# Seismic migration

The processing is the processing of seismic data with seismic reflection studies. It is conducted to the raw data obtained from a seismic experiment, a useful image of the subsurface geology. In a seismic experiment two-way propagation times are ( Two Way Travel Time, TWT) recorded, so the time of delivery of the pulse to the seismic source down to the reflector and back to the earth's surface geophone. The data must be processed so that to be from the time measurements information about the depth of the reflectors to enable successful drilling (eg in hydrocarbon exploration ) or deep -related findings in the geological and geophysical basic research.

- 2.1 speed determination 2.1.1 method
- 2.1.2 method
- 2.1.3 Dynamic correction
- 2.1.4 Average Speed
- 2.1.5 effective velocity
- 2.1.6 interval velocity
- 2.1.7 stacking velocity

- 2.2.1 Constant Velocity Scan
- 2.2.2 Constant Velocity Stack
- 2.2.3 velocity spectrum

- 3.1 motivation
- 3.2 types of migration

## Preprocessing

For the subsequent processing of the measured data is first sorted into common midpoint gathers.

The subsequent preprocessing comprises the analog band-pass filtering of the signal, static corrections, and deconvolution of the signal.

### Band-pass filtering

The band-pass filter comprises a high pass filter and a low pass filter. In digitizing the analog seismic signal may result in aliasing. This occurs when the sampling was injured and one has chosen a too low sampling frequency. To prevent this is given to using an analog low -pass filter prior to sampling the signal. In addition, we apply a high-pass filter to suppress noise, hum and long periodic components of surface waves.

### Static correction

Through the static correction, the seismic experiment is reduced to a reference level, since the uppermost layer of the substrate ( weathering layer ) has elevation changes and strong Geschwindigkeitsinhomogenitäten. To calculate the static correction you need the power and the seismic velocity of the layer between the reference level and the surface. This can be determined by means of refraction at the surface layers or a seismic source in a borehole.

## Velocity analysis

The normal MoveOut (NMO ) is the deviation of the running times of the period of use. The speed of analysis must be done before further processing of the seismic data to determine the normal MoveOut and correct. The duration curve of a seismic wave is given by. The moveout velocity includes both the type of shaft, and the model assumption (see table).

### Velocity determination

#### Method

The method is based on the parabolic approximation. The underlying equation. Now you carry on against and can be calculated from the slope of the MoveOut speed.

#### Method

The method is more accurate than the method since it is based on the Hyperbelnäherung. You can enter the travel time curve in a graph and determine the speed. As with the method is the slope of the line.

#### Dynamic correction

The dynamic NMO correction reduces the travel time curve offset depends on the operating time. For each CMP gathers a NMO correction is performed for each. This gives a rough estimate of seismic velocities in the subsurface, however, poses problems. The NMO correction is a non- linear stretching of the time axis along the offset. Corresponding to the signal is stretched by increasing the offset ( NMO stretch ) and the frequency is decreased.

The data are corrected for the NMO stretch by applying a linear filter that sets the shares Gather zero. The data are to be stacked ready now.

#### Average speed

To determine the average speed, sunk one geophones in a borehole and ignites a seismic source near the borehole (vertical seismic profiling ). The average speed refers to the geometrically shortest path from the source to the reflection point.

#### Effective velocity

The effective speed ( root mean square ) is the root mean square of the velocity.

#### Interval velocity

The interval velocity results from the Dix - Krey Dürbaum formula:

Is the speed of the layer between and. A homogeneous layer is the layer for a layered velocity and area is the effective velocity

#### Stacking velocity

The stacking velocity results together with the pseudo- time use the term curve

.

This hyperbola approaches to the true duration curve in such a way that the surfaces of the positive and negative deviation balancing each other. The stacking velocity and the pseudo operating time vary with the offset of the maturity curve.

### Automatic velocity analysis

The method must be performed by hand, they can not automate nor is it objectively. These claims are, however, placed on modern processing algorithms.

#### Constant Velocity Scan

There are constant speed from an adequate range of speeds used for NMO correction applied. The speed is then selected for each reflection, which shifts the MoveOut times on the operating time. This procedure must be evaluated visually and is very subjective especially in bad signal to noise ratio.

#### Constant Velocity Stack

When Constant Velocity Stack or brute stack constant speed from a speed range are related to the NMO correction to the CMP gathers apply again. Subsequently, the individual traces of the CMP are summed. By destructive overlays duration curves, which are corrected with the wrong NMO velocity, wiped out in the summation ( stacking ). Are all MoveOut times in phase with the use of time, the amplitude of the resulting trace is greatest and the selected speed is optimal.

This procedure must still be visually evaluated and subject to a certain subjectivity.

#### Velocity spectrum

The calculation of Geschwindigkeitssprektren automates the selection of the velocity function.

For each period of use the NMO correction is applied for a speed. Then, the semblance coefficient is calculated for a time window around.

Here, N denotes the number of traces in the CMP gathers, and M is the number of discrete values in the j- th track. The Semblance coefficient normalizes the energy of the incoming track on the energy of all tracks, so it can only take values between 1 and 0. This calculation is entered for all and all and repeated in a diagram. This gives the function for the velocity spectrum. Now can be automated for each reflection of the highest Semblance coefficient are selected, which characterizes the optimal stacking velocity.

The Semblance coefficient is a measure of the coherence and is used because of the high resolution in time and speed.

### Result of the velocity analysis

From the velocity analysis and NMO correction of the CMP gathers with subsequent stacking is obtained called a time section from Lotzeiten, even zero- offset section or stack section. These simulate a vertical incident beam from the source to the reflector. The data volume is reduced by a factor between the contact ratio of the CMP Gathers.

## Migration

Migration (from Latin: migratio, hike, move ) referred to in the seismic method for creating an underground image from the measured wave field at the surface.

The objective of migration is to create from the reflections in the subsurface geological image of the subsurface with the correct slopes, lengths and positions of the reflectors.

### Motivation

CMP gathers may include Triplikationen and other intersecting layers that do not occur geologically. In addition, these times are migrated to depths to allow precise drilling.

### Types of migration

One can use different criteria to distinguish the migration.

Firstly, a distinction is made between the time of migration in the region and the depth migration in the region. The depth migration reacts more sensitive to errors in the velocity model of the subsurface, provides in contrast to the migration time for an authentic image of the subsurface. On the other hand, one can distinguish between the pre-stack and post- stack migration. The poststack migration is made after the velocity analysis and stacking. The pre-stack migration takes place before the velocity analysis and stacking, to take into account also the amplitudes of the received signals.

Through specific prestack migrations statements about the reflection coefficient in the substrate can be made.