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The Location Map F i g ure 1 shows a location map in which all the lines were shot during one survey, with line numbers less than 100 reserved for dip lines and line numbers greater than

Picking: General Rules

For contouring to be valid, the picking must be done with the contouring in mind,

and this leads to the following rules.

1 The pick must represent a geologically meaningful surface This means that we must be able to see a pick as one of the following.

· A bedding plane, across which a contrast of acoustic impedance has been

formed by a change of sediment type, a change of sediment supply, or a change of relative sea level All points on such a surface were receiving

sediment at the same time; we remember that the surface is called time-

stratigraphic (or chronostratigraphic).

· A rock-stratigraphic (or lithostratigraphic) surface, in which a highly

particular set of circumstances has formed a rock contrast that cuts across the bedding planes Usually this contrast is actually a transition,

generating a recognizably low-frequency reflection above an unconformity surface However, diagenetic processes (leading to different cementation

in different zones of the rock) cab lead to abrupt contrasts and discrete reflections.

· A hiatus or gap in deposition, caused by a break in sediment supply or a

change in relative sea level or currents A hiatus increases the chances of

a significant rock contrast; today there is increasing evidence that most strong reflections, even in conformable sequences, represent such gaps in the deposition of sediments.

· An erosional unconformity as it exists at present, within the earth It is

important to remember that the constraints on the reasonableness of this surface are not those constraining an exposed surface undergoing erosion today; within the earth an unconformity was buried in one place, while still

being eroded in another At no time did the complete unconformity

surface seen on a seismic section ever exist as a continuous surface.

Thus, in picking, we are not just marking a pulse-to-pulse alignment; we

are choosing a mappable surface.

2 Each of these surfaces, according to its type, must terminate along

some line If we fail to recognize this termination, or force a pick across it, the surface we contour is no longer geologically meaningful, and

considerations of reasonableness are no longer available to guide the contouring.

3 Wherever possible, we start the picking deep in the basin, and work updip; we are picking in the direction of onlap Of course, the surface we are following may rise to a high and then reverse; then we are picking

against the direction of onlap, and the general picking rule is to stay low.

This is illustrated in F i g ur e 1 ,

Figure 1

F i gu re 2 ,

Figure 2

F i gu re 3 and F i gu re 4

Figure 3

However, we must remember at all times that such rules are no more than

convenient simplifications; the real rule is that the surface we follow must

be a geologically meaningful surface.

Figure 4

Thus, in Figu re 5 ,

and F i g u r e 8 the geologically meaningful surface is the unconformity; it would be quite wrong to follow the pick down a truncated interface merely because someone had told us to stay low.

Figure 8

4 On sections known to have been brought to zero phase, reflections

known to be positive must be picked on a white trough and reflections known to be negative must be picked on a black peak (for the SEG 1975

convention) In less clear situations, we may be driven to picking any peak, trough, or zero crossing that (a) is near the envelope maximum and (b) shows geologically plausible continuity.

5 Whenever changes of character are observed along a picked reflection, those changes are probably due to interference; all we can do is to stay on

the "same" peak or trough, but we must recognize that we are not staying

on the "same" time-stratigraphic surface Therefore we are careful to make some annotation on the map (even " CC" will do) to indicate this change of character If a closed loop indicates a mistie, this is the first place to look for the explanation.

6 The tying of loops is an essential discipline, of course Also essential is the meticulous tying of the seismic picks to the well control For all the reasons given above, the continuous seismic picks between wells usually

tie levels of the same geological age, except where unconformities

intervene; if this is not so we must either understand why not, or be suspicious of the picks Wherever possible, we refine the seismic ties to

the well by constructing synthetic seismograms; if the seismic grid ties

more than one well, we are careful to use the same variables in

constructing the corresponding synthetics (unless, of course, the relevant seismic lines used different variables) Tying all the wells is critically

important if we are to have confidence in the final contour map.

7 We must recognize that sometimes the amplitude of a reflection falls to zero The same time-stratigraphic surface is still there, but the type or condition of the rocks has changed, and there is no acoustic contrast Thus, in Figu r e 9 (an example in which we have to choose between

phantoming the pick and inserting a fault) the temptation is to stay on

reflection aa until it stops, and then to fault it up to bb.

Figure 9

This could be correct if there is other evidence for faulting above or below, but in the absence of such evidence the reflection must be "phantomed,"

following the generally conformable grain of reflections.

