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A short while ago I re-read Anstey's "Simple Seismics" - even though it was over a quarter of a century since I last took it off my bookshelf. I read it once more with all the interest of my first read in 1982, for it is like a junior detective manual, or the taste of 'first blush' Darjeeling tea - fun of fun and anticipatory excitement. A beginner in seismics should read this book first and ten go onto a modern text to figure out the ramifications of the seismic method - source, instrumentation, earth anomalies, amplitude, wavelets and how to eliminate and interpret a seismic signal.
Analysis of the patterns of reflections can be used in many areas of subsurface geology, including the following.
Identify geologic features such as structures, faulting and folding.
Establish the time of faulting and folding.
Identify the presence and timing of uplift and subsidence in the area. Hydrocarbon traps are formed before or during the migration of hydrocarbons NOT AFTER. Late traps are of NO interest to the investor!
Locate a sediment source.
Identify depositional conditions.
Suggest rock type.
Establish the sequence stratigraphy of the region.
Directly locate hydrocarbons.
Estimate the thickness and size of the reservoir.
Estimate the 'interpretational' appeal of a prospect.
Estimate the 'interpretational' risk of a prospect. At one level this can quickly discard bad leads and avoid wasting time and at another it is used to aid the potential investor in assessing his financial risk.
Seismic interpretation MUST be consistent with the geological framework.
The common mid point method [cmp] of acquisition involving many geophones recording each shot is the basis of seismic analysis. However, to get to this stage a number of processes are used. A first look can be seen by gathering the near-traces for each shot to establish a near trace section, usually after the first breaks [representing the upper layers of the earth] have been removed or muted.
The cmp gather brings together all of the traces representing one mid-point coordinate. The cmg has a curvature because the traces are not yet aligned in time - as the source to receiver effect increases so does the travel distance [time] i.e. the nmo [normal move out]. Determining the nmo is called velocity analysis. It provides the best fit of velocity to the cmg. After further tweaking the final gather is summed to provide a single trace [composite trace] in a process called the cmp stacking.
In uniform media the seismic energy per squared area is inversely proportional to the distance between the shot point and reflector. Thus the amplitude is inversely proportional to time, disappearing when it drops below the noise level. In uniform media it is easy to correct for amplitude reduction using a multiplier based upon time [distance] carried to the point where ambient noise is greater than amplitude. Velocity ordinarily increases with depth which means the wavefront does not expand at a constant rate [SNELL'S LAW] thus increasing the decay of the amplitude with depth.
A second important correction to make is location [field static] correction, which involves removing the effect of the near surface and topography. Corrections are performed by selecting a datum plane from which to 'hang' the seismic traces e.g. sea level. The static corrections are designed to remove the effect of the earth above the datum chosen. The thickness and characteristics of the regolith must be accounted for - these include both weathering and water table location, which provide a low velocity layer near the surface.
There are many other factors to take into account during processing of the seismic traces e.g. the depth and strength of the charge, the lateral variation in the sub-surface. To improve the data set the traces are normalized and then balanced. These are common statistical procedures applied to many kinds of data sets ranging from test grades standardization to environments parameter analysis and the errors and assumptions are well known. The procedure in seismic standardization is to calculate mean amplitudes for traces and then standardize to a fixed amplitude. The result removes a variety of irregularities all in the same process.
Trace balancing [automatic gain control] adjusts the amplitude within a single trace - one method to accomplish this is a moving average - adjustments are made for drop off at the beginning and end of each trace. AN abnormally strong reflector should not be allowed to unduly influence the trace data set and this can be done by decreasing the corridor length and using a weighting technique. To preserve the anomalous amplitude a cut-off value can be established above which an amplitude is left unaltered and not included in the moving average.
To improve the signal to noise ration source generated [coherent noise] and spatially /temporally generated [incoherent noise] are removed or attenuated. The adding of adjacent traces [array simulation] can emphasize the s:n ratio [except in laterally rapidly changing terranes]. Frequency filtering passes the trace through a digital prism to provide the amplitude spectrum of the trace. This is used to reduce the ground roll, wind effect etc. Wavelet processing attempts to re-shape the wavelet so that it is symmetrical with a compressed range. If we can determine the source signature w have a better starting point. Dephasing removes the effect of instrumentation and deconvolution statistically evaluates all of the trace reflections to view a more desirable wave form i.e. a wavelet that is as close to a spike as possible. With a better wavelet the events are easier to time and better resolved.
