GUNDALF help index

Contents

• Introduction

• Getting in

• ... and getting out

Gundalf Designer

Gundalf Optimiser

GUNDALF - Airgun array modelling

Introduction

GUNDALF is an abbreviation for GUN Design And Linear Filtering. It consists of two interlinked products:-

• GUNDALF Designer

• GUNDALF Optimiser

This documentation covers both products. To see which product you click on the main menu (the one underneath your email address) and select "Licence Details". This will show the version, calibration date, calibration epoch date (the last date when any calibration was carried out) and the product.

The Designer product contains basic airgun array modelling options whereas the Optimiser contains advanced optimisation options as well as all the Designer options. Both options use the same state of the art airgun modelling engine.

A common help system is used. There are two levels: a pop-up help system for most labels and a detailed set of help messages on various topics under the menus. These are intended to be sufficient for any use of the toolset. There is no hard copy. If there is any aspect which you find unclear, please tell us and we'll improve it. We take this very seriously.

Tooltip Help

Most of the labels have standard tooltip help. Just hold your mouse over a label if you need more information.

Help in general

Check the menus at the top under KnowledgeBase and Help.

Menu configuration

Some of the menus in Gundalf would be very long indeed if all possible options were shown. The two largest are the filters menu, (some 3,000 external filter options in C8.3l onwards) and the airgun menu which contains 25 items as of C8.3m. All of these are available to the user but the menus are user-configurable so you are not swamped by them. In the case of the filters, this is done using the "Edit filter menus" option underneath the main menu tagged by your email address. This allows you to select which filters you would like to appear in the filter menus when modelling. Similarly the "Edit airgun menu" option allows you to choose which airgun types you would like to appear in the airgun palette.

Since Gundalf is stateless as part of its security infrastructure, it does not store these choices in between invocations. If you would like to save these choices for all sessions as a personal configuration, it is recommended to set both filter and airgun menus as you would like to have them and then use the "Save Session" option under the main menu to create a local session file on your own machine, say personalMenus.ses. Then, when starting any future session, begin by uploading this file using the "Restore Session" option and the menus will be restored to your personal choices.

Getting in ...

This is a Cloud application. On entering the portal you will be presented with a user/password dialog. When you pass this you will enter a geometry section where you can construct an array gun by gun or by importing arrays or sub-arrays. The array format is identical to that of Gundalf Windows if you want to upload old arrays.

The graphics area

The graphics area in the geometry section simply shows the current working array in plan view and from the side. This is dynamic so changes as you add / modify / remove guns. x increases to the right away from the boat heading, y increases from top to bottom and depth (z) increases downwards forming a standard right-handed Cartesian system. you have studied so far.

And getting out ...

Just choose the Logout option under your username on the right of the Menu system at the top. Here you can also save or restore your session by downloading or uploading to your local machine and you can also reset your session which sets all of the parameters to their defaults.

GUNDALF - Designer

Designing an array

Creating an array from scratch

In the geometry section you can add, delete or modify guns one by one. There is a button to add, and edit/delete buttons for each gun in your working array, which is the one you see. You can also import or export ready-made arrays either as one or sub-array by sub-array using the Library and sub-array management buttons.

Note that sub-array numbers are allocated by Gundalf based on the separation of guns in the crossline or y direction. They allow you to do operations on grouped guns, such as moving a subarray. They are in other words simply a convenience as arrays are often built up from identical or near-identical sub-arrays. They have no effect on array modelling as Gundalf automatically includes all interactions.

Note that in the "Add gun" menu, guns can be either active (green) or spare (blue). If they are spare, they are ignored by Gundalf except in drop-out modelling if you check the option to allow Gundalf to use spares wherever possible to mitigate drop-outs.

Saving an array

It is important to realise that the working array is not by default saved by Gundalf because no client information is stored on the Cloud servers. If you wish to save your working array, you must use the Library array management in the geometry section to download it to your local computer. You may then upload it by the same mechanism in the future. Array files are in a simple text format described in the KnowledgeBase article on File formats.

The sub-array number is only for the convenience of manipulating sub-arrays as a whole. When Gundalf computes the signature of the array, it always uses all guns so that all interactions are correctly included.

Importing an array

You can import an array by uploading it from your local computer using the library array management button in the geometry page.

