Bryce Hutchinson & Sumit Verma Refraction Statics Objectives: Upload kill file Define Layer 1 and Layer 2 velocities in refraction statics Pick semblance velocities Bryce Hutchinson & Sumit Verma

Upload a kill file 1. Bring up your seismic traces. 2. Select the kill traces icon on the main seismic toolbar. 3. The kill-trace toolbar is now open. Click the open file icon. 4. Click and hold save then click save kill picks to data headers 5. Close the seismic trace window

Refraction and first arrival Hope you remember it from the class!!! Direct wave Refracted wave First break pick

Refraction statics correction A simple approach: Assume a refractor to be horizontal. If this is the first arrival after the linear correction should be flat in common shot, common receiver, or common midpoint gather. Estimate velocity of the refracted event (head wave) Compute ΔT of the weathering zone for each trace. This is theory , in software we do it little differently!!! Assumptions: Elevations of the source and receiver to be approximately the same. Constant weathering depth.

Refraction Statics 1. Open up the Elevation/Refraction Statics window by clicking the icon in the main project dataset window. First break picks Here you can see we are first asking the software to 3. A window appears upon start up with some settings. For now, these settings are alright, we can change them later. Click OK Initially the statics window appears as it does above.

Refraction Statics Note: The first step in analyzing refraction statics is setting up control points. These control points define a model which is the velocity and intercept time for each layer. 1. Click the add control point(s) icon . We will add control points in a grid like fashion instead of one by one. 2. Change radius to 1500, and change In-line incr and X-line incr to 2500. Click OK Note: You want to cover as much of the shot-receiver layout as possible. Don’t worry too much about the overlap of control points. However, if the majority of a control point circle is off the grid, it will make velocities harder to pick due to less data being included in the control point.

Refraction Statics 1. Click the Parameter Options icon this will take us back to the options window that popped up upon opening the window. 2. Be sure the number of layers is set to 2. 3. Change the Model Time Range to 40, this adjusts the width of your velocity picking line. 4. Click OK

Refraction Statics 1. Double click a control point circle to display the graph of offset versus time in the adjacent window. The selected control point is highlighted in yellow. Notice how a control point in the center of the survey has a denser plot of first break picks than the control point on the edge of the survey.

Refraction Statics – picking velocities 1. Select a control point in the middle of the survey by double clicking. 2. Click the V icon on the right side of the window. You can now define the velocity of Layer 1. 3. Click the icon to switch to picking the layer 2 velocity. When you are picking layer 2 velocities, this icon will be displayed: Pick layer 1 and layer 2 based on a break in slope of the first breaks. This break in slope represents the point where the refracted wave overtakes the direct wave.

Refraction Statics – picking velocities 1. Select a new control point, a red grid representing velocity shows a guide for defining layer 1 and layer 2 velocities. 2. Define layer 1 and layer 2 velocities for each control point in the survey. Start from control points in the middle of the survey then move out to the outer control points. Just check the slide 3, here we are trying to find the velocity of fist and second layer… Note: When you’re defining these velocities, try to pick so that the majority of the first break points fall within the blue grid. Also, if you ever get confused about which layer you are defining, make sure that layer 2 is a higher velocity than layer 1. The velocities are displayed in the right window.

Refraction Statics 3. Click save as a control file then save it to the headers as well 1. Once you have designated layer velocities for each control point, click the calculate refraction statics icon. 2. Click the calculate residual statics icon. A dialog box appears. Click OK. Calc. Refraction Statics [Ctrl Points - Calculates refraction statics using the currently defined control points to define the model. Calc. Residual Static [Short Wave]- Calculates the residual short wave statics. Click F1 to learn more about the other options You can look at the statics as the following: FINAL STATIC = LONG WAVE STATIC + SHORT WAVE STATIC

Semblance Analysis – Sort Flow In order to evaluate the correctness of the layer velocities, we must run a velocity semblance. This requires creating a couple more flows. 4. Double click the input icon and select the CMP crop you created last week as the input. Double click it and the window will close. 1. Open a new flow file. 2. Bring up the flow command window. 5. Reopen the window. Click the Sort tab. Set the Sort Order to CMP_NO. Click OK. Change the name of the output to Sort1. Set the execution on in the 3. Add an Input and Output to the flow window.

