Introduction to Marine Seismic Processing - ProMAX

Июль 26, 2022

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Introduction to Marine Seismic Processing - ProMAX

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2. ProMAX Exercise What is ProMAX? A software package for processing reflection seismic data Commonly used in
3. Where did the data come from? A short seismic reflection survey in Papua New Guinea September
4. Woodlark Basin A rift basin in Papua New Guinea A classic place to study the orogenic
5. High Pressure Air Sources: The Air Gun Ready Fire! Fired Lower chamber has a top diameter
6. Airguns suspended from stowed booms Single Air gun – note air ports Air Guns Other source?
8. Summing the signal of multiple guns creates a more desirable signal Note the relative scales of
9. Listening Hydrophone Piezoelectric material Pressure changes in the water generate small currents which are amplified Geophone
10. How is a Marine Seismic Reflection Survey Shot? Definition of shot and common mid-point (CMP) gathers
30. Starting ProMAX Type Promax on the command line Select the survey Select the line that includes
31. Anatomy of ProMAX Click on the EW9910 area, then EW9910 Line ODP11 List of flows –
32. Anatomy of ProMAX Click on 01 – Display Shots List of individual processes – things we
33. Anatomy of ProMAX Click on Disk Data Input with the middle mouse button. This will parameterize
34. Anatomy of ProMAX Clicking on the data file 11 Shots w/geom with the left-hand mouse button
35. Anatomy of ProMAX Clicking on the data file 11 Shots w/geom with the left-hand mouse button
36. Displaying a Shot Clicking on Execute with the LHMB Direct ray path – sound travels directly
37. Water Velocity Clicking on the zoom icon. By holding down the LHMB and dragging a box,
38. Near-Trace Plot When we are collecting data we want to see it as quickly as possible
39. Near-Trace Plot Go back to the flow 02 – Near Trace Plot and uncomment Automatic Gain
40. Power Spectrum Go back the list of flows. Click on the flow 03 – Power Spectrum
41. Filtering We can use a bandpass filter to remove frequencies below and above a certain range.
42. Filtering For each shot a filtered and unfiltered (Control copy) version of the data is displayed.
43. Removing NMO The reason for having so many (24 in this case) traces in a CMP
44. Removing NMO in Practice – Velocity Analysis From Yilmaz, 1989 Go back to the flows list
45. Velocity Analysis Semblance plot CMP Click on the zoom icon and zoom into this area Dynamic
46. Velocity Analysis Click on the pick icon to pick velocity/time point on the semblance plot Click
47. Stacking Go back to the list of flows. Click on 06 – Stack This flow uses
48. View Stack Go back to the list of flows. Click on 07 – View Stack Execute
49. Migration In an un-migrated time section reflectors do not represent the true subsurface geometry. See examples
50. Migration Go back to the list of flows. Click on 08 – Migration Using a velocity
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ProMAX Exercise
What is ProMAX?
A software package for processing reflection seismic

data
Commonly used in the energy industry
Not free!
There are many other programs that will do the same sort of thing – they differ mainly in their user interface (or lack thereof)
Runs on many flavors of UNIX

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Where did the data come from?
A short seismic reflection survey in

Papua New Guinea
September 1999
R/V Maurice Ewing
1.2 km streamer
48 channels
25 m group interval
1395 inch3 tuned six airgun array
25 m shot interval
24-fold CMPs 12.5 m apart
1 Arc-second (~30 m) grids
Downloaded from the USGS Seamless server (http://seamless.usgs.gov/) and converted to a GMT format (NetCDF) grid
Re-sampled (using grdfilter) to a 0.0005 degree grid
Also available from NASA (ftp://e0srp01u.ecs.nasa.gov)

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Woodlark Basin
A rift basin in Papua New Guinea
A classic place to

study the orogenic rifting to seafloor spreading transition
The line that we will look at crosses a large rift basin close to the transition to seafloor spreading
The survey was carried out as part of an Ocean Drilling Program site survey

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High Pressure Air Sources: The Air Gun
Ready
Fire!
Fired
Lower chamber has a top

diameter that's smaller the bottom diameter - air pressure forces the piston down and sealing the upper, firing chamber. High pressure air is filling the firing chamber through the T-shaped passage, and the firing, or actuating air passage is blocked (solid black) by a solenoid valve.

