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MESA Training This series of exercises will introduce you to many of the options that are available in MESA for the design and QC of 3D surveys. It is a good idea to refer to the MESA user’s manual for more details about the features described in these exercises.

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Program Purpose ..................................................... 2 GMG Database Files ................................................ 2 Steps in Survey Design ............................................ 4 Land 3D Seismic Survey Classifications .................. 6 Geometry Examples ................................................. 7 Seismic Data Processing Issues ............................ 13 Information Gathering............................................. 16 Equations Used in Survey Design.......................... 17 Survey Analysis and QC ........................................ 20 Shooting Techniques Compared............................ 25 Ex #1: Basic MESA Usage.................................... 28 Ex #2: Line/Brick Layout Options ......................... 40 Ex #3: Unit Template Layout ................................. 48 Ex #4: The Design Guide ...................................... 53 Ex #5: Importing Survey Files ............................... 60 Ex #6: Marine Design ............................................ 65 Ex #7: Using GMG Image ..................................... 70 Ex #8: Source and Receiver Editing...................... 74 Ex #9: Offset and Rectangular Shooting............... 79 Ex #10: Automatic Template Centering ................ 83 Ex #11: Salvo Shooting ......................................... 88 Ex #12: Label Shooting ......................................... 90 Ex #13: Multi-Survey Capability ............................ 93 Ex #14: Using Advisor ........................................... 99 Ex #15: Attributes and Filtering ........................... 102 Ex #16: Displaying Data ...................................... 112 Glossary of Terms ................................................ 119

Program Purpose MESA provides a great deal of flexibility in 3D survey design and analysis, whether the survey is a land, transitional area, ocean bottom cable, or marine design. Imagery, contour information, and cultural features (provided from .dxf files, for example) can be used as backgrounds to aid in the design of the survey. In this way, permit and logistical problems can be anticipated at the planning stage, reducing the time and cost of field acquisition. Besides flexibility in design methods, MESA provides flexibility in shooting methods and bin attribute analysis. Additionally, a number of output file formats are supported, including SEGP-1, UKOOA, and SPS, in addition to shooting scripts for Input/Output and ARAM acquisition systems. The completion of a survey design in MESA generates a Green Mountain Geophysics GeoScribe geometry database, thereby completing a major portion of the initial pre-stack processing work while still in the field. These database files are transportable across various hardware platforms, making MESA a practical tool for field and office environments.

GMG Files and the Database The following set of files represents what Green Mountain Geophysics refers to as the MESA or GeoScribe database. These files are a combination of ASCII and binary files and combine to hold all of the information needed to define the geometry (and refraction statics) for any 2D or 3D survey. Not all of these files will be found with every database. The *.bin and *.mid files are required only for bin attribute displays and can be deleted before archiving the database, if necessary.

File Extension

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Format

Description

*.atr

Binary

Attribute information for receivers

*.ats

Binary

Attribute information for sources

*.bin

Binary

Bin information, sizes

*.bmp

Binary

Picture for open database preview display

*.cf1

ASCII

Configuration file for the receiver spreadsheet

*.cf2

ASCII

Configuration file for the source spreadsheet

*.def

ASCII

Default values for MESA to use with this survey

*.fbt

Binary

FFID info. and space for 1st break picks

*.hdr

ASCII

Header information for SPS outputs

*.idd

Binary

Image ray attribute information

*.inr

ASCII

Instrument information for receivers

*.ins

ASCII

Instrument information for sources

*.mar

Binary

Marine survey information

*.mas

ASCII

Database parameters and status flags

*.mdd

Binary

Model attribute information

*.mdl

ASCII

Aperture model information

*.mid

Binary

Midpoint information, offsets, azimuths

*.mrl

Binary

Streamer marine receiver locations

*.mut

ASCII

Mute function information

*.ndd

Binary

Normal ray attribute information

*.pat

Binary

Source/receiver template relationships

*.rdd

Binary

Offset ray attribute information

*.rfi

ASCII

Filter settings for receivers

*.rln

ASCII

Line names for receivers

*.seq

ASCII

General shooting sequence description

*.sfi

ASCII

Filter settings for sources

*.sln

ASCII

Line names for sources

*.sor

Binary

Source numbers and coordinates

*.sta

Binary

Receiver numbers and coordinates

*.tpl

Binary

Source to receiver patch relationship

*.unt

ASCII

Configuration of the unit template

*.xcl

ASCII

Exclusion zone type, size, and all coordinates

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Other GMG Files used or created in MESA File Extension

