extended seismic processing sequence lecture 24

EXTENDED SEISMIC PROCESSING SEQUENCESEISMIC DATA PROCESSING ZGE 373

Data Format

As a result of the large amount of seismic data, these data should be stored for computer usage using certain formats. These formats must be space efficient and also must be standard. The space efficiency here means to use less disk space. The standardization means its definition is constant and thus can be read on any machine. The latter is important for data exchange.

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Fortunately this is obtain through SEG (Society of Exploration Geophysicists. Since early days ; in the 80’s, SEG-Y format was introduce to format data on magnetic tapes. SEG-Y format is no well documented and standardized that all processing packages can use its stream as input. Off course these packages sometimes some local formats. SEG-Y now can be stored on other media than magnetic tape including disks. SEG produce two formats SEG-D and SEG-Y.

SEG-D Format

This format is usually used in data acquisition stage. Meaning that the data delivered to processing unit are in SEG-D format. It is converted to SEG-Y format and further actions are carried out on it. But why SEG-D format is used for data acquisition? Because it supports multiplexing and demultiplixing.

What is multiplexing and demultiplexing mean?

In multiplexing data are stored in the manner that the first samples from all traces being stored together, then the send sample and so on. The data is not saved trace by trace this method reduce the space and effort to store the data.

Demultiplexing on the other hand, extract the data and store it trace wise format.

SEG-D Format

Another benefit from using SEG-D format is that it can support many words length!!! Which make it useful in making reduction in data space on storage devices.

Seg-y format

It is the standard format for oil and gas industry. The format consists mainly of two main blocks; the header and the traces blocks. The header is also subdivided into two blocks one for metadata and the other for specific important data like the line number, sampling rate and so on. The trace block contains the data.

SEG-Y Format

Surveying information and field geometry

The next step is to put the surveying information into SEG-Y volume. This is an important task and sometime it take six months to be carried out. Surveying information include data about the locations of the shot points and geophones. It also include data about elevations. Field geometry is a collection of all information related to sources used, geometrical parameters of the layout. These information is vital and the quality of the obtained models depends greatly upon it.

Do you have any idea where this data is stored in the SEG-Y volume?

Trace editing and amplitude balancing:

This part is related to the data quality. For trace editing, it happens that some shots are bad or some traces are malfunctioning. These type of data should be removed before any further step. Trace editing id made based on field report similar to the one shown in next slide.

Amplitude balancingHere we do several algorithm to correct for the loss of energy due to geometrical spreading and absorption (reflection energy attenuates more as it travel longer distances). Geometrical spreading are applied such that the amplitude is decaying at a rate equal to 1/r. Other energy loss due the experiment itself is taken into consideration. These energy loss can be summarized as:

Trace (geophone) problemsDue to bad coupling with the ground, or problems in geophone. This is solved by modelling or averaging the energy of traces and increasing the gain for this certain trace.

Shot problems:Some shot due to some local conditions may have different energy distribution. In solving this we follow same processes above for shot energy equalizatin

AGC

Used generally to enhance the view of the trace by appling various gain.

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