8 We also recognize that seismic artifacts (for example, poor statics, incorrect mutes, loss of stack fold, wrong stacking velocities, changes of source pulse, changes of surface) can cause loss of continuity in

reflections With the interactive workstation, we have some ability to determine whether an observed loss of continuity is caused by such artifacts or is real But often some doubt remains This leads us, again, to

the same ultimate rule: if there is doubt on the section, note it on the

map When we come to the contouring, it may be of enormous significance

to see part of a line marked "phantomed," or "forced through," or "could

be a leg higher."

9 The picking operation can sometimes be confused by sideswipe from

anomalous features (faults, steep structure, salt domes) off the side of the line Often, sideswipe events identify themselves by being geologically improbable, and we do our best to pick through them In other instances the events are more difficult to identify, and inevitably lead to picking errors; then we hope that the errors are revealed and located by tying around loops In general, if conflicting dips lead us to suspect sideswipe,

we defer the picking in that part of the line until all the lines have been worked, and the major features seen on other lines have been marked on the map; then the source of the sideswipe can usually be identified With the location (and, possibly, the orientation) of the sideswipe source established, it is easier to recognize and discard its effects on nearby sections.

The Location Map

F i g ure 1 shows a location map in which all the lines were shot during one survey, with line numbers less than

100 reserved for dip lines and line numbers greater than 100 reserved for strike lines

Figure 1

From the scale, we can see that the line spacing is appropriate to a large feature.

More typically, the location map would include lines shot in several different surveys, with the line numbers

coded to show the year This is likely to reveal the history of the prospect: early reconnaissance lines

(perhaps at 5-10 km spacing on a regular north-and-east grid), followed by semidetail lines (perhaps at0.25-1 km spacing in the same directions) and a line to tie a well, followed by individual short lines oriented

to resolve specific zones of uncertainty in the previous interpretation

For preference, the source-points from different surveys should be plotted with different symbols; this aidsthe reading of the map where lines cross If different symbols are not used, or lines of the same survey intersect at a very acute angle, or a line is caused to deviate across others by a fishing boat, the symbols may be supplemented to show the line direction

Source-points are usually shown every 10 or 20, and numbered every 50 or 100 Posting errors are

minimized if the system of marking and numbering is the same on the map and on the section

The scale of the map is typically 1:100,000 (that is, 1 cm to 1 km) for reconnaissance work, 1:50,000 (2 cm

to 1 km) for semidetail work, and 1:25,000 (4 cm to 1 km) for prospect delineation It is an advantage if

sections and maps are displayed at the same scale Thus, detail sections are often made at a vertical scale of

10 cm/s and a horizontal scale of 1:25,000; the section may be folded horizontally along the time origin, andthe fold laid with exact correspondence along the line on the map

(In the U.S onshore, where topographic maps are usually plotted at 1 in to 2000 ft, the scales for bothlocation map and section become 1:24,000.)

Transferring the Features

The first thing we must do in making a structure map is to transfer from the sections to the map all the highsand all the lows (more strictly, all the dip reversals), and all the significant faults This is not an option but a

necessity; it is our only protection if the frequency of dip reversals or faults causes us to violate the sampling

theorem Then we transfer the arrows signifying truncation, toplap, and baselap We also annotate zones of

near-constant dip, zero dip, loss of the reflection, change of character, phantom picks, and any recognizable

characteristic that is likely to aid correlation from line to line F i g u r e 1 is a simple example of this sort; a typical prospect would have much more annotation

Figure 1

Misties

The operation of picking around a closed loop involves folding one section vertically at the line intersection, overlaying it on the next section at the right place, and continuing the pick from one section to the other Forthis purpose, we are not fazed by a mistie if we can see that we are indeed on the "same" reflection However, we can no longer be so tolerant when we are preparing to make a contour map Obviously, errors

in the contours must arise if we ignore the mistie, or average it near the line intersections The only truly sound course is to solve the mistie

To do this we must learn to recognize the types of mistie While we can hope to eliminate misties from

unmigrated sections, a dip line and a strike line must always mistie on migrated sections Therefore we look

first at the sources of mistie on unmigrated sections.

