The Seismic Method

Slide 1

• This unit gives a very simple introduction into reflection seismology
• A little about:
• The reflection of seismic data
• Seismic acquisition
• Seismic processing
• The goal of having data to interpret geology & HC systems

Slide 2

• This is what people in exploration do at a very high level
• They identify opportunities (high potential basins where we might gain access – licenses)
• They capture prime areas – typically through lease sales)
• If they get 1 or more blocks, they may acquire better seismic data
• The seismic data has to be processed
• Then the data are interpreted
• During interpretation, interesting features/anomalies that might be associated with undiscovered fields are noted
• Initially they are called ‘leads’
• If some detailed work indicates that there is a good chance they hold economic amounts of HCs, they are called prospects
• The use of the term prospect varies – some say it is a lead good enough to present to a manager as a drilling candidate
• Prospects have to be assessed – to help decide on whether or not to drill it
• Will it hold oil or gas?
• How much?
• What is the quality of the reservoir?
• An economic analysis would then be performed
• Is the value of the predicted HCs greater than the costs involved?
• All this is presented to the manager – he/she decides if it should be drilled
• The first well is called a wildcat
• Of course we do good work – and the wildcat is a success – we find evidence for enough HCs so that we want to proceed
• Exploration may need to drill 1 or more confirmation/delineation wells
• We will illustrate this with a hypothetical example in a few minutes
• Once the economic viability is established, the new FIELD is turned over:
• To the development department/company if lots of money needs to be spent on facilities (platform, pipeline, etc.)
• To the production department/company if only a little money is needed (e.g., 2 land wells and an extension of a pipeline a couple of miles)

Slide 3

• This slide illustrates the basics of reflection seismology
• We start with an energy source – like an explosion
• Acoustic energy radiates down through the Earth (represented by the half-circles and the arcs)
• For simplicity, geophysicists use rays (lines with arrows) to represent the acoustic energy traveling through the Earth
• At a boundary between one unit and the next deeper unit, some of energy is reflected – most is transmitted (continues to travel down)
• The reflected energy travels towards the surface where we have set out “listening” devices
• In this cartoon example, some energy is reflected off the top of the orange unit and is “heard” (recorded) at 0.4 seconds at the receiver near the middle of the diagram
• Most of the energy is transmitted through the orange layer
• At the top of the brown layer, some energy is reflected and is recorded at 0.8 seconds at the receiver on the right side of the diagram

Slide 4

• This slide shows what the raw seismic data would look like
• The seismic energy sent out by the explosion looks like a sine wave
• The filled in (blue) portions would be compressions – material is being pushed together – usually recorded as positive numbers and dispalyed to the right of the center (zero) line
• The unfilled portion to the left of the center line would be rarefactions – material is expanding out – usually recorded as negative numbers and displayed to the left of the center (zero) line
• Device #1 recorded the energy reflected off the top of the orange unit and shows a response at 0.4 seconds
• Device #2 recorded the energy reflected off the top of the brown layer and shows a response at 0.8 seconds
• To get a good image of the subsurface, we use hundreds of shots (explosions) and millions of receivers (listening devices) arranged in lines either on land or in the offshore environment

Slide 5

• Today many of our seismic surveys (both land and marine) are to collect 3D seismic data
• However, there still are 2D surveys being collected
• We start by asking the ones familiar with the area about what they want to image
• E.g, an anticline at 9876 ft covering 10 sq miles and sands believed to be 150 feet thick
• This determines the survey parameters
• Survey area, the fold, shooting direction, etc.
• With seismic acquisition it is always a balance
• Good data quality is expensive
• What level of quality do you need to answer the business questions?
• The left picture is of a land seismic acquisition operation
• Four vibrator trucks work in tandem
• The right picture is of a marine seismic acquisition ship
• In the water you can see 2 airgun arrays
• You can also see 4 streamers (cables with hydrophones)