A section should be annotated to show the scales. Normally a trace line is displayed East to the right and North to the right with a side table showing the field variables, processing history, datum, elevation velocity, cut-off time of spreading correction, trace normalization window, and length of trace balancing window.
For detailed work a scale of 10 cm/sec [two-way] which is approximately 4" per second] is used. Regional analysis uses half this scale and the location of subtle traps twice this scale.
The base map scale is generally 1:25,000 [4cm/km] or 1:24,000 [1inch/2000ft].
A fence diagram has a time scale of 5cm/sec on a 1:50,000 base map [2cm/km] which gives a vertical exaggeration of approximately 2:1.
Maps are generally produced at a contour interval of 10 msecs.
Normally there are a variety of display modes. The original standard was the variable area wiggle, but in 3-D seismic there are numerous methods available each of which offers a slightly different viewpoint. Commonly a color bar is used in which strong reflectors are red/orange/yellow and weak reflectors are blue/green.
The processing starts with the Field Tape. The near traces are selected, the first breaks are muted and the spherical spreading is corrected. Using the field notes the data is corrected to datum, the amplitude adjusted and then displayed. Amplitude adjustment takes place during the processing of the s:n ratio with array simulation, correcting for instrument response, dephasing, applying wavelet extraction, deconvolution, correction for water depth, deverberation, and filtering.
The basic structural tuning process is migration with the repositioning of reflectors so that their optimal relationships are as correct as possible. Effectively the reflection is moved from the trace that recorded it to the trace that would have recorded it if the source had been on the reflector, and the travel path was vertically upwards. Inversion attempts to convert a seismic section to represent the properties of the earth LAYERS, correctly positioned in space.
A borehole check shot or velocity survey correlates reflection events to geological markers at known depths. The velocity survey aids in interpretation in that:
The product of sonic velocity [sonic log] and formation density [density log] provides a measure of the acoustic impedance [hardness] and can be used to calculate the synthetic seismogram.
After the data is loaded it is important to ask "What is the polarity?"
When correlating it is best to pick the point near the maximum of the amplitude envelope whether it is a trough or a peak. If done correctly the final seismic line has true amplitude processing with a zero phase wavelet so that the peak is at the maximum of the seismic envelope. The top and bottom of structure can be picked from vertical seismic lines or time-slices. With a saddle between the top and the bottom the top of the bed is picked as a trough and the bottom of the bed is picked as a peak. A reflection train that does not show a saddle is probably the result of 3 or more reflections merged together.
A phase display does not show amplitude but gives a sense of continuity to locate faults.
The derivative of the instantaneous phase is the instantaneous frequency display and us a useful correlation tool showing the manner in which component frequencies interfere with one another. The display can be used as a HC indicator because a low frequency shadow is often seen below and accumulation of HC's.
Compaction [density effect] tends to exaggerate the real plane towards concavity.
In unconsolidated sediments HC presence tends to produce a ''bright spot'. In consolidated sediments HC presence tends to produce a 'dim out'.
Examining the energy characteristics derived from the amplitude is of use in interpretation. The wavelength of the seismic energy can determine whether a thin bed can be observed as a reflection, whether the top and bottom of reflectors can be resolved; or, whether the structure gives a response at all! Features larger than 1/4 WL can be seen [the Raleigh Limit]. The wavelength is the ratio of velocity over frequency and on a seismic section the wavelength can range from 10:1. Shallow reflections [<2km] usually are a result of low velocities [6,600 ft/sec or 2km/sec] and moderate frequencies [ca 50 hertz] and the wavelengths approximate to 135 ft [40m]. Deeper reflectors higher velocities [e.g. 16,400 ft/sec or 5mk/sec] and frequencies of 20 hertz and the wavelengths are approximately 820 ft [250m]. The result is that a large deep feature has the same seismic expression as a shallow small feature. i.e. resolving power at shallow depths is 35 ft [10m] compared with 820 ft [60m] at depth.