Building an array from regular sub-arrays

If (as is often the case), your array is built from a number of identical sub-arrays, you can build these sub-arrays independently by creating them from scratch and then saving them as described above. To build an array from regular sub-arrays, use the "add local array to Working array" option in Library array management. This will allow you to add it to the working array with an (x,y,z) offset. Gundalf will assign a sub-array number depending on the crossline separation.

Translating an array or sub-array

You can move the entire array or any selection of sub-arrays by an x, y and z increment using the translate option under the "Sub array management" button in the geometry section. This option will preserve relative distances and can be used for example to change the depth of an entire array (z), without disturbing the relative distances in clusters for example.

Deleting an array or sub-array

You can delete the entire array or any selection of sub-arrays using the delete option under the "Sub array management" button in the geometry section.

Changing the depth of an array or sub-array

You can change the depth of an entire array by using the change depth option under the "Sub array management" button in the geometry section.

Randomising the depth of an array or sub-array

You can randomise the depth of any or all sub-arrays by using the randomise option under the "Sub array management" button in the geometry section.

The purpose of this option is to allow the user to simulate the inevitable gun depth variations in the field. Simply enter the level of uncertainty in gun depth, (0.5m is typical), choose which sub-arrays to apply it to and then select OK. Gundalf will randomly perturb the depths of the guns in the selected sub-arrays by up to +/- this amount.

Delaying an array or sub-array

You can delay all the guns of any or all sub-arrays by a set amount by choosing Change delays option under the "Sub array management" button in the geometry section.

The purpose of this option is to allow the user to delay an entire sub-array by the same amount. Simply enter the delay, choose which sub-arrays to apply it to and then select OK. Gundalf will then delay all the guns in the selected sub-arrays by this amount.

Aligning the bubbles in an array or sub-array

Choose the "Align bubbles" option under the "Sub array management" button in the geometry section. This uses a standard relationship for bubble period in terms of pressure and volume to attempt to delay individual guns so that their bubbles are approximately aligned.

This option was introduced as there was some interest in the industry in trying to boost low frequencies for sub-basalt imaging. It was believe that aligning bubbles would facilitate this. Our own experiments suggest that this is not the case but the option has been left in for the user to perform their own experiments. Simply choose which sub-arrays to apply it to and then select OK. Gundalf will then line up all the bubbles. Note that this is only an approximation because of the effects of interaction.

Clearing an array

Simply use the Delete all guns button on the geometry page.

Modelling gun controller variations

Use the RMS variation in gun controller delays on the physical parameters page to set a level of variation in the gun controller. Gundalf will generate a Gaussian distribution based on this value and randomly vary the gun timings to simulate a real gun controller. This has the effect of reducing the high frequencies. Note that the random seed is based on the system clock so each random distribution will be different and the report will therefore be non deterministic - repetitions of the same parameters will have a slightly different distribution. Note also that even if the gun geometry is symmetric, random variations in gun timings will make the overall directivity asymmetric. A value of zero means that timings will not be varied.

Generating a signature report

Choose the "Signature model and statistics only" option in the Service Choice section , follow the parameter pages presented to you setting them as appropriate, run the model and download the resulting report. If you wish to include directivity plots, choose the "Signature model and statistics and directivity" report before proceeding.

Report options can be set using various parameter pages.

Generating a full drop-out report

Choose the "Drop-out analysis" option in the Service Choice section , follow the parameter pages presented to you setting them as appropriate, run the model and download the resulting report.

Note on timing: 3 gun dropout calculations require the complete array to be simulated n(n-1)(n-2)/6 times where n is the number of guns in the array unless there are symmetries which Gundalf can exploit. Unlike Gundalf Windows, there is no restart option as the Cloud version does not store user data.

After the run completes, Gundalf will give you the option of generating an Intranet compatible report.

Note on timing: 4 gun dropout calculations require the complete array to be simulated n(n-1)(n-2)(n-3)/24 times where n is the number of guns in the array. A single full-size array run might take a 30 seconds so 4-gun dropout tests can take many days even on high-end PCs. Gundalf shows you the run progress and you can stop it at any time and restart it later. Please note the restart option is provided for convenience only and it is important not to change the drop-out parameters in between a stop and a restart.

Gundalf gives you the option of saving drop-out data in the form of .csv (comma separated variable) file which can be imported into a spreadsheet such as Excel for special purpose plotting.