Slide 6

• Here is a display of raw seismic data – what would be recorded for one shot/explosion (marine example)
• The horizontal scale is receiver number which can be translated into ft/miles or meters/km
• The vertical scale is two-way travel time
• The receiver nearest the boat is on the left; receiver furthest away on the right
• Notice the hyperbolic shape of the reflections
• This is because near the boat the energy travels almost straight down and up – very little lateral distance (red arrow on right figure)
• For receivers far from the boat (perhaps 4 km) the energy not only has a vertical component but also a horizontal component (blue arrows on right figure)
• Thus the distance traveled by the blue rays is longer than the red rays – and takes more time
• Based on the hyperbolic shape of the reflections, we can calculate the average velocity along the ray paths

Slide 7

• We obtain the raw seismic data for energy traveling from each shot into each receiver
• The raw data goes to the seismic data processors
• They have methods to manipulate the raw data so that we get images of the subsurface that can be interpreted
• With data processing, the saying “you get what you pay for” is true
• Simple corrections are fast and relatively cheap (in dollars, manpower and time)
• If the subsurface is complex, there are very sophisticated algorithms to “focus” the subsurface image – but these are very expensive to extremely expensive
• The processing that is applied is (hopefully) enough to give images that answer the business questions without spending more money/effort than necessary
• As with acquisition, we strive to achieve the correct balance

Slide 8

• This slide shows a marine operation
• For simplicity we will consider 1 shot (S1) and 5 Receivers (R1, R2, ... R5)
• The shot record on the right shows 2 events:
1. the direct arrival – where acoustic energy travels horizontally through the water and is detected by the receivers
• What is recorded is shown by red detected energy (reflections)
• Note this event shows as a straight line
• The slope of the line is controlled by the velocity of sound in sea water
2. A reflection off the top of the grey layer
• What is recorded is shown by blue detected energy (reflections)
• Note this event has a hyperbolic shape
• The shape of the hyperbola can be used to estimate the average velocity between the shot and the top of the grey unit

Slide 9

• The first thing the data processors do is to sort the data
• What they want to do is to collect all the reflections that “bounce” off the same subsurface point
• For example, they want all the information related to the red box “A”
• Different combinations of shots and receivers have a “bounce” point at A (e.g., shot 5 into receiver 5)
• They display these combinations as a function of lateral distance to get the figure on the right – a common midpoint gather (CMP)
• It looks like a shot record, but instead of the shot being the common feature – the “bounce” point or midpoint is the common factor
• Whereas on the shot record the traces are evenly spaced, the traces on a CMP gather may not be equally spaced

Slide 10

• On the CMP gather, we again have reflections with a hyperbolic shape
• The travel times differ since the path for a near offset trace is shorter than the path for a far offset trace
• If we know or can estimate the correct velocity, we can correct for the difference in travel time for each trace
• From the shape of the hyperbola, we can estimate the average velocity down to the depth of the reflection

Slide 11

• With the speed of computers, we can iteratively try different velocities and see which value is best
• We know the velocity is correct when all the reflections are at the same time valve – they are FLAT
• This is shown in the middle figure on the right
• If the velocity is too slow, the reflection curves down – we have not corrected the gather enough (upper right)
• If the velocity is too fast, the reflection curves up – we have over-corrected the gather (lower right)

Slide 12

• The next step is to sum all of the (moveout) corrected gathers
• In this case, we have 10 traces that are from a common midpoint – each with a different amount of lateral offset
• We add the 10 traces together – since we are adding 10 traces, we say that this is 10 fold data
• Why do we add the traces?
• Each individual trace has information about the midpoint (like the red A box) and a certain amount of ‘noise’
• The receivers pick up the reflected seismic energy, but also other sound energy
• On land, it might be noise from passing cars/trucks, wind, etc.
• In marine operations, it might be waves, noise on the boat, weather, etc.
• The geologic information from the midpoint (e.g., box A) should be about the same on all the traces
• Much of the ‘noise’ is random
• By doing the summation, we enhance the signal and we cancel out the noise
• Stacking is one of the best ways we have to improve the signal-to-noise ratio

Slide 13

• Thus far we have kept everything quite simple by assuming the boundaries that generate reflections are horizontal (flat)
• Problems develop if the boundaries are not flat – they have some dip
• When the boundary is dipping, what we record is the energy that travels from the source and hits the boundary at 90 degrees
• In the figure on the left, we record the energy that follows the dotted black line
• In this case, the energy travels 0.2 seconds down to the bounce point and 0.2 seconds up to the receiver – a total of 0.4 seconds
• When this is displayed, we would plot the reflection vertically below the shot at 0.4 seconds
• So we capture the reflected energy, but do not place it in the correct position
• This is another thing we have to correct – but, don’t worry, we have methods to apply this correction!

