Changes

===Misties===

===Misties===

[[file:mapping-with-two-dimensional-seismic-data_fig1.png|thumb|{{figure number|1}}False structure (dashed lines) created when misties are averaged. Dots show times of events on seismic lines A, X, and N where those lines intersect line Q. Solid lines show true attitude of beds. If dashed events were mapped, false structure would appear.]]

[[file:mapping-with-two-dimensional-seismic-data_fig1.png|left|thumb|{{figure number|1}}False structure (dashed lines) created when misties are averaged. Dots show times of events on seismic lines A, X, and N where those lines intersect line Q. Solid lines show true attitude of beds. If dashed events were mapped, false structure would appear.]]

[[file:mapping-with-two-dimensional-seismic-data_fig2.png|thumb|{{figure number|2}}(a) Crude base map illustrating seismic line intersections. (b) Table showing misties at seismic line intersections (times in milliseconds). Circled lines constitute a group having small misties. A group can be used as a base to which times on all other lines are adjusted. For example, times on line A561–80 could be shifted down about 52 msec.]]

[[file:mapping-with-two-dimensional-seismic-data_fig2.png|thumb|{{figure number|2}}(a) Crude base map illustrating seismic line intersections. (b) Table showing misties at seismic line intersections (times in milliseconds). Circled lines constitute a group having small misties. A group can be used as a base to which times on all other lines are adjusted. For example, times on line A561–80 could be shifted down about 52 msec.]]

Usually, raw times do not match perfectly at the intersections between two lines. These differences in time are known as misties, and can range in magnitude from 1 msec to tenths of 1 sec. Misties often result between lines that are processed differently, particularly when different datums are chosen for the statics routine. Small misties may also result because normal migration routines cannot compensate perfectly for the geometry of dipping events. Commonly, misties result from “end-of-line” effect, where one line crosses the poorly migrated end of another line. Misties can also arise from errors in the interpretation or in the reading of two-way time.

Usually, raw times do not match perfectly at the intersections between two lines. These differences in time are known as misties, and can range in magnitude from 1 msec to tenths of 1 sec. Misties often result between lines that are processed differently, particularly when different datums are chosen for the statics routine. Small misties may also result because normal migration routines cannot compensate perfectly for the geometry of dipping events. Commonly, misties result from “end-of-line” effect, where one line crosses the poorly migrated end of another line. Misties can also arise from errors in the interpretation or in the reading of two-way time.

Misties are corrected mathematically. Differences of a few milliseconds are normally small enough that they can be averaged. However, when the differences approach the magnitude of the structures that you want to resolve, they must be treated by other methods. Averaging large misties (“splitting the difference”) is ill-advised because doing so may introduce nonexistent structures to your map (Figure 1).

Misties are corrected mathematically. Differences of a few milliseconds are normally small enough that they can be averaged. However, when the differences approach the magnitude of the structures that you want to resolve, they must be treated by other methods. Averaging large misties (“splitting the difference”) is ill-advised because doing so may introduce nonexistent structures to your map ([[:file:mapping-with-two-dimensional-seismic-data_fig1.png|Figure 1]]).

In general, adjusting for large misties in a group of lines starts by identifying a subgroup with very small misties. This subgroup is used as a base to which the times of events on all other Unes are adjusted (Figure 2). Adjustments are made by shifting all the times of all the events on a given line by the same amount. Properly honoring the data requires that, for a given line, all the times for every event be adjusted by a constant value ([[:file:mapping-with-two-dimensional-seismic-data_fig2.png|Figure 2]]). For example, do not adjust one event up 5 msec and another 15 msec on the same Une; this would introduce more time (i.e., depth) between the two events.

In general, adjusting for large misties in a group of lines starts by identifying a subgroup with very small misties. This subgroup is used as a base to which the times of events on all other Unes are adjusted (Figure 2). Adjustments are made by shifting all the times of all the events on a given line by the same amount. Properly honoring the data requires that, for a given line, all the times for every event be adjusted by a constant value ([[:file:mapping-with-two-dimensional-seismic-data_fig2.png|Figure 2]]). For example, do not adjust one event up 5 msec and another 15 msec on the same Une; this would introduce more time (i.e., depth) between the two events.

The end result after adjusting for misties will not be perfect. In cases where scores of lines are being used, some of the final misties may actually be quite large. However, the object is to minimize the error, which is an attainable goal.

The end result after adjusting for misties will not be perfect. In cases where scores of lines are being used, some of the final misties may actually be quite large. However, the object is to minimize the error, which is an attainable goal.