Generating an array comparison report

Choose the "Array comparison" option in the Service Choice section , follow the parameter pages presented to you setting them as appropriate, run the model and download the resulting report.

The far-field signature

Filtering the signature

Most options allow the signature to be filtered and the type of filtering can be chosen in the Filter parameters section if presented.

Note that there are three kinds of filter

• (Optional) Q filtering: This allows you to specify approximate filtering effects of the earth's sub-surface by specifying a value for Q and a two-way travel time. This form of attenuation is widely used to model earth filtering. Values of Q vary between around 10 for some very absorptive rocks such as shales up to several hundred for limestones.

• (Optional) Wiener filtering: This allows you to specify a Wiener filter using standard parameters such as whether the filter is a spiking filter or a shaping filter, the filter gap in samples, the filter length in samples and the amount of white light. In addition, a scaling can be applied optionally which preserves the energy of the input wavelet but it should be noted that deconvolution always distorts amplitudes in some way.

• (Optional) band-pass filtering: These can be specified in two ways.

  • They can be specified by their low-cut and high-cut values with the appropriate slopes in db./octave. In this case, the filter is constructed directly in the frequency domain, cosine smoothed and then applied either as a minimum phase filter or as a zero phase filter.

  • An external instrument filter can be specified from a pop-up list. These are filters supplied by the manufacturer. Gundalf applies them without any alteration other than a single normalising scalar and this requires the signatures to be modelled at the same sample interval as the filters. The sample interval is encoded in the filter name, so that a filter with a sample interval of 0.5msec appears with 0p5 in the file name.
    
    In both cases, the filters are scaled to have an amplitude spectral value of 1.0 in the pass-band. This means that the Fricke maximum scaled amplitude (see below) is almost independent of the chosen filtering.

  In both cases, the filters are scaled to have an amplitude spectral value of 1.0 in the pass-band. This means that the Fricke maximum scaled amplitude (see below) is almost independent of the chosen filtering.

The amplitude spectra are always be displayed in the standard industry format based on the work of Fricke, Davis and Reed (1985) "A standard quantitative calibration procedure for marine seismic sources, Geophysics (50), number 10, p. 1525-1532. These authors described a way of computing the amplitude spectrum for a transient which allows sensible comparisons to be made independently of the sampling interval or the duration of the transient. The resulting amplitude spectral scale is in db. relative to 1 microPascal per Hz. At 1m. The peak amplitude of a typical full seismic array lies between 200 and around 208 db. on this scale. All spectra in the html report use this scaling.

Note that the filtering applied to either the signature or the spectrum is the concatenation of all active filters, optionally Q filtering, followed by optionally, Wiener filtering followed by an obligatory band-pass filter.

Saving your signature

When your Gundalf report is ready, you will see a link at the top giving you the option of downloading the signatures and amplitude spectrum created. Signatures are available in both ASCII and SEGY format. The format of the ASCII text files is described in the KnowledgeBase File formats menu. The text files have a simple header in which the sample interval, index of time zero and number of samples are stored. The amplitude values then follow in standard floating format.

Saving the notional sources

These are automatically part of the downloadable signatures file when the "Signature model and statistics and directivity" option is chosen in the service section.

The term notional sources dates back to the original paper by Ziolkowski, Parkes, Hatton and Haugland (1982) on airgun interaction, (Geophysics, v. 47, p. 1413-1421). In essence, notional sources are those sources which when superposed linearly produce exactly the same radiation field as the original interacting sources. In other words, if you know the notional sources, the radiation field can be determined at any point by simply linearly superposing them with the appropriate geometric factors (not forgetting that each notional source will have a ghost). The resulting radiation field includes the full effects of interaction. Notional sources are the natural output of sophisticated airgun modelling packages like Gundalf which account for interaction between guns and allow the user to 'roll their own' directivity functions or otherwise manipulate the acoustic radiation field.

Note that the notional sources are output as bars at 1 m. To calculate the radiation field anywhere else, divide by the appropriate geometric factor to get the field in bars.

The energy budget

Calculating the energy budget

This is generated automatically for the "Signature model and statistics only" or the "Signature model and statistics and directivity" options .

Help

The Help and KnowledgeBase menus provides both general information about using the toolset as well as specific discussions of key topics in airgun array modelling. Simply explore them according to your needs.