Full pressure has built up in the upper chamber. The Solenoid has been triggered, releasing high-pressure air into the active air passage, which is now yellow. The air fills the area directly below the piston, overcoming the sealing effect of the air in the lower, control chamber. The piston moves upwards, releasing the air in the upper chamber into the water.

A large bubble of compressed air is expanding into the surrounding water. The air in the lower control chamber has been compressed. The triggered air, released into the space below the piston, is fully expanded, and can now exhaust at a controlled rate through the vent ports. As this takes place, the piston rapidly but gently moves downward, re-sealing the chamber, and readying the sound source for refilling.

From: http://www.ldeo.columbia.edu/res/fac/oma/sss

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Airguns suspended from stowed booms
Single Air gun – note air ports
Air

Guns

Other source?

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Summing the signal of multiple guns creates a more desirable signal
Note

the relative scales of the left and right plots

Tuning An Air Gun Array

From http://www.ldeo.columbia.edu/res/fac/oma/sss/tuning.html

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Listening
Hydrophone
Piezoelectric material
Pressure changes in the water generate small currents which are

amplified
Geophone
Mechanical
Motion of coil relative to magnet generates a small current which is then amplified

From Kearey, Brooks, and Hill, 2002

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How is a Marine Seismic Reflection Survey Shot?
Definition of shot and

common mid-point (CMP) gathers

Shot gather: All the data recorded on all the channels by a single shot

CMP gather: A collection of traces that have been recorded at the same location.

Shot and CMP gathers are simply different ways of sorting the data.
What is the natural CMP spacing relative to the group interval?

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Starting ProMAX
Type Promax on the command line
Select the survey
Select the line

that includes your name

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Anatomy of ProMAX
Click on the EW9910 area, then EW9910 Line ODP11

List of flows – operations that we will apply to the data

This area tells you what each of your mouse buttons will do – ProMAX uses three mouse buttons

Status of any jobs that are running

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Anatomy of ProMAX
Click on 01 – Display Shots
List of individual processes

– things we can do to the data. Flows are built up of a sequence of processes. Click on one and it will appear in the left-hand window. Delete and processes accidentally added by clicking on delete in the left-hand window

This flow reads in the seismic data and displays it

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Anatomy of ProMAX
Click on Disk Data Input with the middle mouse

button. This will parameterize the process. Here we see that we are reading in 11 Shots w/geom (raw shot file with navigation added to the headers), sorting the data by Source index number (shot number), reading in every 10th shot from shot 300 to the end of the file

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Anatomy of ProMAX
Clicking on the data file 11 Shots w/geom with

the left-hand mouse button (LHMB) will take you to a list of data files. Click on 11 Shots w/geom with the middle mouse. The details of the data file are now displayed

We can now see how many traces there are in the file, the sample rate (in milliseconds), how many samples there are per-trace, the minimum and maximum CDP, and the minimum and maximum shot (SIN). Moving your cursor to the top of the screen will take you back to the flow

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Anatomy of ProMAX
Clicking on the data file 11 Shots w/geom with

the left-hand mouse button (LHMB) will take you to a list of data files. Click on 11 Shots w/geom with the middle mouse. The details of the data file are now displayed

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Displaying a Shot
Clicking on Execute with the LHMB
Direct ray path –

sound travels directly from the airgun array to the hydrophones – forms a straight line

Reflected ray path – sound bounces of the seafloor and underlying layers – forms a hyperbola

Water column noise

Reflected ray path – sound bounces of the seafloor and underlying layers

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Water Velocity
Clicking on the zoom icon. By holding down the LHMB

and dragging a box, zoon into the area where we see the direct wave. The gradient of the direct wave gives us the water velocity. Click on the gradient icon. By holding down the LHMB, drag a line that follows the first arrival of the direct wave. The corresponding velocity will be displayed at the bottom of the screen

Zoom icon

Gradient icon

Which channel is nearest to the ship?