On unmigrated sections from the same survey, the source, the field settings, and the processing are likely to

be uniform In this case there should be no systematic misties The sources of unsystematic misties can then

be identified as follows

• At sea, positioning remains the most likely culprit Before we blame the navigation

as such, however, we must check that the setback has been properly computed and applied In tidal waters, where significant feathering is likely on some lines, we also need to check whether the antenna, the near group, or the common midpoint has been kept on the programmed line If these are the source of the trouble, the maps or the setback must be corrected before we go any further If they are not, the standard procedure is to test empirically whether variations in the line intersection yield a better tie At the interactive workstation it is easy to move the line intersection on both lines simultaneously (manually, it would be done by trying several folds on one section, sliding the fold backward and forward across the nominal point of

intersection on the other section) Because the degree of mistie introduced by a navigation error is a function of the dip, what we are searching for is a revised line

intersection that eliminates the mistie on all reflections particularly the dipping ones

Where the evidence for a revised line intersection is clear, the map must be changed (and annotated to show that it has been changed) Where it is less clear, we usually make some compromise adjustment, acknowledging the physical realities of boat speed

and feathering, and the significance of the change on the final contours.

Misties appearing to be unsystematic can also be caused by what is actually a systematic error between the positioning of two or more surveys This can occur, for example, where two base stations of an early survey were on the mainland (perhaps in different countries, with different geodetic datums) and the third was on an island that, at the time of the survey, did not have a good geodetic tie to the mainland Or a subsequent survey may use different positions for the basestations, with different path lengths over land; again the two surveys may be systematically offsetfrom each other The best approach to this problem is to go through all the lines of the most recentsurvey, marking on the map every recognizable feature of the geology (highs, lows, flexures, and particularly faults), and then to do the same, on transparent paper, for all the lines of the older survey; then we can move one map over the other until we get the best fit of the geological features Normally we would then replot the older survey to accord with the positioning of the morerecent survey

• On land, the most likely culprit for unsystematic misties is static corrections The mistie is then the same at all reflection times Of course, datum corrections calculated by deterministic means should tie precisely at the line intersections; however, those calculated from the reflection data themselves (including all forms of autostatics) may not do so This is because of certain

mathematical indeterminacies in the statics solution (often called "long-wavelength" statics) A good autostatics program yields a section after auto-statics that is a best fit (in a least-squares sense) to the section before autostatics, but this in itself does not guarantee that the fit is actuallythe same at the line intersections The interpreter is then left to "fudge" the mistie as best he can

Typically, he adopts the average reflection-time value at the line intersection, and then distributes

half the mistie over a reasonable distance each side of the intersection, on each line

Can we improve on this rough-and-ready approach? First, we can use our understanding of the phenomenon to tell us over what distance we should do the distribution; this is half a spread length(or, more properly, half the length of that portion of the spread within the mute line over the

window used in the statics solution) Second, the processors can constrain the statics solution to tie

at all the line intersections Although there is still no guarantee that the statics solution is actually

correct, this does eliminate the misties as an obstacle to contouring.

In any situation where we eliminate the misties introduced by an autostatics program whether by machine or by hand we must remember that the resulting contours, although smooth, are not right This tells us that, whenever we "fudge" a mistie, we should add a note "M" at that

intersection; this will remind us, when we come to read meaning into the contour map, to have lesstrust in the map near that intersection

In working on a mistie problem caused by statics, we sometimes find that it helps to plot the misties on a map; this may reveal an areal pattern, suggesting that the assumptions of the datum-correction technique are not appropriate everywhere

• Another culprit causing unsystematic misties is obviously noise If the tying reflection on one orboth sections is seen to be noisy, we just do whatever reasonable smoothing is necessary to remove the mistie If one of the sections is noisier than the other, we concentrate the mistie correction on the noisy section

• Although we are here concerned with unsystematic misties, and although processing variations generally introduce systematic misties, we should note that processing variations from line to line, within a survey, can yield misties that appear unsystematic The most common culprits are changes

in the mute pattern from line to line and discordant stacking velocities Time-variant statistical deconvolution programs can also cause apparently unsystematic misties

This leads us to a discussion of systematic misties those where all reflections of one

survey have a constant mistie with the corresponding reflections on another survey; the mistie may be of time, or character, or both.

• A constant mistie of time (for example, where one survey is uniformly 10 ms earlier than another) suggests a simple difference of datum correction For example, one marine survey may be properly corrected to a sea-level datum, whereas another (particularly an old one) may not be corrected for source and streamer depth Or one land survey may use one elevation velocity above one datum, while another may use

a different elevation velocity or a different datum, or both All these differences should be apparent from the section labels.