GUNDALF - Optimiser

This product contains revolutionary ways of designing airgun arrays which are in some sense optimal in a geophysical sense. For example, a very simple optimisation criteria would be to find an array with the best primary to bubble ratio, a common goal in acquisition. Another example might be to minimise the ripple in a certain passband to improve the way the signature responds to various deconvolution techniques.

The most important aspect of the optimiser is that it can do this from a gun inventory. A gun inventory is simply a list of guns which the acquisition crew has available to build arrays. With Gundalf, the deployment geometry can be specified along with the gun inventory and Gundalf will find an optimal array according to the chosen criterion. It will only use guns from the inventory so that the resultant arrays are instantly useful. Note however that under option, Gundalf will overdraw on the inventory to handle those cases where an externally supplied specification must be met. Gundalf then reports on any necessary guns additional to the inventory. Please note that Gundalf only overdraws on gun types which already exist in the inventory. If you want to use additional types, you must add them manually to the inventory using the inventory section.

The optimiser also offers additional facilities for analysing array stability as well as a unique capability to generate environmental noise impact reports to estimate any potential impact on marine life.

Starting an optimisation run

Just do the following:-

1. Define a geometry in the geometry section by importing an existing file or defining it manually as described above. The gun volume details do not matter as they will be changed by the algorithm as it runs according to the contents of the supplied inventory.

2. Proceed to service selection and choose Optimiser analysis,

3. Define a gun inventory in the inventory section.

   Either built this up gun by gun or import an existing inventory file (.gnv).

   Since version 2.1, there is an option to overdraw on the inventory. This proves to be very useful when trying to design an array to meet some externally defined specification. In normal use, when an inventory gun count reaches 0, Gundalf ignores that category of gun. If the overdraw option is set, Gundalf will continue drawing guns out as necessary. In the report which is available at the end, Gundalf will give a summary of the inventory highlighting any categories which were overdrawn. This allows the user for example to ask Gundalf to meet a particular design specification and then find out which additional guns are required

   Gundalf will only use categories where the gun volume is non-zero. If for example, you suspect that you might need some gun types you don't currently have, define them in the inventory with count 0 and then select the overdraw option. Note also that Gundalf does not attempt to merge two instances of the same category entered separately. If you have say, Bolt 100 cu.in 1500C guns entered twice, they remain distinct as Gundalf operates.

   Note that the existing array is automatically merged with the inventory before optimisation starts.

4. Set your optimisation parameters. For a quick start, just accept the defaults which will optimise the primary to bubble. For a more sophisticated optimisation, you can use any combination weightings. Just use the tooltip help over each label to understand each parameter. There are essentially two kinds of optimisation parameter:-

   • Time-domain parameters such as finding the combination of guns in the inventory which maximises the primary to bubble ratio with the supplied geometry.
   • Frequency domain parameters such as the minimum ripple within a specified passband or the spectral minimum and maximum to be obeyed within a specified passband.

5. Set your run parameters on the optimiser parameters page. These allow you to set constraints on the optimisation such as the minimum and maximum total gun volume.

6. If you are using frequency domain parameters, you may want to set the bandwidth in which they are to operate using the modelling parameters section when you are presented with it.

7. Run Gundalf from the Model checking page, (Confirmation of modelling details).

After the run completes, you will be able to download the optimisation report as a pdf file.

The tooltip help provides considerable detail on each kind of optimisation which Gundalf is capable of achieving.

Generating an environmental noise impact report

• Choose the Environmental report option on the service choice page , and proceed.

• Environmental reports are very specialised and contain many options. Familiarise yourself using the extensive tooltip advice available on every label on the environmental parameters section.

• When on the Confirmation of modelling details page, choose to run the model.

After the run completes, you will be able to download the environmental report as a pdf file.

Starting a stability run

• Choose the array stability analysis option on the service choice page , and proceed.

• When on the Confirmation of modelling details page, choose to run the model.

After the run completes, you will be able to download the array stability report as a pdf file.

Starting a theta/phi signature sweep

• Choose the Theta/phi signature sweep option on the service choice page , and proceed.

• You will be invited to define the theta and phi values to define the sweep. These are documented with extensive tooltip advice. Simply select your parameters.

• When on the Confirmation of modelling details page, choose to run the model.

After the run completes, you will be able to download the theta/phi signature sweep report as a pdf file. You will also be able to download the generated sweep signatures (ASCII or SEGY format), as well as the generated sweep amplitude spectra.

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