PROCESS NMO, NMO2, NMO3 ------- --- ---- ---- Parameters, alphabetically: addwb dstretch fno hires ivtp lno newx opath stretc type vadd vintpl vmul vtp vtrkwb xfactor Document Date: 19 July 2012 Doc mods: Add parameter IVTP, May 2003 Add parameter DSTRETCH, August 2002 Add parameter XFACTOR, September 2008 Add parameter HIRES, July 2012 Processes NMO apply a travel-time correction to each trace. Process NMO2 and NMO3 are identical to PROCESS NMO, enabling NMO to be given three times in a job. The following corrections are available: 1) Normal MoveOut or NMO: T(0) = SQRT(T(X)**2 - X**2/V(T)**2) 2) MoveIn or deNMO: T(X) = SQRT(T(0)**2 + X**2/V(T)**2) 3) Slant MoveOut or SMO: T(0) = T(X) + X/V 4) XMO: MoveOut followed by MoveIn: T(0) = SQRT(T(X)**2 - X**2/V(T)**2) followed by: T(XEW) = SQRT(T(0)**2 + XEW**2/V(T)**2) where x is the range (shot-receiver distance) in the SEG-Y trace header and V is the velocity specified by the user. It is usually thought that all traces common to the same reflection point should have the same velocity function. Thus, the velocity function given by the user may be specified at various points along the seismic line (control points). Traces lying between the user specified control points are obtained through interpolation of the adjacent specified velocity functions. The times associated with the velocity function may be relative to the water bottom rather than time zero. SPATIAL VARIATION OF VELOCITY FUNCTIONS RPs that do not have a velocity function defined by the user are assigned a velocity function generated by one of two spatial variation schemes (parameter vintpl). (Also see the document on spatial variation.) ISO-Velocity spatial variation (vintpl 1) The program calculates the time associated with each velocity when the velocities are the same. e.g. nmo fno 1 vtp 1500 0. 2000 .4 2500 1.0 end fno 3 vtp 1500 0. 2200 .6 2400 1.1 end end then rp 2 will get vtp 1500 0. 2000 .414 2450 1.050 Layer cake spatial variation (vintpl 2) The program interpolates both the velocity and the time of each velocity-time pair. Every velocity function must have the same number of pairs. e.g. nmo fno 1 vtp 1500 .2 1600 .4 2000 1.0 end fno 3 vtp 1450 0. 1800 .6 2100 1.1 end end then rp 2 will get vtp 1475.0 0.100 1700.0 0.500 2050.0 1.050 The velocity function (VTP) applies to all shots/rps with FNO to LNO of the parameter list (recall that each FNO/LNO list is terminated with the word END). Spatial variation is accomplished by giving multiple FNO/LNO lists, each with a unique VTP and terminated with END. The velocity function is constant for all shots/rps from FNO to LNO. *** Note *** spatial variation of velocities with velocity inversions has problems!!!! Each parameter list must be terminated with the word END. The entire set of NMO parameters must be terminated by the word END. Process NMO2 is identical to PROCESS NMO, enabling NMO to be given twice in a job. e.g. procs diskin nmo nmo2 diskoa end THE PARAMETER DICTIONARY --- --------- ---------- VTP - Velocity-time-pairs. A list of stacking velocity and two-way travel time (in seconds) pairs. VTP must be given in each NMO parameter list. A maximum of 25 pairs may be given. Data times before the first given time in VTP are held constant from the first given time. Likewise, data times exceeding the last given time in VTP are held constant from the last given VTP time. Velocities and times with VTP MUST INCREASE. Default=none. e.g. vtp 1490 1.0 2000 2.0 IVTP - Interval velocity-time-pairs. A list of interval velocity and two-way travel (thickness) time pairs. Spatial interpolation is done before ADDWB is applied (before the interval velocities are converted to rms velocities). When using ADDWB YES, the first interval should have 0 thickness and a velocity of water. Each IVTP function must have the same number of intervals described. Default = none e.g. ivtp 1500 0 1600 .1 1700 .05 FNO - The first shot (or rp) to apply the velocities to. Shot (rp) numbers must increase monotonically. Preset=1 LNO - The last shot (rp) number to apply the velocities to. LNO must be larger than FNO in each list and must increase list to list. Default=fno VMUL - Velocity multiplier. Every velocity is modified so that: vel = (vel - vadd) * vmul + vadd Preset = 1. e.g. vmul .9 VADD - Velocity additive. Every velocity is modified so that: vel = (vel - vadd) * vmul + vadd Preset = 0. e.g. vadd -200 DSTRETCH - The maximum stretch allowed as a percentage of the ratio of the NMO over the zero offset time t0. i.e. (tx-t0)/t0. Move-out of long ranges or shallow data change the frequency of the waveform and will cause interference