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Near-Trace Plot
When we are collecting data we want to see it

as quickly as possible – one way of doing this is by displaying a near-trace plot. This is simply a display of the channel nearest to the ship for each shot. This will give us the first glimpse of what we are looking at in terms of geology. Go back to the list of processes and click on 02 – Near Trace Plot. Execute the flow.

Seafloor

Basement

Graben bounding faults

Multiple

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Near-Trace Plot
Go back to the flow 02 – Near Trace Plot

and uncomment Automatic Gain Control by clicking on it with the right-hand mouse button (RHMB). This will add gain to the section, enhancing the deeper reflectors

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Power Spectrum
Go back the list of flows. Click on the flow

03 – Power Spectrum and execute it. This flow is setup to show the frequency content of every 10th shot. We use a plot like this to determine characterize the range of frequencies in data, and possibly identify noise

Frequency content by channel

Frequency range

Phase

Shot gather

Click on the arrow to go to the next shot

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Filtering
We can use a bandpass filter to remove frequencies below and

above a certain range. We are now going to test some filter parameters using the process 04 - Filter

The filter defined in Parameter Test will remove all frequencies below 6 Hz and above 80 Hz. All frequencies between 10 and 70 Hz will be kept. A ramp is applied to intermediate values

The number 99999 next to filter values indicates that the actual filter value comes from the Parameter Test process

Execute the flow

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Filtering
For each shot a filtered and unfiltered (Control copy) version of

the data is displayed. Advance to the next shot by clicking on the arrow.
Zoom in to look at the data in detail
Try some different filters

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Removing NMO
The reason for having so many (24 in this case)

traces in a CMP is so that we can stack (sum) the traces for a given CMP.
Noise cancels out
Real signal (geology) is amplified
Signal to noise ratio increase
First we must remove Normal Moveout (NMO) – the difference in travel time that is the result of varying ray path lengths

From Yilmaz, 1989

From Kearey, Brooks, and Hill, 2002

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Removing NMO in Practice – Velocity Analysis
From Yilmaz, 1989
Go back

to the flows list in ProMAX and select 05 – Velocity Analysis – click on Execute

Nowadays velocity analysis is carried out using semblance plots – these show how well the data stacks (i.e. a reflector is coherent across a stack after NMO is applied) for a given two-way travel time and velocity

CMP

Semblance plot

NMO has been removed correctly and the reflector is now coherent

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Velocity Analysis
Semblance plot
CMP
Click on the zoom icon and zoom into this

area

Dynamic stack

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Velocity Analysis
Click on the pick icon to pick velocity/time point on

the semblance plot

Click on Gather – Apply NMO to see NMO applied as you pick

NMO

Add velocity/time points to the semblance plot such that the NMO is removed for the major reflectors.
Zoom in and out as necessary
Do not pick the multiple
Save your picks

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Stacking
Go back to the list of flows.
Click on 06 – Stack
This

flow uses your velocity picks and other that were picked earlier to stack the data
The traces in each CMP are summed to form one trace

Removes some residual noise and spikes

Applies a bandpass filter to the data

Applies the NMO correction using the picked velocities

Trace mutes to remove stretched traces and attenuate multiple

Traces in each CMP are stacked

Execute the flow – this will take some time…..

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View Stack
Go back to the list of flows.
Click on 07 –

View Stack
Execute the flow
You will see that the image is now much better than our original near trace plot
You can start to see stratigraphy
However, the are lots of diffractions and reflectors are not in their correct subsurface location – we need to migrate

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Migration
In an un-migrated time section reflectors do not represent the true

subsurface geometry.
See examples below…

From Kearey et al., 2002

Seafloor

Time section

Geological Cross-section

Dipping reflectors

Bow-tie effect

(A)

(B)

(C)

(A) a syncline on the seafloor is imaged as a “bow-time section
(B) The addition of diffractions from the end of reflectors results in a very complex time section
(B) A dipping reflector is shallower in a time section

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Migration
Go back to the list of flows.
Click on 08 – Migration
Using

a velocity model that was made earlier we will migrate the data
There are a number of ways of migrating the data – all are mathematically very complex….

Execute the flow. This will take some time
When it has finished running, Click on 09 – View Migration and execute the flow