• Another simple source of constant mistie, which should also be apparent from the section labels,

is the display polarity The 1975 SEG polarity standard requires that a compression in the water (or

an upward motion of the geophone case on land) should produce a white trough on the section.This standard may need modification for water guns and for Vibroseis, depending on the processingapplied Of course, not everybody agrees with the SEG convention; some hold, with good reason, that the dominant lobe of a positive reflection should be a black peak All these considerations oftenmean that sections of different vintage, or from different companies, have opposite polarity.The correction, of course, is that a reflection picked as a peak on one set of sections must be picked

as a trough on the others It is not correct to keep the "same" picking point and to add or subtract

a fixed time

• We turn now to apparent misties caused by differences of reflection character, from one survey to

another This is a very common phenomenon; different surveys use different sources or different source depths, different geophones or different hydrophone depths, different instruments or different recording filters, and different processing

F i g ure 1 reminds us of the effect of these agencies

Figure 1

The figure represents successive stages in the life of the reflection pulse, excluding those effects

which occur in the body of the earth

The source, in this marine illustration, is a small air-gun array After the command to shoot, at nominal zero time, there is a delay during which the air-gun valve is accelerated Then we observe the outgoing pulse as it would be recorded with a near-field hydrophone (pulse 1) As this signal is transmitted downward, it is followed by the free-surface ghost (2); in the illustration the source is about 6 m deep, so the ghost (which is negative, of course) comes after 8 ms At this stage we imagine that the pulse is turned back by a perfect reflector, and enters the hydro-phone array It is followed by a new free-surface ghost, in this case corresponding to a streamer depth of 15 m Pulse

3 is the notional reflection signal at the input to the recording instruments

The antialias filter in the instruments removes the highest frequencies, and has a marked effect onthe pulse (4) Thereafter the minimum-phase deconvolution in the processing reshapes this pulse, and removes the vestigial bubble pulse (5) Finally, time-variant zero-phase cosmetic filtering is applied; the illustration shows the resultant pulse at representative shallow and deep levels ([6] and [7], respectively) Clearly, the dependence of the final reflection pulse on the intermediate stages is major even without the inevitable contributions from the earth

F i g ure 2 shows the same sequence for a different source Even without a change in the sourcedepth (or anything else), the final reflection pulse is quite different

Figure 2

Obviously, surveys shot with these two sources must yield a major mistie of reflection character;indeed, in this case, the change is so large that it could almost be taken as a reversal of polarity

F i g ure 3

effective, provided that the picks we make on all data are close to the envelope maximum Indeed,

this is one of the reasons for always picking near the envelope maximum

We can arrive at a figure for the time correction by looking at many line intersections at the level ofreflections known or believed to be discrete (that is, for a single major contrast of acoustic impedance, without the complications of thin beds causing interference above or below) Then, on our chosen pick near the envelope maximum, we estimate an average figure for the mistie, and usethat as the correction As always, we use the convention that a positive correction increases the reflection time

The second approach involves reprocessing the older data to zero phase In so doing, we may choose to check whether the bandwidth can also be improved by modern deconvolution, but this isnot a necessary part of the approach Our object is merely to measure whatever reflection

spectrum exists on the stacked data, and thereby to derive and apply a phase-correction operator

to bring the section to zero phase This is straightforward and inexpensive if we have kept the stacked but unfiltered data, and all the processing applied up to this stage has been minimum phase, or close to minimum phase

With this done, we pick all reflections on the peak or trough actually at the envelope maximum, andthe misties become much smaller If they are still significant (for the type of structure being delineated), we again estimate a mistie correction as an average of the misties observed ondiscrete reflections It is still important to work on discrete reflections; because the bandwidths onthe different surveys remain different, the complexes produced by reflection interference have different character, and cannot tie leg for leg F i g u r e 4

The third approach involves reprocessing of all the sections to have the same bandwidth, again with

zero phase The advantage of this is that the mistie corrections now become very small, and yield a full correction of all picks (including those on reflection complexes); only with this approach can we really talk about the "same" pick on different sections The disadvantage, of course, is that the final bandwidth of all sections can be no larger than that of the worst; all other sections are degraded in resolution, to this smallest common bandwidth This approach is therefore a technique of lastresort, applicable only to structures whose relief is so small as to be jeopardized by the mistiesremaining with any other approach

In practice, mistie problems often have several causes If the structures are large, we tend to

do what can be done with a little thought, and to "fudge" the rest Perhaps we get a discovery Then we need to understand more of the detail, and the statics must be given more attention.

