Description

Pick first breaks from the min offset stack to calculate the uphole times

The main objective of the Uphole survey is to get to know the thickness and velocities of the weathering layer or low velocity layer (LVL). To exercise the uphole survey, generally a shot hole is drilled and places an explosive source at the defined depth. Receivers are placed along the line and records the seismograph. From the field, we get the uphole times and depths. However these values may be inaccurate often times due to wrong measurements or the uphole depths are not correct etc.

To overcome this, the user can compute the uphole times by picking the first breaks from a near trace stack (minimum offset stack). In g-Platform we can perform this by using the module “Uphole time modification”.

Procedure:

The user should read the geometry assigned gathers and make the necessary connections/references to the uphole time modification module. Generate the Vista items. It will display the selected source gather, Min offset stack and Location map. In g-Platform, we have two options to pick the first breaks. Either auto picking or Manual picking.

Auto picking

By clicking on the Auto picking option, it will automatically picks the first breaks. Adjust the parameters to have a better picks.

In case the user prefers to pick the first breaks manually, then the user should follow these steps

Manual Picking

Once the user happy with the first break picking, we can execute the module “Uphole time modification” and generate the updated uphole times.

Input data

Input SEG-Y data handle

Input trace headers

Parameters

Create updated headers

By default it is unchecked. The user can check this option to create the updated headers which got latest uphole times.

V0

Provide the near surface or replacement velocity.

Manual picking params

Manual picking window

This parameter helps in adjusting the picks where these picks are either too far/less from the current mouse position. The user can adjust parameter to have a better control on the first break picks.

Pick type

Choose among the two. Negative phase & Positive phase

Threshold

Provide the threshold value

Magnet type

Select the available magnet from the drop down menu.

Nearest

Max Energy

Auto picking window

Auto picking type

By default First break. Choose among the 3 options. First break option works well.

Start time for 'First break' option

Define the start time for first break picking

Map creation params

Map step X

Define the map step size in X direction. This map is the updated uphole time.

Map step Y

Define the map step size in Y direction

Map margin

Interpolation method

Kriging covariance type

Choose the interpolation method. Kriging and Abos. If the user selects the Abos then the next few parameters are obsolete.

Kriging range

By default Exponential

Kriging number of points

100000 by default