like this (the upper plot shows the time-function, and the lower shows the frequency content of the signature):-

Hardly a perfect energy source!

Although the initial energy burst is reasonable, a complex pressure

interaction between the air "bubble" and the water causes the bubble to oscillate as it floats towards the surface - this

produces the extraneous bursts of energy following the initial burst.

The amplitude and period (time

difference) between these bubble pulses depend on the depth of the gun and the size of the main chamber in the gun.

We can't do much about the depth (see Ghosts in the next Chapter), but, if we build an array of guns, made of different chamber sizes, and fire these simultaneously, we gain several advantages.

1. We obviously increase the total amount of energy being directed into the ground for one "shot".

2. The different chamber sizes will produce different bubble responses, and these will tend to cancel out.

3. We improve the directivity of the source. Other than directly below the source array, some frequencies will be attenuated by the spatial design of the array.

45 Here's a typical gun array, and the sleeve guns being deploy ed from the rear of the vessel.

Once again, here's a plot showing the time and frequency response of the entire array.

When the entire array is fired, the bubbles "cancel-out" (more or less), and the frequency content is much flatter over the range of typical seismic frequencies.

This is now close to an ideal source, and is very repeatable.

It has to also be very reliable as shots are normally fired roughly every 5-10 seconds - possibly up to 10,000 shots in 24 hours!

We have mentioned before the concept that an array or group of shots or geophones can improve the spatial response of our source and/or receivers.

This plot shows the hemisphere of energy emanating from the source array shown above (viewed from below).

The grey line shows the direction of the seismic line (the direction the boat is moving in) with the arrow showing the vertical output from the array. The colours show the total energy, red being the highest.

Change the frequency of interest and see how, although the downward energy is always maximum, the energy at other angles is attenuated by the array design.

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Although the actual "shooting" of Marine data is made simple by modern Air-gun arrays, there are still some operational difficulties associated with these sources. The high

operating pressures are very dangerous. Air at 80-100 psi is used in industry, with an appropriate abrasive, to remove paint from metal. Air at 2000-5000 psi will remove almost anything (including skin) without any abrasive.

The guns must be properly maintained - any gun failure will damage the desired array output response and re-introduce bubbles into the signature. Deployment of the arrays is made relatively simple by the use of floats etc., but the position of each array (multiple arrays may be used for alternate shooting etc.) must be carefully monitored in all three dimensions. We'll discuss the positioning of arrays in more detail when we discuss shooting geometry.

Again, just like on land, many different types of seismic source have been used in the marine environment. However, as almost all modern data is acquired with variations of air-guns, we will restrict our discussion to those and move on to the recording instruments.

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Recording the data - Offshore

The recording of offshore seismic data is complicated by the fact that all of the recording equipment must be encased in an oil-filled cable or streamer (about 10 cm in diameter) that is towed behind the vessel.

Unlike land geophones, the hydrophones used in marine recording normally use a piezo-electric device to record the incoming energy. These hydrophones are connected together in groups (just like land recording) and may be placed every metre or so along a 3000 metre streamer.

The front-end of the streamer is connected to the vessel by a complex system of floats and elastic stretch-sections, which are designed to eliminate any noise reaching the streamer from the vessel. The end of the streamer farthest from the vessel is connected by similar stretch-sections to a tail-buoy. This buoy may contain its own GPS receiver and radar reflector so that its position can be established.

There are several problems associated with marine acquisition:- 1. Keeping the streamer at a constant depth is important.

2. Determining the position of (possibly) multiple gun arrays, and every hydrophone group in multiple streamers (vessels are now on the drawing board with up to 20 streamers!).

3. Getting all of the seismic signals from all of the recording groups (in all of the streamers) back to the recording system on the vessel.

All of these problems are solved by the complex system of mechanical and electronic systems inside or on the outside of the streamer itself. Here's some of them:-

Although the oil used within the streamer is designed to give the streamer neutral-buoyancy, changes in tides and currents can effect the depth of the streamer.

Mechanical depth controllers (known as birds) are placed at intervals along the streamer which (using pressure as a

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guide) adjust the angle of their "wings" to correct for undesirable changes in depth.

Combinations of acoustic transceivers (transmitter-receivers), operating at frequencies above the normal seismic frequencies can be used to establish

distances from one part of one streamer to another and to the source array.

Compasses are also used within the streamer itself to check the angle of the streamer at several positions along it's length.

Complex electronics within the streamer filters the incoming signal from a whole group of geophones, and then converts the resultant voltage into a numeric value (digitisation).

The numbers corresponding to the values for all of the groups at any one time are multiplexed (interleaved) together and sent digitally down just a few wires to the recording instruments.

This process is repeated roughly every 2 milliseconds (2 ms or 1/500th of a second) and the resultant digital recording is eventually sent to the processing centre. (Which may also be on the vessel!).