when stacked with short range traces. (tx-t0)/t0 approximates the ratio df/f where f is frequency and df is the change in frequency due to NMO. Preset = not given e.g. dstretch 50 STRETC - The maximum amount of NMO (delta t), in seconds. Data exceeding stretch will be muted. Valid with NMO only (type 1). Preset =1. ADDWB - A yes/no switch. YES indicates that the two-way water bottom time from SEG-Y trace header floating point word 50 will be added to each time of the velocity function, after spatial variation has been done. Preset=no TYPE - The type of travel time correction to apply. = 1, Normal MoveOut or NMO: T(0) = SQRT(T(X)**2 - X**2/V(T)**2) = 2, MoveIn or deNMO: T(X) = SQRT(T(0)**2 + X**2/V(T)**2) = 3, Slant MoveOut or SMO: T(0) = T(X) + X/V = 4, XMO or Normal MoveOut followed by MoveIn. Requires parameter NEWX. Preset = 1 e.g. type 2 VINTPL - The type of velocity interpolation between successive velocity control points. = 1, Velocity spatial interpolation according to "iso-velocity" = 2, Allows regions of constant velocity to be interpolated correctly, however VTP pairs must be equal across control points. = 3, "Iso-velocity" spatial variation except on the first velocity time pair, which is interpolated in both velocity and time, making the first velocity time pair (water bottom reflector usually) spatially smooth. = 4, Both the velocities and times are interpolated. Each VTP function must contain the same number of points. Preset = 1 when VTP is give, = 4 when IVTP is given. VTRKWB - Velocity Tracking Waterbottom. When given a positive value, velocity interpolation is cued from the water depth at the source (long SEGY header word 16). This is typically the centerbeam depth value which is placed in the header during acquistion for each individual shot. The value of VTRKWB is the maximum depth change between adjacent cmp gathers. If the change in water depth exceeds VTRKWB, the new water depth is "bad" and the previous depth is used instead. Although each trace in a gather may have a different centerbeam depth values, the depth value from first trace will be applied to the entire gather to minimize static shift problems (process gather normally sorts so that the trace with the smallest range is first). This option is useful during realtime shipboard processing and allows a pre-defined 2-D velocity field to be used during processing, with fno and lno values representing water depth, vtp pairs defining the stacking velocity function for a given water depth. This scheme allows more flexibility for varying velocities along a reflection line (as opposed to hanging a single velocity function from seafloor). Preset = -9999.0 e.g. vtrkwb 100.0 OPATH - The pathname of an output file containing the velocity used in move out after spatial and temporal interpolation have been done. Each seismic sample has a velocity associated with it and is written to OPATH. The velocities are written in ASCII, one velocity per line, unless the last four characters of the pathname end with '.segy'. When an SEG-Y file is written, the SEG-Y headers are the same as the input and the trace length is the same; the seismic samples are replaced with velocities. Preset = none e.g. opath velocities.1234 NEWX - The constant range that all traces will be MovedIn when using TYPE 4. The use of NEWX automatically changes parameter TYPE to be 4. NEWX causes every trace to be MovedOut so that the trace is time corrected as if it was at normal incidence (x=0), and the MoveIn is applied so that the trace is time corrected as if it was at range NEWX (x=newx). This could also be accomplished using processes nmo1 header nmo2, where nmo1 did nmo, header changed the range, and nmo2 did MoveIn to range NEWX. Preset = none. e.g. newx 1000 XFACTOR - A ranger multiplier. The SEGY specification for range is a thirty two bit integer. If the range isi to be expressed with greater precision, such as centimeters, then the range can be multiplied by a factor of 100 in process geom and then removed in NMO by using xfactor .01. Preset = 1. HIRES - A yes/no switch when set to YES uses a polinomial interpolation of trace values to obtain "high resolution". Normally, nmo uses the closest sample of the TX trace. This is like resampling the data before NMO. Shallow data recorded at too coarse a sample interval will have repeated amplitudes that result is a "square wave" appearance. Preset = NO e.g. hires yes END - Terminates each parameter list. Written and copyrighted by: Paul Henkart, Scripps Institution of Oceanography, April 1980 ALL RIGHTS RESERVED.Go to the list of seismic processes. Go to SIOSEIS introduction.