Geoshare Data Model, Version 13.0

217-SEISMIC-LOCATION-3D

Description    Attributes    Comments    References   

Description

The 217-SEISMIC-LOCATION-3D object contains seismic coordinate extent, and optionally the geometry information, for 3-D seismic surveys. An assumption about such surveys is that they consist of lines and CDPs (or shot and/or receiver locations for prestack data, which is less common in Geoshare) laid out in a rectangular grid. Since a major component of the data for such a survey is its set of traces - data values indexed along a roughly vertical time or depth axis - this is often thought of as a `cube' of data. Some minor departures from this definition are allowed, and they are spelled out below. For seismic data not exhibiting this regular geometry, Geoshare provides the 2-D survey.

With the GDM 11.0 changes that made geometry optional for seismic surveys, all of the attributes of this object giving geometry information became optional - including the INLINE-AZIMUTH, DISTANCE-BETWEEN-INLINES, MAP-COORDINATES, etc. Senders in possession of thus information are strongly encouraged to encode it unless it is known to be unnecessary for a given data exchange.

Survey, View and Subset: Why Logical Files May Contain Less Than the Whole Survey

An exchange of survey data need not convey the entire survey, which can be extremely large. A view of the survey can be defined, which allows restriction to particular geographical areas of interest, and within that area, may only contain some of the lines and CDPs defined within the larger survey. For example, an exchange may only pass data from the first 41 lines from a much larger survey. Or the exchange may only deliver every other line, or every tenth CDP. This concept of a view is a restriction of visibility of data from the original survey to just that part which is relevant to the purpose for which the data is being transmitted. A seismic survey view is that part of the survey conveyed in a single Geoshare dataset.

Seismic survey data, even as restricted in a view, are typically quite large, and generally require further decomposition for exchange using Geoshare. In such a decomposition, a small part of a survey view is placed in a distinct logical file along with its lines, traces, interpretations, etc. Multiple logical files are exchanged in this manner until the entire seismic survey view is exchanged. See the guidelines in the description of the 217-SEISMIC-SURVEY object, and the section 'Logical Composition of Large Datasets'.

The term survey subset, or just subset, will be used to mean that part of a seismic survey view being exchanged in a given logical file. Each such subset has a 217-SEISMIC-SURVEY as its data model top, and a 217-SEISMIC-LOCATION-3D to describe the overall survey extent and geometry. A receiver only knows about the contents of the subset by looking at the set of 217-SEISMIC-TRACE objects and the values of some of their `header' fields.

The decomposition into subsets is typically done by restricting a given logical file to just a few seismic lines or cross-lines, tending down to a single line, or even just a part of one line. The latter case would be required for very long lines with many interpreted horizons per line, as may be typical in marine surveys.

Recomposition takes place in the receiver, in an operation in which the view is reconstructed from the collection of subsets in a dataset.

The next sections describe the attributes in this and other objects defining the surveys, views and subsets relevant for exchange.

Attributes That Define the Survey

The purpose of the survey attributes is to establish the identity of the survey, define which line numbers, CDP numbers and shot point numbers are in the survey, and optionally establish the mapping from these seismic coordinates to actual locations on the earth.

The labelling of lines and CDPs follow the standard model for seismics. The labelling of lines in the 217-SEISMIC-LOCATION-3D object does not depend on whether the actual survey lines in the data are in-lines or crosslines.

The identity of the survey is established through the name 217-SEISMIC-SURVEY .SURVEY-NAME. This will be the same anytime data from that survey is exchanged.

A common frame of reference within the survey is its corner point geometry. Attributes are provided to describe these four points (in no particular order), both in seismic coordinates, line number, CDP number, etc., and in absolute location. The corner points are those four points defined as:

The seismic coordinates of these four points are defined by the values of the attributes:

• LOCAL-LINE-NUMBER
• LOCAL-CDP-NUMBER

which each must have four values, one per corner point, all in the same order. The attribute:

• LOCAL-SHOT-POINT

is provided for cases when shot points are specified.

The seismic coordinates in these attributes correspond to the geometry given in the attribute:

• MAP-COORDINATES

which also has four values corresponding to the corner points, and specified in the same order as is used to specify the seismic coordinates.

Even if a survey consisted of just one line, or a number of lines with just one CDP each, there should still be four corner points.

