Acquisition of Seismic Data

Geophone Hydrophone

Parameters of data acquisition

Some parameters of a seismic acquisition program are:

· maximum offset: distance from the source to most remote receiver;

· minimum offset: distance from the source to nearest receiver;

· group interval: distance between geophone arrays. Constant for a survey;

· shot intervals: distance between holes;

· fold coverage: number of times a subsurface point is surveyed by different sources and detectors;

· sample interval: the time interval between digital samples of the signal, which varies from less than 1ms to 4ms. This sample rate is chosen not to limit the vertical resolution and to record the desired maximum frequencies;

· choice of source and geophone arrays;

· number of recording channels.

Acquisition of parameters and noise

Fig. 1 below is a schematic seismic section showing the major signal and noise features. It has been corrected for normal movement (NMO) which is the time correction applied to each trace to account for its offset. The first arrivals at the top of the record are labeled P-waves; these are typically refracted waves from near-surface formations. These are followed by two coherent noise patterns. The first one, characterized by low frequency and a velocity that varies from 3500 to 5500ft\sec., is the surface wave (ground roll). The second one is the air wave, which is energy traveling from the source to the detectors through the air having a high-frequency component and low velocity of 1100ft\sec. The third type of noise is the multiple, which is a repeat reflection from the same interface. It may be either a simple multiple from one of the shallow reflectors or an intrabed multiple bounced between two reflectors and back to the surface or one of several other types in which the reflected ray reverses direction at some point.

Elements of a data acquisition system are:

1. sources and arrays;

2. detectors and arrays;

3. instrumentation;

4. field geometry or survey configuration;

5. surveying, positioning and navigation.

Profile

AIR GUN

four- gun array

Vibroseis

Five- vibrator array

SHOT HOLE

Surface

Fig. 1 Source arrays

There are a number of types of

1. Seismic sources used in exploration. They fall within one of the two principal categories:

1. impulsive (explosives);

2. distributed or diffuse in nature (vibrators).

2. Seismic detectors. A variety of receiver types are available for detecting seismic waves. On land we use geophones, which respond to either vertical displacement or rotational motion. Vertical – displacement geophones are commonly used in land seismic data acquisition. They measure the rate of change of displacement or velocity In fact, they can measure derivative of displacement which is acceleration.

Multiple receivers can be connected in arrays to enhance the signal and to reduce noise. Types of arrays used include:

1. linear array – a line of single geophones;

2. weighted\ tapered array – a line in which the number of geophones at each position varies so that the outer elements have the smallest number of geophones (less weight) and the center element has the greatest number of geophones (most weight). The change in number of geophones from position to position is the taper of the array.

3. Instrumentation represents great advancement in the recording of seismic signals without loss of information and with the ability to recover field amplitudes.

- Amplifier – filter – recorder

- Alias filter (assure us that our digital sampling is properly accomplished and that high-frequency noise components do not masquerade as contributions of lower frequency by aliasing: is a property of sampling systems in which an input signal of one frequency canyield the same values as a signal of another frequency).

Alias filter frequency = 0.6 alias frequency

72 dB + amplitude

Discrimination from 0.6 FA – 1.0 FA

- Digitizer (converts the analog electrical signals from the geophones into discretesamples)

- Gain ranging and control

- Sample–and-hold aperture (in order to accomplish such conversion; it is necessary to hold a short portion of the analog signal for conversion to the digital equivalent)

4. Field geometry – common depth point (CDP) gather is a group of seismic traces that represent a single point on a flat reflector. A common midpoint (CMP) gather is a group of traces from a dipping reflector. The traces of a common depth point (common midpoint) gather are put together in one family. Each trace is from a different source and different receiver, but represents the same subsurface point. This reordering of the traces and stacking (summing them together) enhances the signal-to-noise ratio by attenuating random effects and undesired events, such multiple reflections whose variation with offset offers from that of the primary reflections.

SOURCE HI Я2 R3 R* Rn * RECEIVERS

12 End-on pattern channels used for simplicity

Fig. 2 Common depth point field geometry

The diagram (Fig. 2) illustrates the ray path geometry for six shots taken and twelve geophones in the line.

By progressively moving the sources and the receivers, traces are obtained for a given subsurface point. These traces use different sources and receivers. The number of traces obtained for a common depth point is called the fold. At surface location R4, you can observe that the common depth subsurface point does have six traces. This type of shooting pattern is called end-on. If the TRACE 1 is the closest to the source, the shooting configuration is called pushing the cable. If TRACE 1 is the farthest from the source it is called pulling the cable. Another type of shooting pattern is split spread or straddle shooting, in which the source is in the middle, the middle spread is symmetrically split and the two banks of the spread are the same length. A technique called shooting through the cable may be used to place depth points as close to the line termination as possible.

(Wikipedia. The free encyclopedia. http://www.wikipedia.org/)

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