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Format

Description

*.cfg

ASCII

Configuration file used to import survey files

*.csi

ASCII

Color scale settings

*.cyr

Binary

GMG contour file

*.lyr

Binary

GMG image file

*.nop

ASCII

MESA midpoint exclusion output file

*.ptn

ASCII

MESA pattern output file

*.scr

either

Input/Output script file

*.sts

ASCII

MESA land statistics/cost output file

*.tdf

ASCII

Trace data format file used to import SEGY files

*.vyr

Binary

GMG vector file

Steps in Survey Design

Step 1: Building an “idealized” survey MESA provides several methods for defining a survey. 1. Direct layout and shooting Using the source and receiver layout dialogs, you can create orthogonal (brick or straight line), zig-zag, slash, button patch, and radial surveys. The surveys are created by specifying information such as inline and crossline spacings, bearings, and survey size. Several shooting options are then available to define the sourcereceiver template relationship. 2. Unit Template You can create a unit template, a group of sources which are fired into a common receiver template, in the Unit Template window in MESA. This unit template is then repeated throughout the design area to simultaneously define and shoot the survey. The unit template is good for creating brick, orthogonal, button or swath surveys. 3. Importing ASCII files ASCII files containing coordinates and source or receiver numbers can be imported directly into MESA. Examples of these files are UKOOA, SEG-P1, and SPS. If ASCII relational files or ASCII or binary shooting scripts are also available, they can be imported, as well.

Step 2: Creating a “real world” survey Once the initial design parameters have been set for the survey, aerial and satellite imagery, scanned topographic maps, contour displays, and/or files containing cultural information (.dxf files, for example) can be used to modify the design to take into account physical and cultural obstacles. Exclusion zones which exclude sources, receivers, and/or midpoint information can be defined as circular, linear, or polygonal zones. These zones can be defined graphically, by manually entering coordinates, or by importing coordinates from an ASCII file. Once the exclusion zones have been defined, the survey can be designed around them. Editing functions allow the user to selectively deactivate sources and receivers as well as relocate them in groups or individually using the mouse or keyboard. The 'redesign a line' function allows the source or receiver lines to be re-drawn maintaining the inline group interval and thus preserving the stack response - extra receivers may then be required to fill the gap. A snap to grid function may also be used to ensure that source and receiver moves maintain the group interval. Thus, the survey is as close as possible to the real world conditions before any equipment is deployed, minimizing the time in the field for equipment and crew.

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Step 3: Updating with surveyed coordinates The theoretical survey design can easily be updated with actual coordinates from the survey crew on a shot by shot, swath by swath, or daily basis through the ASCII file import option. The new coordinate information may be provided as absolute values or shifts from the original position. Analysis of the ongoing acquisition, via the bin attribute displays, allows for the repositioning and/or addition of sources and receivers in order to compensate for any deficiencies which may have appeared in the desired fold, offset, or azimuth distributions because of conditions in the field.

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Land 3-D Seismic Survey Classifications Jim Musser, Director GMG Energy Services Type In-Line Swath

Applicable Areas Open Terrain

Advantages Narrow azimuth data can be processed and analyzed like 2-D seismic

Disadvantages Poor cross line statics, high SRC and RCV line density, very sensitive to obstructions

Orthogonal

All Terrains

Wide azimuth, good for 3-D DMO, can solve cross line statics, industry standard, economic operations

Must use 3-D algorithms, cannot use simple 2D F-K algorithms

Brick

Open Terrain

Like orthogonal, plus improves near offset and overall offset distributions

Discontinuous source lines are difficult in jungle and in some other terrains

Slant

All Terrains

Improves overall offset coverage, better offsets for AVO

Surveying and line clearing on source lines are longer due to diagonal line orientation

Button Patch

Open Terrains, Allows sparser source points, Farm Land, Arctic, efficient use of large channel Desert systems

Variable Line Spacing

All Terrains

Modification of orthogonal, brick, or Complex to plan slant design with similar advantages to each, plus guarantees surface consistency

Asymmetric Spread

All Terrains

Modification to orthogonal, brick, or Same as for orthogonal, slant design with similar brick, and slant designs advantages to each, plus longer offset with less recording equipment

Random

All Terrains

Surface consistent, minimizes acquisition footprint

Complex to plan

Complex to plan and operate

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Geometry Examples Any of the following geometries can be built in MESA by using the unit template option or by directly placing the sources and receivers before shooting. For each geometry, there is a view of the Unit Template window followed by a view of the Design window, as well as the main points for and against each survey type.

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Inline Swath shooting - Marine like Pros: Cons:

Simplest geometry for DFSV type recording systems. Poor azimuth distribution, poor coupling, high fold.

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Orthogonal or straight line shooting Pros: Cons:

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Very simple geometry to lay out in the field. Comparatively expensive and yields largest Xmin. Requires good access for sources and receivers.

Brick shooting Pros: Cons:

Smaller Xmin reasonable azimuth and offset distribution with potential for statics coupling. Requires good access for both sources and receivers so not suited to areas with access problems. Excessive long offsets may result with whole survey, or replanting of geophones.

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Button Patches Pros: Cons:

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Efficient utilization of large channel systems with minimum source access and effort. Can require large numbers of sources. Requires computerized planning. CMP fold does not yield same offset/azimuth distributions in adjacent bins.

Zigzag (including mirrored, double, triple and shifted double zigzag) Pros: Cons:

Smaller Xmin with good offset and azimuth distribution. Only good in conditions of open access such as deserts.

