Topic:
Practical Uses of SeisAn, Attribute and Mappable Seismic Statistics.

Description:
 Outlined below is a list of practical uses for the seismic trace (SeisAn),
 windowed trace (CSA) attributes and Mappable Seismic Statistics that can be
 generated in IESX Computation Manager. This list should not be regarded as a
 definitive guide but rather as a general overview of the potential uses of
 attributes. Used in conjunction with each other, attribute analysis can be a
 powerful tool in the search for hydrocarbons.
  

Solution:
SeisAn :
--------
Instantaneous Frequency -  may provide information on the cyclicity of geologic
intervals. Laterally increasing instantaneous frequency can indicate bed
thinning or pinchouts. Like Instantaneous Amplitude, a dramatic instantaneous 
frequency change can indicate a change in the type of fluid in a rock.
 
Instantaneous Phase - this attribute may make it easier to pick weak events
due to its independence from reflection magnitude. It can be used to assist
picking of horizons in low amplitude/high noise areas.  Note, however, that
it does show abrupt changes at +90 and -90 degrees (due to the arctan function
used to calculate the instantaneous phase).
 
Cosine Inst. Phase - this attribute is similar to Instantaneous Phase. It may
be easier to use because peaks and troughs align with peaks and troughs in the
original seismic data.   It does not show abrupt changes at +90 and -90 degrees.
All peaks and troughs of instantaneous phase and cosine of instantaneous phase
have the same amplitude.

Reflection Magnitude -  the seismic trace envelope is computed in terms of
decibels below a reference value, which may be either the maximum amplitude
envelope or a user defined value. The attribute may be used as a Direct
Hydrocarbon Indicator that is independent of amplitude sign. A high Reflection
Magnitude is often associated with major lithologic changes, for example an
unconformity, and gas accumulations. Because decibels involve logrithmic
conversion, high amplitudes appear muted.

AGC Scaling - Automatic Gain Control is the automatic time variant amplitude
scaling of seismic data. It may be used to improve visibility of weak events. A
drawback is that relative amplitude information is lost, particularly if short
AGC time window(s) are specified. The process may also create false shadows
within an AGC window above and below strong reflection events.

Amplitude Normalization - may be used to rescale seismic data to a "User" or
"Seismic" reference value. This function may be used to correct differences in
amplitude between seismic lines so that amplitude mapping or comparative
attribute generation is possible.

Phase Rotation - a phase rotation (0-360 degrees) may be applied to seismic
data. Different seismic vintages may require phase rotation to improve
consistency between datasets.

Filter - may be used to apply time variant or time invariant filtering to
seismic data. Seismic data may require filtering to enhance particular seismic
events, reduce unwanted noise and thus facilitate more accurate interpretation.

Seismic Bulk Shift - may be used to apply positive or negative time shifts to
seismic data. This may be required when correlating surveys with different
datums.

Remove Bias -this option removes DC bias from seismic data. DC bias occurs when
the average of the trace values departs from zero and may be caused by
processing artifacts and/or geological factors. There are no input parameters
for this routine.
 
Computed Seismic Attributes :
-----------------------------
Rms Amplitude - is a measure of reflectivity within a time window. This windowed
attribute may be used to map Direct Hydrocarbon Indicators in a zone. 

Energy half-time - is the proportion of time required for the "energy" within a
window to reach one-half of the total energy within the entire window. The
attribute may indicate changes in lithology or porosity within a specified
zone.

Average Magnitude - is a measure of the reflectivity within a time window, but
is less sensitive to large numbers than RMS Amplitude. It's uses are similar to
those for RMS Amplitude.

Maximum Magnitude -  another measure of reflectivity within a time window and
may be used to map the strongest Direct Hydrocarbon Indicator within a zone.
 
Computed Inst. Frequency - may be regarded as the Average Instantaneous
Frequency within a window. A Low Inst. Frequency values may occur in zones
below gas accumulations.

Computed Inst. Phase - may be regarded as the Average Instantaneous Phase. A
zero length window may be used to track event discontinuities such as
pinchouts, angular unconformities and oil/gas and oil/water contacts along a
user defined horizon.
 
Max. Amplitude - is a measure of reflectivity within a time window. It is the
maximum positive number in the window and is used to detect positive DHI such
as bright spots. Maximum Amplitude is the largest positive value (peak) in 
the window.

Min. Amplitude - a measure of the reflectivity within a time window. This is the
maximum negative number in the window and is used to detect negative DHI such
as bright spots. Minimum Amplitude is the largest nrgative value (trough) in 
the window.

Mean Amplitude - this is the arithmetic mean of the amplitude and is a measure
of trace bias. Positive or negative bias may indicate the presence of bright
spots. In the event that the trace data has an artificial bias caused, for
example, by processing artifacts, it may be adviseable to remove such effects
prior to generating the Mean Amplitude attribute. This can be done in the
Remove Bias option in SeisAn.