The corner points may be given in any order, but the correspondence between the attributes must be maintained. Additionally, the sender can arbitrarily choose one of these four corners as an `origin', whose seismic coordinates are given by:

• INLINE-ORIGIN
• CDP-ORIGIN
• SHOT-POINT-ORIGIN

The corner points and origin are meaningful, even if they do not correspond to traces conveyed in the view, or even existing in the survey (since data may have been `cut out' for a number of reasons; see below).

The origin defines the starting point of one seismic line. This is not necessarily the point with minimum in-line number and minimum CDP number. The azimuth of that line, measured at the origin is given by the two attributes:

• INLINE-AZIMUTH
• AZIMUTH-REFERENCE

Because of the size of the survey, this azimuth will probably not be exact for all points along all seismic lines, but it should be correct at the origin.

The attributes:

• INLINE-INCREMENT
• CDP-INCREMENT
• SHOT-POINT-INCREMENT

define the way these seismic coordinates step from one line/CDP/shot to the next, and DISTANCE-BETWEEN- INLINES and DISTANCE-BETWEEN-CDPS give the (positive) distance between successive lines and CDPs in the survey.

The figure titled 'Survey Extent and Geometry' collects these concepts in a concrete example.

Attributes That Define the View

The above attributes describe the attributes that define the overall survey. The following set of attributes define the survey view, or just that part being exchanged in a given dataset.

Views are parts of 3-D surveys being communicated in a single Geoshare data exchange. The view is identified with the attribute:

• VIEW-NAME

This should be used in all 217-SEISMIC-LOCATION-3D objects conveying data for this view. There should be no more than one 217-SEISMIC-LOCATION-3D object in a logical file with the same VIEW-NAME. A given logical file may include parts of more than one view.

As with the survey, the view has a defined origin defined in terms of seismic coordinates:

• VIEW-INLINE-ORIGIN
• VIEW-CDP-ORIGIN
• VIEW-SHOT-POINT-ORIGIN

These coordinates fix a seismic location, and the survey attributes (see above) suffice to verify that the coordinates are valid in the survey, and determine the location of this origin. Starting from that view origin, the view follows the survey geometry, e.g., view inlines are coincident with, and identified with the same seismic coordinates as the survey inlines. The view however can skip over survey lines, CDPs and shot points as long as this is done in a regular pattern. The following attributes establish this relation:

• VIEW-INLINE-INCREMENT
• VIEW-CDP-INCREMENT
• VIEW-SHOT-POINT-INCREMENT

For example, if the INLINE-INCREMENT attribute has a value of 2, it means that the lines in the survey increment by 2.0 from one line to the next. So if one line has inline number 22, then the lines on either side would be 20 and 24. It's not that lines were skipped, but rather that adjacent lines are labelled this way. If VIEW-INLINE-INCREMENT has a value of 6, it means that the labels for successive view inlines would increment by 6. So the first line after 20 would be 26. Another way to look at this is that the view only used every third line from the survey.

The total number of lines and CDPs in a view are established via:

• VIEW-NUMBER-OF-INLINES
• VIEW-CDP-PER-INLINE

These are actual in-line and CDP counts, and are independent of the seismic coordinate labelling.

Attributes That Define the Subset

Recall that a survey subset is that part of a seismic survey being conveyed in a single logical file. Unlike survey and view, no objects and attributes exist solely to define the subset. Attributes that implicitly define the subset can be found in other objects.

The actual lines in a logical file can be found by looking for the 217-SEISMIC-LINE objects that are descendent of a single 217-SEISMIC-SURVEY object. These may be in-lines, crosslines, or random lines from a survey. The identity and type of the lines can be found in the 217-SEISMIC-LINE attributes LINE-TYPE and LINE-TYPE or LINE-NUMBER.

Finally, since individual traces may be missing because of hardware failure, lack of coverage or processing mishaps, a reader must review the seismic coordinates on a trace by trace basis to know definitively which seismic locations are represented with data in a subset.

Examples of Surveys and Views

The following survey example will be used to illustrate the principles and use of surveys and views.

Here are some examples of valid views of this survey, where each column represents a single view:

The following datasets would not be legal for the same survey:

Datasets 4 and 5 specify inline and CDP increments that are incompatible with the source survey. Dataset 4 would require non-existent lines and CDPs, and dataset 5 reverses the order of arrival of lines and reverses the order of arrival of CDPs. Dataset 6 specifies lines and CDPs that lie outside the extent of the survey.