Semblance Analysis – Sort flow 1. Change the name of the output to Sort1. 2. Click OK. 3. Right click in the flow window and Set All Execute ON 4. Click the flow connect icon and link the flows with an arrow. 5. Click Go to run the flow.

Semblance Analysis – Statics flow 1. Now we will create another flow. For this flow we need Input, StatShft, ExpTPower, and Output flow commands. 4. Set the Exponential Time Power to 2. This is increasing the amplitude with time. 5. Set the Output name to static1 2. Set the input parameter to be the sort we just created 3. Set the Defined Header Static on StatShft to be STATIC_TOTAL 6. Right click and Set All Execute to On, be sure your flow commands are linked with arrows. Click GO to run.

Semblance Analysis – Velzone flow We are now moving to a slightly more complex flow. It will have 1 input and 3 outputs as shown here. 1. VelZone is located in the inputs category. In the image above you can see where to find the other flow commands. CVS ( Constant velocity stack), and Offset sort stack helps to pick the best velocity of the seismic data. Next slide will show On the next few slides I will go through the input for each flow command.

Semblance Analysis – Velzone flow 1. Set the input on velzone to be the statics file we just created 5. Manually enter the numbers above then click OK. Super gather formation(common offset stack) is a common process to do, in velocity analysis work flow. By a combination of CMP’s that lie close together (Super gather), the accuracy is increased when a small number of traces per CMP are available (low coverage). 2. Click the Super-Gather Options tab and select 3-D Analysis. Set in-line and X-line bins per zone to 5. 3. Right click the Velzone icon and under Data input, click Data/Header Selections 4. Click the icon then the green plus icon to make the screen appear as it does above.

Semblance Analysis – Velzone flow 2. In OffSrtStack, make certain the maximum offset is 8400 and the offset increment is 100. Click OK 1.Under CVS, click the velocity tab. Double click whatever is in the velocity function list and change it to range from 4000 – 18000. Click OK 4. Change the name of the output for semblance to be semblance1 5. Change the name of the output for OffSrtStack to be offset stack1 6. Change the name of the output for CVS to be cvs1 7. Click GO to run the flow. 3. Enter the correct Start Velocity and End Velocity to match the stack. Click OK

Semblance Analysis 1. From the options at the top of the screen in Vista, select Velocity then click Interactive Velocity Analysis. Velocity semblance panel (next slide) shows what velocity has the highest semblance. That means if I choose that velocity to NMO correct the reflector at that time will be flat. 2. Shift-click semblance1 then cvs1 to select all three datum as inputs for velocity analysis. Click OK

Semblance Analysis Super gather or Offset gather Semblance panel CVS - “Constant velocity stacks” : is NMO-corrected. This is carried out for several CMP gathers and the NMO-corrected data is stacked and displayed as a panel for each different stacking velocity. Stacking velocities are picked directly from the constant velocity stack panel by choosing the velocity that yields the best stack response at a selected event. Used for quality control velocity picks. Super gather or Offset gather Constant velocity stacks Semblance panel 1. Scroll to Inline 201 and Xline 101. 2. Click the pick velocities icon 3. Click the areas of highest semblance to better define the velocity.

Semblance Analysis Decrease in velocity : overlap of Refraction and direct wave. You are trying to make your reflectors appear as flat as possible.

We will cover the rest in the next lab! You if you want to try NMO Correct the data 2. Stack the data with cmp_stack and then 3. Follow the next slide.

Semblance Analysis Now you will try to flatten the reflectors further by adjusting either the weathering zone velocity or the refraction replacement velocity. Take screenshots of the NMO Stack and insert them into powerpoint slides after you adjust the weathering zone velocity or refraction replacement velocity to see how the stack changes. 1. Return to the Refraction/Elevation statics by clicking the icon for the original d_sg_stratton3d dataset. 3. Change either the weathering velocity or the refraction replacement velocity 2. Click the options icon 4. Click these icons to calculate residual and refraction statics. Then create a new NMO stack and compare it the previous one you created.