9.1 ntroduction: The seismic method has been greatly improved in the both in the areas of data acquisition and processing. Digital recording along with the CMP multifold coverage was introduced during the early 60’s. Data acquired from the field are prepared for processing by the field party itself and then it is send to the processing centre. Processing is required because the data collected from the field is not a true representation of the subsurface and hence nothing of importance can be inferred from it.
With the advent of high end computing systems modern day processing has become a lot easier than it really used to be. Turnaround times have therefore come down with lot of processing taking place in-field or onboard.
9.2 Why Processing?
Field record which we obtain contains:
• reflections,
• coherent noise, and
• random ambient noise.
9.2.1 Reflections: Reflections are recognized by the hyperbolic travel times. If the reflection interface is horizontally flat, the reflection hyperbola is symmetric with respect to zero offset. On the other hand if it is dipping interface, then the reflection hyperbola is skewed in the up dip direction.
9.2.2 Coherent Noise: Under the coherent noise category there are several wave types.
• Ground roll is recognized by its low frequency, strong amplitude and low group velocity. It is the vertical component of dispersive surface waves i.e.
Raleigh waves. Typically we try to eliminate ground roll in the field itself by array forming of receivers.
• Guided waves are persistent, especially in shallow marine records in areas with hard water bottom. Guided waves also are found in the land records.
These waves are largely attenuated by CMP stacking. Because of their prominently linear move-out, in principle they also can be suppressed by dip filtering techniques. One such filtering technique is based on 2D Fourier transformation of the shot record.
• Side Scattered noise commonly occurs at the water bottom, where there is no flat, smooth topography.
• Cable noise is another form of coherent noise which is linear and low in amplitude and frequency. It appears on shot records as late arrivals.
• Another form of coherent noise is the air wave which has a velocity of 300 m/s. It can be a serious problem when shooting with surface charges. Notch muting is the only way of removing them. Power lines also give rise to noisy traces in the form of a mono frequency wave (50 or 60 Hz).
• Multiples are another type of coherent noise. They are secondary reflections having inter- or intra- bed ray paths. They propagate both in sub and super-critical regions.
• Power lines also cause noisy traces in the form of a mono-frequency wave. A mono-frequency way may be 50 or 60 Hz, depending on where the field survey was conducted. Notch filters of ten are used in the field to suppress such energy.
9.2.3 Random Noise: Random noise has various sources. Poor planting of geophone, wind, transient movements in the vicinity, wave motion in the water (marine) and finally electrical noise of the recording instrument.
One important aspect of data processing is to uncover genuine reflections by suppressing all unwanted energies (noise of various types) .The objective of seismic data processing is to convert the information recorded in the field to a form that can be used for geological interpretation. Through processing we are enhancing the signal to noise
ratio, removing the seismic impulse from the trace (inverse filtering) and repositioning the reflectors to its true location (NMO, DMO and migration), thereby making it into a more palatable form.
9.3 Seismic Data Processing
Seismic data processing is composed of basically five types of corrections and adjustments:
• Time,
• Amplitude,
• Frequency-phase content,
• Data compressing (stacking), and
• Data positioning (migration)
These adjustments increase the signal-to-noise ratio, correct the data for various physical processes that obscure the desired (geologic) information of the seismic data, and reduce the volume of data that the geophysicist must analyze.
The geologic information desired form seismic data is the shape and relative position of the geologic formations of interest. In areas of good data quality it is possible to produce estimates of the litho logy based upon velocity information. From the amplitudes of reflections, it is even possible to make estimates of the pore constituents, since gas accumulations often generate amplitude anomalies. Knowing the shape of the structures at depth allows oil company explorationists to assign probabilities of finding commercially exploitable hydrocarbons in the area surveyed.
The velocities of seismic waves in the earth can be derived from seismic data or measured in wells, and they are used to convert the known reflection times into estimated reflector depth.
9.3.1 Time Adjustments: Time adjustments fall into two categories:
Static and
Dynamic
Static time corrections (normal move-out) are a function of both time and offset and convert the times of the reflections into coincidence with those that would have been recorded at zero offset, that is, to what would have been recorded if source and receiver were located at the same point.
9.3.2 Amplitude Adjustments: Amplitude adjustments correct the amplitude decay with time due to spherical divergence and energy dissipation in the earth. There are two broad types of amplitude gain programs:
Structural amplitude gaining or automatic gain control (ABC), and
Relative true amplitude gain correction
The first scales amplitudes to a nearly alike and is generally chosen for structural mapping purposes. The second attempts to keep the relative amplitude information so that the amplitude anomalies associated with facies changes, porosity variations, and gaseous hydrocarbons are preserved.
9.3.3 Frequency-Phase Content: The frequency-phase content of the data is manipulated to enhance signal and attenuate noise. Appropriate band-pass filters (one-channel filtering) can be selected by reference to frequency scans of the data which aid in determining the frequency content of the signals. De-convolution is the inverse filtering technique used to compress an oscillatory (long) source waveform, often seen in marine data, into as near a spike (unit-impulse function) as possible. Ghosts, seafloor multiples, and near-surface reverberations can often be attenuated through convolution approaches. Many de-convolution techniques use the autocorrelation of the trace to design an inverse operator that removes undesirable, predictable energy.
9.3.4 Data Compressing (Staking): The data compression technique generally used is the common midpoint (CMP) stack. It sums all offsets of a CMP gather into one trace.
Forty-eight to 96-fold stacks are common. Conventional 2D seismic data initially exist in a 3D space: the three axes are time, offset and a coordinate x along the line of survey. Three-dimensional data consist initially of a 4D data set; the coordinates being time, offset and two horizontal spatial coordinates, x and y, which lies on the midpoint axis.
9.3.5 Data Positioning (Migration): The data positioning adjustment is migration.
Migration moves energy form its CMP position to its proper spatial location. In the presence
of dip, the CMP location is not the true subsurface location of the reflection. Migration collapses diffractions to foci, increases the visual spatial resolution, and corrects amplitudes for geometric focusing effects and spatial smearing. Migration techniques have been developed for application pre-stack, post-stack, or a combination of both.
9.4 Objectives of Data Processing
The objectives of data processing may be summarized as follows:
• To enhance the signal to noise ratio (S/N).
• To produce seismic cross section representative of geology.
• To meet the exploration objectives of the client.
9.5 Basic Data Processing Sequence
Since the introduction of digital recording, a routine sequence in seismic data processing has evolved. There are three primary steps in processing seismic data
1. De-convolution, 2. Stacking, and 3. Migration,
in their usual order of application. Figure 21(a) represents the seismic data volume in processing coordinates – midpoint, offset and time.
All other processing techniques may be considered secondary in that they help improve the effectiveness of the primary processes. The secondary processing steps include corrections (statics, geometric, NMO, DMO, velocity analysis, filtering etc.). Many of the secondary processes are designed to make data compatible with the assumptions of the three primary processes. De-convolution assumes a stationary, vertically incident, minimum-phase, source wavelet and white reflectivity series that is free of noise. Stacking assumes hyperbolic move-out, while migration is based on a zero-offset (primaries only) wave field assumption.
Conventional processing of reflection seismic data yields an earth image represented by a seismic section usually is displayed in time. A conventional processing flowchart is shown in the figure 21(b) on the next page.