Slide 14

• Time for an exercise!
• Here we have a dipping seafloor
• We will consider 6 shot locations and only the direct down and direct up ray path
• That direct down – direct up raypath hits the seafloor where it is 90 degrees – as shown for shot #1
• We want to figure out where the reflected energy would be displayed (without corrections)

Slide 15

• To figure out where the reflected energy would be displayed, we can use a compass
• Place the point on the shot point (here shot point #1)
• Place the pencil point at the place where the raypath hits the seafloor at 90 degrees – as shown for shot #1

Slide 16

• Next we swing an arc so that the pencil point is directly below the shot point
• Thus on the previous slide we captured the distance (for a seismic section that would be related to the time)
• And on this slide we have determined where it would be displayed – directly beneath the associated shot

• NOW THE STUDENTS SHOULD SWING ARCS FOR THE OTHER SHOTS
• Where is the “bounce” point – where the ray path is at 90 degrees
• Swing an arc to locate where the recorded reflection would be plotted/displayed
• This should take about 5 minutes

Slide 17

• ANSWER – This is what you should have found
• The seafloor (reflection surface) is the black line
• BUT the seismic reflection is displayed where the red line is located
• We considered only 6 points – if the shots were more closely spaced, we would get a continuous reflection

• NOTE: a continuous surface in the subsurface will result in a continuous reflection on the seismic section

Slide 18

• There is a data processing procedure that we call seismic migration that corrects this mis-positioning problem
• For each trace, the computer swings arcs (based on the velocities) to find all the possible locations from which a reflection could have originated
• To illustrate, consider the black peak circled by a green oval in the upper left – we will call this the “green” peak
• In the lower left we have swung arcs to define all the possible reflections points
• The dotted green arc shows the possible reflections points for the “green” peak
• On the right, we show 3 possible shot-receiver pairs that could be the reason for the “green” peak
• But which one is correct – of these 3 or some other case?
• Seismic migration will answer this for us

Slide 19

• Since we are dealing with waves where we have some positive and negative numbers and closely-spaced traces, as we migrate all the traces something wonderful happens
• For the small piece of the arcs from the true position (the right answer mentioned on the previous slide) there is constructive interference and the wave shape is preserved and enhanced
• For all other places along the arcs, there is destructive interference – positive numbers are canceled by negative numbers
• So in simple terms, all the incorrect places along each arc are wiped out, but the correct location is preserved
• On the left side of this slide we show 2 seismic reflections
• They are dipping, so they are not is the correct position
• Since the correction has NOT been applied, we call this unmigrated data
• On the right side of this slide we show the 2 seismic reflections after they have been migrated
• Both reflections have been moved
• The red dotted lines show where the unmigrated reflections (left figure) are for reference
• Migration moves the reflections deeper and further updip
• Low dips -> slight corrections moving events deeper and updip
• High dips -> larger corrections moving events deeper and updip
• The cancellation is not perfect, so you see some ‘noise’ away from the reflections

Slide 20

• Here is a comparison of the same seismic line before (upper) and after (lower) seismic migration
• Note there is poor imaging – lots of smearing – of the structure
• Performing even a simple migration has improved the image of the structure – in this case there is a thrust fault

Slide 21

• After seismic acquisition and processing, we have seismic interpretation
• Here is where we take the images and deduce the subsurface geology
• This includes:
• Map faults and other structural features
• Map unconformities and other major stratal surfaces
• Interpret depositional environments
• Infer lithofacies from reflection patterns & velocities
• Predict ages of stratal units
• Examine elements of the HC systems