Once we have the depth interval velocity model, we calculate the travel times using the depth interval velocity model. For this we need to use the module "Time tables calculation"
Prior to that, the user should QC the source and receivers that are at the constant datum by using QC trace geometry module. Also we need to check that the Bin (CMP) elevations are actual values and are lesser than the datum value.
Input(s) required for Time tables calculation
Depth velocity Depth interval velocity which we created from Grid tomography 2D/3D
Delta param Provide Delta volume
Epsilon param Provide Epsilon volume
From the above image, we see that the Source & Receivers are having a constant datum value and Bin (CMP) elevations are the actual values and lesser than the datum value. We have to annotate the Source_Elev, Source_Datum, Receiver_Elev, Receiver_Datum, Bin_Elev, Inline (In case of 2D it shall be zero) and Crossline to confirm that all the required headers are present and correct.
Once the QC part is over, the user can input the initial depth interval velocity model to the Time tables calculation module.
Within the Time tables calculation, it calculates the travel times for each source and receivers and generates the tables separately. There are no visual outputs generated from this module.
Parameters:
3D In case of 3D, the user can check this option.
Make anizo If the anisotropic parameters like Delta and Epsilon are provided as an input then it will consider that as an anisotropic medium in calculating the travel times of the source and receivers. If Delta and Epsilon are available then only the user should check this option.
DX (meter) This should be the bin grid in case of 3D. User can provide DX/DY as per their bin grid definition.
In case of 2D, it is should be the CMP interval.
DY (meter) For 2D case, the user can ignore it however for 3D it should be the bin grid as we mentioned earlier.
Aperture This parameter is somewhat related to the migration aperture. As a rule of thumb, the user should put this value as 1.5 times the migration aperture to have enough distance to measure the travel times. It will consider the range in X&Y direction to compute the travel times.
Datum Datum value. It takes automatically from the input.
Depth step factor This is crucial in case of 3D data. It will compress the travel time tables by this much factor. 2 shall be alright.To explain it better, let us consider that the input depth velocity model is having 5m sample rate. When we use the Depth step factor as 2 then it will make it as 2.5m sample rate.
In case of 2D data set, it doesn't have any significant impact on the overall travel time calculation.
Source calculation area
Step along inline number If the user provides say for an example step along inline number as 5, that means it will calculate the every 5th inline within the volume. It is kind of a decimation to calculate the travel times in much faster and efficient manner.
Step along xline number Same as the inline.
Receiver calculation area
Step along inline number Same as source calculation area. By default it's 1.
Step along xline number Same as source calculation area. By default it's 1.