Other equipment, either in the streamer or in the instrument room on the vessel, can be used for real-time processing of the seismic data. Other, more complex processing, can be done while the vessel is turning-around between seismic lines. (This can take a time - remember the 3 kilometres of equipment out the back!).

Other techniques can be, and are used for the recording of marine data. Ocean-Bottom Cables, where the receivers are actually placed in contact with the seabed, are becoming increasingly common. These allow for the recording of S-Waves (remember, these don't travel though water), and can be fixed in position to allow for the re-recording of the data in future years. This technique, or 4D recording (3 dimensional seismic data recorded at

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time intervals), can be used to measure changes in an Oil or Gas field caused by the actual extraction of the hydrocarbon.

Although the "per-kilometre" rate for offshore acquisition is much less than that for onshore, there are still considerable costs involved. A single, modern, seismic streamer costs about $1,000,000, and the running costs of personnel and equipment are also high.

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Chapter 3 - Waves and Raypaths

We now move on to the two ways that seismic energy can be described, either as waves or as ray paths, and look at some of the wave and ray path phenomena that can appear on our sections.

Page 03.02 - Waves

The general parameters associated with waves, in both time and space.

Page 03.03 - P & S Waves

The two types of waves encountered in seismic exploration, their differences, and all their possible names!

Page 03.04 - Wavefronts

One way of looking at seismic energy. A very brief mention of the wave equation used (in an approximate form) in some of the more complex processing.

Page 03.05 - Ray Paths

The other way of looking at the energy, and the one used most of the time. Some of the physics associated with reflections and refractions.

Page 03.06 - More Ray Paths

More on ray paths. Reflection coefficients, and the arithmetic of reflection and refraction. Page 03.07 - Ghost Reflections

One of the more common problems associated with marine data. The duplicate reflections produced by energy that has reflected back down from the sea surface.

Page 03.08 - Multiple Reflections

The biggest single problem for the processing geophysicist! Events on our seismic section produced by the energy reflecting more than once.

Page 03.09 - Diffractions

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Another problem that can sometimes be useful data. Energy that is reflected back in all directions from abrupt changes in the physical parameters of the Earth.

Page 03.10 - Questions

Another set of questions to conclude this Chapter.

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Waves

Whenever an acoustic source is detonated on or near the surface of the Earth, an acoustic wave is produced that propagates away from the source.

Apart from effects very close to the source, this wave moves through the medium without causing a net movement of the material - the medium (more or less) returns to its normal state once the wave has passed through.

Waves can travel through a body (body waves), or along the surface of a body (surface or interface waves), and, although we are not directly interested in surface waves they can be used for shallow investigations (refraction studies) or they can cause problems by masking body waves within seismic data.

Here are some of the parameters associated with a wave recorded as a function of time:-

A simple sinusoidal wave can be described by three parameters. Its amplitude, or maximum

excursion from the zero level, its frequency, or the number of "cycles per second" of the wave, and its

phase - the offset of the maximum value from time 0 measured in degrees along the cycle (1 cycle =

360 degrees). For more complex waves, the RMS or root-mean square amplitude may be more useful - we'll come back to that later.

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A wave shown as a function of distance has similar parameters:-

The wavenumber is usually measured in "cycles per 1000 metres" and represents the frequency of the wave in space. For a wave measured in both time and space (on, for example, a seismic section) the velocity relationship shown above gives the horizontal velocity of the wave through the earth.

Here's another time-domain wave, use the scroll bars to change frequency, amplitude and phase:-

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P and S Waves

Energy that is applied exactly at right-angles to an elastic body produces an elastic body wave in which the particle motion is in the direction of propagation - a P-Wave.

The pressure wave pushes the particles of material ahead of it, causing compression and expansion of the material.

The P-Wave is the type of wave assumed for conventional seismic exploration.

The P-Wave is also known as:-

 Primary wave  Compressional wave  Longitudinal wave  Push-pull wave  Pressure wave  Dilatational wave  Rarefaction wave  Irrotational wave

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A wave in which the particle motion is at right-angles to the direction of

propagation is known as an S-Wave.

S-Waves have been used for some forms of seismic exploration but they do not propagate through fluids so are not

recorded (directly) in conventional marine acquisition.

The S-Wave is also known as:-

 Secondary wave  Shear wave  Transverse wave  Rotational wave  Distortional wave  Equivolumnar wave  Tangential wave

Whenever a P-Wave or S-Wave hits a change in velocity or density, a process known as mode conversion takes place and some part of the wave converts into the other form (P to S, S to P). The use of this phenomenon is just beginning to be investigated but, at the

moment, it is ignored in conventional seismic processing. It can, however, explain some of the artefacts seen on seismic sections.

The (somewhat gross) assumptions made in seismic processing assume only P-Waves, travelling through isotropic* and homogeneous* material.

* isotropic - the physical properties of the material don't depend on the direction of the wave - it's the same in all directions.