Changes

Begin mapping by recording the two-way times of the event being mapped for every shotpoint along each line and for all line intersections. The precision of time picks will vary with the scales of lines, but be as precise as possible. The numbers that are recorded are raw times. It is advisable that the times be recorded on a rough draft base map and in tabular form.

Begin mapping by recording the two-way times of the event being mapped for every shotpoint along each line and for all line intersections. The precision of time picks will vary with the scales of lines, but be as precise as possible. The numbers that are recorded are raw times. It is advisable that the times be recorded on a rough draft base map and in tabular form.

In some cases, the distances between the shotpoints printed on the lines are too large for resolving relatively small structures. In this case, it might be necessary to interpolate a ground position and then record a time. However, be aware that a relative position between two shotpoints on a line may not correspond to the same relative position between the same two shotpoints on the basemap. In short, the only ground positions that you can locate accurately on the base map are those that are printed on the base map. If highly accurate intermediate ground positions are important for your interpretation, refer to the person who drafted the map for help or check the field records yourself.

In some cases, the distances between the shotpoints printed on the lines are too large for resolving relatively small structures. In this case, it might be necessary to interpolate a ground position and then record a time. However, be aware that a relative position between two shotpoints on a line may not correspond to the same relative position between the same two shotpoints on the basemap. In short, the only ground positions that you can locate accurately on the base map are those that are printed on the base map. If highly accurate intermediate ground positions are important for your interpretation, refer to the person who drafted the map for help or check the field records yourself.

===Misties===

===Misties===

[[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.]]

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]]).

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 ([[:file:mapping-with-two-dimensional-seismic-data_fig2.png|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.