Basic Depth Conversion in OpendTect

Version: OpendTect 6.0.1

We routinely track a horizon in OpendTect to make time structure and horizon amplitude maps. But we would also like to make at least a basic contoured depth map. This is the simplest possible depth map since we are using a constant velocity for depth conversion. In areas of subtle structure it should be reasonably accurate. Here is the workflow:

1. This example is in the Wild Creek 3D survey, the horizon is a Lower Penn (LPenn) horizon, just below Oswego (Fig. 1) and the well is the Modica_1A-17. Even though this well is projected from 1.5 miles away, we will use it for this example. (Seismic data credit: Osage Nation Minerals Council; Well data credit: Spyglass Energy).
1. Numbers we need.... 
1. Wild Creek Survey = +1200 ft = SRD... seismic reference datum
2. Modica_1A-17  = +954 ft = KB ... kelley bushing
3. Event time at Modica Well = 0.533 sec = T ... reflection time
4. Event depth at Modica Well = 2575 ft = MD ... measured depth from KB
5. Event depth at Modica Well = 953 - 2575 = -1622 ft TVDSS ... depth sub-sea
2. calculate depth conversion velocity
1. V = 2*(MD - KB + SRD)/T = 2*(2575 - 953 + 1200)/0.533 = 10589 ft/s.... this is our constant velocity for depth conversion. The velocity will scale horizon times to depth from SRD. This depth will need to be subtracted from SRD to get TVDSS.
2. In the project tree add your 3D horizon and make the usual time maps
1. Horizon time structure with contours (Fig. 2)
2. Horizon amplitude with time structure contour overlay (Fig. 3)
3. On the top toolbar choose the Edit Attributes icon 
1. Choose the <All>/Horizon attribute for your horizon and data type and Z output. Name this attribute T_LPenn and Add As New (Fig. 4)
2. Choose the <All>/Mathematics attribute for your horizon and implement the depth conversion equation TVDSS = SRD - V * T / 2 and for 'T' use T_LPenn.  Name this attribute TVDSS_LPenn and Add As New (Fig. 5)
4. On the project tree under the LPenn horizon add the T_LPenn attribute, then right click on T_LPenn and select Save As Horizon Data... 
5. On the project tree under the LPenn horizon add the TVDSS_LPenn attribute, then right click on TVDSS_LPenn and select Save As Horizon Data... 
6. Add contours for T_LPenn (Fig. 6) and TVDSS_LPenn (Fig. 7)
1. Zoom near Modica well of TVDSS_LPenn color and contour map shows sub-sea depth near well is within 5 ft of the correct -1622 ft value (Fig. 8)

Figure 1. LPenn event (red) and Modica well with formation tops

Figure 2.  LPenn time structure (color and 4 ms contours)

Figure 3. LPenn amplitude and time structure contours

Figure 4. Horizon attribute defining T_LPenn

Figure 5. Mathematics attribute defining TVDSS_LPenn

Figure 6. LPenn time structure (T_LPenn) with 2 msec contours

Figure 7. LPenn TVDSS with 10 ft contours

Figure 8. Zoom LPenn TVDSS with 5 ft contours