Data Shape Problems

Some surveys exhibit irregularities that depart from the general assumptions about survey geometry.

Some surveys, because of processing problems, media loss or improper acquisition coverage, have individual traces missing. Geoshare allows this through flagging individual traces as missing. See the 217-SEISMIC-TRACE object, and its TRACE-TYPE attribute.

A more difficult problem arises when large, regular (or worse, irregular) chunks of data are missing. This might be the result of a production platform interfering with a marine survey, for example. Two solutions are possible. The first is to describe the regular geometry as described above, but then only provide traces for part of a defined line in the 217-SEISMIC-TRACE object. For example, lines in the view may contain CDPs numbered from 2200 to 2700, but certain lines may only have traces starting at CDP 2300, and/or ending with CDP 2600. The other solution is to break up the survey view into rectangular blocks, treat them as distinct surveys in Geoshare, and recompose in the receiving system.

Finally, a small percentage of surveys may have lines that are not straight, or successive lines that are not parallel, or survey line/CDP grids that are not orthogonal. These are far outside the mainstream of 3-D data. Such data must at present be treated as 2-D surveys, and described line-by-line

Description    Attributes    Comments    References   

Attributes

Description    Attributes    Comments    References   

Comments

1. DISTANCE-BETWEEN-INLINES   C=1, R=FDOUBL      U
         

   The unsigned distance between successive in-lines in the survey. For example, suppose the underlying survey geometry has 400 meters between successive in-lines. Then the DISTANCE-BETWEEN-INLINES would be 400 meters, regardless of the line numbering in the survey or decimation of lines exchanged.

2. DISTANCE-BETWEEN-CDPS   C=1, R=FDOUBL      U
         

   The unsigned distance between successive CDPs in the survey. For example, suppose the underlying survey geometry has 300 meters between successive CDPs. Then the DISTANCE-BETWEEN-CDPS would be 300 meters, regardless of the CDP numbering in the survey or decimation of CDPs exchanged.

3. ANGLE   C=1, R=FDOUBL      U
         

   Survey in-line ANGLE with respect to true north. ANGLEs are in degrees and open positively in the counterclockwise direction. An ANGLE value of zero indicates that the in-lines run south to true north, with CDP numbering increasing northward. Starting with GDM 9.0, the ANGLE attribute is deprecated. Its function is replaced with the more-specific INLINE-AZIMUTH and AZIMUTH-REFERENCE attributes. ANGLE will be dropped in a future version of the GDM.

4. INLINE-ORIGIN   C=1, R=FSING1   MAND   
         

   The RATIONAL number that is the in-line number of the origin point. Could be the lowest-numbered or highest-numbered line in the survey.

5. INLINE-INCREMENT   C=1, R=FSING1   MAND   
         

   The RATIONAL number which is the survey line number increment. See also DISTANCE-BETWEEN-INLINES. This is the signed increment added to the origin in-line to get the next line number.

6. NUMBER-OF-INLINES   C=1, R=ULONG   MAND   
         

   The number of in-lines in the survey.

7. CDP-ORIGIN   C=1, R=FSING1   MAND   
         

   The RATIONAL that is the CDP number of the origin point. Could be the lowest-numbered or highest-numbered CDP in the survey.

8. CDP-INCREMENT   C=1, R=FSING1   MAND   
         

   The RATIONAL that is the survey CDP increment. See also DISTANCE-BETWEEN-CDPS. This is the signed increment added to the origin CDP number to get the next CDP in the same line.

9. CDP-PER-INLINE   C=1, R=ULONG   MAND   
         

   The maximum (typical) number of CDPs in each in-line in the survey.

10. SHOT-POINT-ORIGIN   C=1, R=FSING1      
          

    The RATIONAL that is the shot point number of the origin point. Could be the lowest-numbered or highest-numbered shot point in the survey.

11. SHOT-POINT-INCREMENT   C=1, R=FSING1      
          

    The RATIONAL that is the signed increment between shot points in the survey.

