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Semblance picking

One of the most basic interactive processing operations is picking move-out. Writing a move-out application involved creating two new classes inherited from the h_data object, nmo_semblance and nmoed.

The nmo_semblance object is initialized with the data object associated with the pre-stack gathers. The nmo_semblance object takes advantage of the fake axis concept mentioned above. Instead of having an offset axis, a sixth axis, velocity, was added to the dataset. The data is generated from the CMP gather so the read_h_data does not perform any action. Finally, the get_h_data is modified. When initialized, every time the position changes the nmo_semblance object grabs the current CMP gather. It performs semblance on the gather and returns the resulting field. The center panel of Figure 6 shows the semblance calculated at the given CMP location.

The nmoed object is initialized with the both the pre-stack data and the nmo_semblance objects. Like the nmo_semblance object, it does not use the read_h_data function. The get_h_data function is also modified. The current CMP gather is grabbed from the data object, and the RMS picks closest to the current CMP are taken from the nmo_semblance object. The picks are then used to form a RMS velocity function and the CMP gather is NMOed with this function. The result is shown in the right panel of Figure 6. Picking is only allowed on the nmo_semblance object, and navigation is only allowed on the data object.

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Figure 6. A typical semblance analysis display. The left panel show a depth-midpoint section. The center panel show the semblance at the given CMP overlain by the user selected $v_{rms}$ function. The right panel is the result of NMO using the selected RMS function.[NR]

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