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 , Figure 5 F i gu re 6 , Figure 6 F i gu re 7 Figure 7 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.