12. LOCAL-SHOT-POINT   C=4, R=FSING1      
    LOCAL-CDP-NUMBER   C=4, R=FSING1   MAND   
    LOCAL-LINE-NUMBER   C=4, R=FSING1   MAND   
          

    Shot Point, CDP and Line Number of the four survey corner points used to establish the relation with a corresponding set of map locations. The first point need not be the origin; there is no requirement about ordering. Shot point should be encoded if it is meaningful for a survey. CDP and line numbers are required.

13. MAP-COORDINATES   C=4, R=OBJREF      
          

    Reference to four 217-MAP-LOCATION objects giving the map locations of the four survey corner points. These must be in the same order as the local coordinates described in the previous attributes.

14. NULL-VALUE   C=1, R=OBJREF      
          

    A reference to a 217-NULL-VALUE object defining null or absent values that can be encoded in the attributes of this object.

15. INLINE-AZIMUTH   C=1, R=FDOUBL      U
          

    Angle of the grid's in-lines relative to a reference established by the AZIMUTH-REFERENCE attribute. From this reference, INLINE-AZIMUTH values increase in the clockwise direction. When true north is used as a reference, an angle of +45 degrees corresponds to northeasterly direction. In this case, this would mean that CDP numbering would increase as one moved to the northeast.

16. AZIMUTH-REFERENCE   C=1, R=OBJREF      
          

    A reference to an object of type 217-AZIMUTH-CORRECTION defining the reference (zero angle) for the INLINE-AZIMUTH attribute. This allows grid north, true north, or magnetic north to be used as the reference, and provides a place to specify the relation between these references.

17. VIEW-NAME   C=1, R=ASCII   COND   
          

    The name of the survey view represented in the current dataset. This is a sender-assigned name that can be used for future reference to this particular collection of survey data. If less than all of a survey's line and CDP data is being sent, this attribute is required.

18. VIEW-INLINE-ORIGIN   C=1, R=FSING1   COND   
    VIEW-INLINE-INCREMENT   C=1, R=FSING1   COND   
    VIEW-NUMBER-OF-INLINES   C=1, R=ULONG   COND   
          

    The origin, increment and number of lines in this dataset. The rules for interpreting the sign of the increment, and the sign itself, must be the same as for the containing survey. The increment must be an integral multiple of INLINE-INCREMENT. Use of multiples other than one would correspond to omission of survey lines in the dataset. For example, if VIEW-INLINE-INCREMENT is two times the INLINE-INCREMENT, it means that every other survey line is sent within the region covered by the survey dataset. The value used for VIEW-INLINE-ORIGIN must be an actual line number that exists in the survey. If less than all of a survey's line and CDP data is being sent, these attributes are required.

19. VIEW-CDP-ORIGIN   C=1, R=FSING1   COND   
    VIEW-CDP-INCREMENT   C=1, R=FSING1   COND   
    VIEW-CDP-PER-INLINE   C=1, R=ULONG   COND   
          

    The origin, increment and number of CDPs in this dataset. The rules for interpreting the sign of the increment, and the sign itself, must be the same as for the containing survey. The increment must be an integral multiple of CDP-INCREMENT. Use of multiples other than one would correspond to omission of survey CDPs in the dataset. For example, if VIEW-CDP-INCREMENT is two times the CDP-INCREMENT, it means that every other survey CDP is sent within the region covered by the survey dataset. The value used for VIEW-CDP-ORIGIN must be an actual CDP that exists in the survey. If less than all of a survey's line and CDP data is being sent, these attributes are required.

20. VIEW-SHOT-POINT-ORIGIN   C=1, R=FSING1      
    VIEW-SHOT-POINT-INCREMENT   C=1, R=FSING1      
          

    The origin, and increment for shot point numbers in this dataset. The rules for interpreting the sign of the increment, and the sign itself, must be the same as for the containing survey. The increment must be an integral multiple of SHOT-POINT-INCREMENT. Use of multiples other than one would correspond to omission of survey shot points in the dataset. For example, if VIEW-SHOT-POINT-INCREMENT is two times the SHOT-POINT-INCREMENT, it means that every other shot point is sent within the region covered by the survey dataset. The value used for VIEW-SHOT-POINT-ORIGIN must be an actual shot point number that exists in the survey.

Description    Attributes    Comments    References   

References

• 217-SEISMIC-SURVEY

Geoshare Data Model, Version 13.0

Send questions and comments to Jim.Theriot@POSC.org
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