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Seismic Data Processing Issues Related to Geometry

Migration Migration creates some profound requirements on survey design. Diffracted events in the subsurface impose the requirement to sample more time and wider areas, in order to capture enough of the diffraction to collapse its energy. This almost always will require the design geophysicist to record seismic data over an area which is much larger than the actual prospect area. The calculation of this “migration aperture” is described in the Equations section.

Refraction Statics If you are designing a survey in an area where significant weathering and statics problems may exist, you will want to focus some energy on optimizing your survey to solve these problems. Several refraction statics algorithms exist. Most of these algorithms are primarily numerical equation solvers, which are dependent on statistical redundancy for the best solutions. Statics coupling does not play a large role with most refraction statics algorithms because the statics are not measured in the midpoint domain, and there is no structural or RNMO term to solve in the standard equation. Therefore, anything which improves the quality of the first breaks will contribute to enhanced refraction statics solutions. A single point dynamite source with no significant receiver field arrays will produce the best results. Geometries with receiver lines which are not straight produce first breaks which can be very difficult to pick. Statistical algorithms will perform best if the statistics provided are consistent and well sampled. This would require the designer to balance source point fold and receiver point fold. Your final design should produce source/receiver fold of 6 or more. Split spread type shooting creates surveys which have reciprocal travel paths. Many types of algorithms depend on reciprocal paths to build stable solutions. Off-end shooting schemes should be considered as a last resort. Shallow refractors will require narrow receiver line spacing or they will not be well sampled.

Reflection Statics It is our experience that most regular 3D designs will decouple in the traditional sense without editing. What saves the designer in most cases is the fact that sources and receivers are shifted around in the field, providing a pseudo-randomized version of the original plan. While this randomization tends to have a coupling effect on the survey, it does not however guarantee that the survey couples. The noise plot in MESA demonstrates the degree of coupling which a survey design possesses.

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If you understand any potential statics problems which may exist in the survey area, you can consider the way in which your design will sample the statics problem. Extremely long wavelength statics or large known statics may affect the sampling decision made by the designer, relating to crossline length or receiver template size.

Velocity Many of the best 3D velocity algorithms are currently using azimuth as well as other information to build plots and aid in the determination of stacking velocities. These types of algorithms require that bins sample offsets and azimuths with enough statistics so the data can be analyzed. Velocity analysis is usually performed on a super bin, so users should be aware of how the bin-tobin relationships of offsets and azimuths will complement each other. Large gaps in offset distributions or absence of near traces on shallow reflectors can contribute to problems in the analysis.

Deconvolution Surface consistent deconvolution presents the same requirements that reflection statics does. Evenly sampled data in both the source and receiver domain will contribute to better solutions. At far offsets, the data often becomes distorted by incidence angle and emergent effects, making far offsets unusable for the derivation of the deconvolution operator. This imposes additional requirements that the near traces need to be well sampled to provide the information required by the deconvolution algorithm.

DMO (Dip Moveout) DMO will function best if a survey is sampled at all offsets and all azimuths. Obviously, this is not possible. Modern processing techniques can make up for the lack of sampling required, but a well-sampled survey in both offsets and azimuths will produce better solutions. DMO is known to create amplitude artifacts in 3D surveys. This amplitude effect is called geometry imprinting, artifacts, or geometry “foot print.” The imprinting effect is reduced if a broad range of azimuths is collected. As has been demonstrated, this effect becomes more pronounced for steeper dips and shallower targets (small reflection times).

Coherent Noise Attenuation Much research has been done in recent years relating to attenuation of coherent noise with acquisition geometry. The “bleed through” effects of source-generated noise differ depending on the acquisition design. Certain geometries will attenuate noise better than others will. Looking at this issue in a post-stack or post-migration environment is the current work of researchers at several major companies. Noise plots and Array Analysis from the Advisor menu can be used to perform source and/or receiver array noise analysis.

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Relative Amplitude (AVO, AVA) AVO (amplitude versus offset) and AVA (amplitude versus azimuth) analyses could be a part of the processing flow in some data areas. Good offset and azimuth sampling within the useful range of analysis is a strict requirement for either analysis. Try to gain an understanding of the useful offset range to observe AVO effects in the survey area. Details like this can help make tough decisions about tradeoffs easier. The tough decisions refer to compromises between the desired source or receiver sampling and the economic limits that exist.

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Information Gathering

Here are some issues to consider during survey design. This list is not exhaustive.

Exploration Objectives Type of feature (anticline, fault, reef, etc.), Exploration method (structural, stratigraphic), Strike, Dip, Lithology of target, and Lithology of overburden

Target Description Depth, Arrival time, Average velocity to target, Interval velocity at target, Dip (expected, maximum), Bed thickness, Required vertical resolution, Desired reflection frequencies, Expected horizontal resolution, and Shallowest reflection

Operational Considerations Expected noise (ambient, source-generated, non-random), Permitting/Positioning, Timing limitations / weather limitations, Access problems, Digital maps or imagery available, and Data processing

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Equations Used in Survey Design Bin Size To avoid spatial aliasing in the data: Bin size at subsurface