In this situation it is not unusual to reprocess all of the surveys over the feature with a

common (or at least harmonious) statics technique The expenditure is justified by the

discovery, and the interpreters can now map detail with much more confidence Indeed, the

number and severity of misties can be used as a quantitative measure of the confidence to

be placed in the data.

We turn now to the accommodation of misties on migrated sections Our first note, obviously, has to be that all the sources of mistie present on unmigrated sections are there also on migrated sections But now we add

a new and entirely systematic type of mistie, characterized by a degree of mistie that increases with the

difference of dip on the two sections This type of mistie, then, is greatest between strike lines and steeply

dipping dip lines Provided that the migration has been properly done, with appropriate velocities, it is the dipline that is correct, and the strike line that misties Unless we have reason to doubt the migration, therefore,

we accept the picks on the dip lines; the question is what we do to the picks on the strike lines

There are several options for picking migrated and unmigrated sections, depending on the geological

complexity We can summarize these options as follows

First, if the geology is uncomplicated, we reaffirm that we prefer to pick the unmigrated sections This avoidsthe problem of migration misties Then we migrate the contour map derived from the unmigrated sections.Second, at an intermediate level of geological complexity, we continue to pick the unmigrated sections, but

we use the migrated sections to guide the picking; in particular, we use the migrated sections to clarify theexistence of the faults, and to verify their locations after the migration of the contour map

Third, in very complex or steep-dip areas, we are forced to work with the migrated sections alone We use the unmigrated sections only to check the line ties, and so to verify that the misties on the migrated sectionsare solely the consequence of the migration We pick the migrated sections in the normal manner on the dip lines We also pick the migrated sections normally on those strike lines where the dip is small (for example,

on the crestal strike line) ; in these cases, of course, there are no migration misties Then we have to tackle the strike lines on the flanks, where the migration misties are material If the character ties are good, these strike lines at least confirm the picking thus far, and so allow the contouring to proceed on the dip lines But

to go beyond this, and actually to use the time values on the strike lines, we have two problems: to decide what times to plot on the map, and to decide where to plot them

Perhaps it helps to have an example before us F i g u r e 6 and F ig u r e 7 illustrate how migration can make the difference between a dry hole and a producer

Figure 6

Note that a well at about SP57 on the unmigrated section would be far downdip from the fault trap

Figure 7

The migration moves the trap some 20 source-points up-dip; this is the distance h in the dip-line diagram of

Fi gure 8

Figure 8

For a strike line to show the updip trap, the line intersection can be no farther updip than about SP57 in

F i g ure 6 and F i g u re 7 The actual reflection "points" on this strike line are approximately under SP37 This isillustrated graphically in F i g u re 9 (the inability of 2-D migration to correct the displacement on a strike line).

Figure 9

These illustrations give us one approximate solution to the problems posed above We do not use the actual reflection times on the strike line, but only their variation; in effect, we force a tie on one dip line, plot the variations in time along the strike line, and hope (as we may) that this yields at least an approximate tie to the next dip line Then these values are plotted, not on the actual location of the strike line, but updip by the

amount h; in F i g u r e 6 and F i g u r e 7 , this would be at about SP37

To do this, one must have a measure of the distance h In F i g ure 6 and F i g ure 7 this is in no doubt because the fault can be seen clearly on both migrated and unmigrated sections In other cases some recognizable feature (a small fault, a thinning, a local loss of amplitude) can be seen on both sections, and similarly used

to give a direct measure of the distance h.

To do better than this, we must calculate the distance h, and calculate the sideways-migrated times on the

strike line; this we can do, from either the migrated or unmigrated sections, using the equations derived inthe appendix

Of course, care is necessary if there is geological complexity (splay faults, for example) along the strike line,

or if the dip changes markedly on the dip line, or if the lines do not correspond fairly closely to the dip and strike directions In a severe situation we must adopt a completely general approach, calculating a new position and new times for every line (or at least for those not substantially on dip) F i g ure 10 illustrates

the process of constructing a track line, and the vertical projection of the reflection "point" to the surface.

Figure 10

F i g ure 11 gives the unmigrated time structure map for a uniformly dipping surface; the five lines show nomisties

Figure 11

F i g ure 12 shows what happens if we map picks from the migrated sections (and convert to depth) ; mistie

"bumps" appear on the map at the line intersections

Figure 12

F i g ure 13 displays the calculated vectors to move the actual line locations on the map to the track-linelocations; all lines are moved except the two dip lines