Average Peak Value - is a measure of the positive reflectivity within a time
window. Large or small values may be used as a Direct Hydrocarbon Indicator. A
small value may indicate gas percolation.

Ave. Peak Value (zero X) - is similar to Average Peak Value, but instead of
taking all peak values, only the largest peak between each pair of zero
crossings is taken. This attribute may give a better measure of reflectivity
when tracking a complex event.

Ave. Trough Value - is a measure of the negative reflectivity within a time
window. Its uses are similar to those of Average Peak Value.

Ave. Trough Value (zero X) -  is similar to Average Trough Value, but instead of
taking all peak values, only the largest negative peak between each pair of
zero crossings is taken. This attribute may give a better measure of
reflectivity when tracking a complex event.

Arc Length - this is a measure of reflection heterogeneity and may be used to
quantify lateral changes in reflection patterns. This is a stratigraphic
sequence indicator. 

Threshold Value - computes the fraction of samples greater than, less than or
equal to the user input threshold value. It is useful for analyzing amplitudes
in seismic data by utilizing user defined values that might infer porosity or
fluid changes.

Average Energy - is the squared RMS Amplitude. This attribute is a measure of
reflectivity within a time window and may be used to map Direct Hydrocarbon
Indicators in a zone.
 
Number of Zero Crossings - this attribute can provide information on the
complexity of the stratigraphy. A high number of zero crossings generally
indicates a greater degree of vertical lithologic complexity. 

Ratio of Pos to Neg samples (RPN) - may be used to look for lateral changes in
thickness as well as lithology, such as a pinchout or a thickening sand within
a shale sequence.

Dominant Frequency - this number is sensitive to the reflectivity sequence and
may be misleading. Changes in the dominant frequency of an event may be
indicative of hydrocarbons.

Bandwidth - this quantity is determined as the distance between the half power
points on both sides of the peak of the smoothed amplitude spectrum (see
Estimating the statistical bandwidth of a time series by A. T. Walden and R. E.
White). This attribute gives a measure of the frequency range of the data. For
example, cyclic sand/shale sequences may be represented by narrow bandwidth,
whereas widely varying lithologies would more likely be typified by broader
bandwidth.

Bandwidth Rating (Bias) - is similar to Bandwidth but is computed from the
smoothed auto-correlation of the data. This attribute has similar uses to those
of Bandwidth.

Bandwidth Rating (Debias) - similar to Bandwidth Rating (Bias), but corrected
for the bias effect of the smoother. This is usually the preferred measure of
the bandwidth.

Sum of Amplitudes - this equals the arithmetic mean of the amplitude multiplied
by the number of samples in the window. This attribute provides a measure of
brightness multiplied by the formation thickness (time) and may be regarded as
a measure of brightness volume. A large value may indicate a high net sand
ratio.

Sum of Magnitudes - sums the absolute values of reflectivity within a time
window. The attribute may be used in a similar way to Sum of Amplitudes but is
independant of amplitude sign.

Window Length - indicates bed travel time thickness above tuning when using peak
above and trough below (or vice-versa) as "Blip Horizons". Tapering does not
affect this attribute because the tapers are centered at the start and end
points of the window.

Blip Horizon - a user may generate two phantom horizons that may be based on one
or two pre-existing horizons. The phantom may be a constant above or below the
horizon, or it may be the nearest peak, trough or zero-crossing above or below
the horizon. Blip Horizon is not really an attribute but a phantom horizon
generator used to delineate where attributes are being generated.

Mappable Seismic Statistics (MSS) :
-----------------------------
MSS is not windowed. All of its attributes are defined for the entrire trace
whereas all CSA operations occur within a specific window comprising only a
portion of the trace length.

All the results from MSS may be posted in Basemap under horizon
name "Linevolume_stat".

The following table describes additional SEGY mapping of Trace header
information to Computation Manager's Mappable Seismic Statistics. Users can
display these attributes if they exist in the SEGY file.  Remember that while
the SEGY file may contain values in these fields, that does not guarantee
that they are correct.
  
CM CS Attribute          IESX trace        SEGY Record
Name                    Header var.         Header var.    Byte Positions
(CM dialog)           (cm_statistics.c)  (segy_bld_thdr.c) (segy_trace_hdr.h)
---------------          -----------    ----------------  ----------------   
CDP Elevation            cdp_elevation  recelevation        41  -  44
CDP Datum                cdp_datum      recdatumelevation   53  -  56
CDP Fold                 cdp_fold       numversum           33  -  34
CDP Weathering Depth     cdp_wdepth     *                   *
CDP Weathering Velocity  cdp_wvel       weathervel          91  -  92
Water bottom depth       water_depth    grpwaterdepth       65  -  68
Time of First Sample     tfs            delaytime           109 -  110


(*)  -   No standard SEGY header name.





 




Last Modified on: 25-AUG-98