Seismic Trace Data

A seismic trace is a fundamental element in seismology, representing the recorded motion of the ground at a specific location over time due to seismic waves. Traditionally, this was visualized as a continuous line on paper produced by a seismograph's needle. In contemporary practice, seismic traces are captured digitally, allowing for precise analysis and interpretation.
 
Each seismic trace corresponds to the seismic wavefield detected at a particular receiver, or geophone, following an energy source event like an explosion or a natural earthquake. The trace records variations in ground motion, which are indicative of the subsurface geological structures the seismic waves have traversed. By analyzing these traces, geophysicists can infer properties such as rock composition, stratification, and the presence of resources like oil and gas.
 
In seismic data acquisition, multiple traces are collected simultaneously across an array of receivers. These traces are then compiled into datasets that provide a comprehensive view of the subsurface. Advanced processing techniques, such as seismic imaging, utilize these datasets to construct detailed models of underground formations. This process is essential in various applications, including natural resource exploration, earthquake research, and civil engineering projects.
 
The analysis of seismic traces involves examining attributes like amplitude, frequency, and phase. These characteristics can reveal critical information about subsurface features, such as the location of fault lines, the density and type of rocks, and potential fluid reservoirs. As such, seismic trace data is a cornerstone of geophysical investigations, enabling scientists and engineers to make informed decisions about exploration and hazard mitigation.

Format History

Seismic surveys are acquired and processed so that geoscientists can interpret the sub-surface structures and geological features.

The improved understanding of the sub-surface can then be used in the exploration and development of resources such as hydrocarbons, minerals and water.  It is also useful for planning other activities like CO2 sequestration, geothermal power generation, etc..

The seismic method was first developed and used for hydrocarbon exploration in the 1930s and there has been dramatic and continuous improvements in the acquisition and processing techniques being applied and the accuracy of the resultant sub-surface imaging.

Seismic data is acquired in the field by having an energy source that generates acoustic waves that penetrate the subsurface, which will be reflected and refracted by changes in sub-surface densities – for example where sandstone meets granite.  This generates acoustic signals that are detected and recorded by sensors at the surface.

Analog Seismic Recording

These systems relied on mechanical or electrical devices, such as galvanometers and photographic drums, to record waveforms on paper or film. Signals from geophones or seismometers were amplified and converted into continuous traces, visually representing seismic waves.

Though limited in dynamic range and resolution compared to modern digital systems, analogue recordings provided valuable data for early seismology and oil exploration.

Advances in digital technology have largely replaced analogue methods, but historical records remain crucial for long-term seismic studies and understanding geological events over time.

Paper

The first analog recording method was using paper, with the paper being pulled at a consistent speed through a printer while one or more evenly spaced pens were being deflected by the signal from its respective sensor. 

These may be separate sheets of paper or separate “pages” on a continuous roll of paper. The result is a page containing a series of visible “wiggle traces”, with each trace being associated with a sensor. 

Typically there was one page per seismic shot, and the acquisition crew would often hand annotate each page with important information such as date and time, shot point, etc..

 

Magnetic Strips

An improvement over paper-based recording was the introduction of “analog strips”.  These were rectangular sheets – typically made of mylar - with a magnetically sensitive coating.  There was one strip per seismic shot and these often contained hand-annotated information relating to the shot on each strip.

There were various strip sizes that were used, with some examples being 5.7cm x 123.7cm (quite long and narrow) and 18.4cm x 62.5cm (shorter and wider).

The strips would be mounted and moved in a recorder in a similar way to the paper technique, with the pens being replaced by multiple, evenly spaced magnetic write heads – similar to those found in an audio cartridge player.  These heads would leave permanent magnetic patterns on the strips, based on the signal coming from the sensor.  There were typically 12 or 24 magnetic heads depending on the model of recorder.

There were two methods of retaining signal amplitude of an analog recording.  Both involve recording a higher frequency oscillating magnetic signal (a “carrier wave”) modulated by the strength of the lower frequency signal coming from the sensor.  

  • The first and simpler method is known as “Amplitude Modulation” (AM) in which the carrier wave signal is fixed and its amplitude (magnetic strength) varies according to the amplitude of the incoming signal.
  • The second method is known as “Frequency Modulation”  (FM), in which the amplitude of the carrier wave remains constant while its instantaneous frequency varies according to the amplitude of the incoming signal.
The FM technique typically offers advantages with respect to the bandwidth of signal that can be recorded as well as improved signal-to-noise and less susceptibility to media degradation.

The data on magnetic strips is not visible to the human eye although can be retained for long periods of time and played back on an appropriate instrument – typically for processing or transcription purposes.



A typical 24-channel Analog Recorder.  Photo: A. Cairns


An example Analog Strip with Protective Sleeve.  Photo: A. Cairns

Magnetic Tape

A more efficient method of magnetically recording analog signals was the introduction of magnetic tape, in which multiple shots could be recorded one after the other down the length of a spool of magnetic tape. 

These spools were typically about 12 inches in diameter with a length of about 2000 feet and could continuously record (e.g. in a marine environment) for several hours.

Tape widths were typically one inch or less and so the number of channels being recorded was somewhat limited.  To complicate matters, it was not unusual to have some channels recorded using the FM technique and other channels using the AM technique. 

In addition, additional supporting information was critical, as the tape could not contain additional hand-annotated detail for each shot. This method of recording was not used extensively as digital recording techniques became available and offered more advantages.

 

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Digital Seismic Recording

Geophysical companies in the 1950s and 1960s realized that the digitization of the seismic trace data during both acquisition and processing would provide enormous benefits with regards to efficiency, storage and the development and application of future processing algorithms.

The digitization process converts each seismic trace to a sequence of numbers with each number representing the amplitude of the analogue signal at a specific point in time. 

 

Sampling

The process of digitizing a trace involves passing the analogue signal through a circuit known as an A-to-D converter (Analogue-to-Digital Converter, also known as ADC) that will measure the instantaneous amplitude of the signal and create the appropriate numeric value at a fixed interval known as the “sampling interval”.

The sampling interval for seismic data is usually measured in milli-Seconds (mS), and typical values for acquiring seismic data have been 4mS (250 samples per Second), 2mS (500), 1mS (1000) and 0.5mS (2000).

During acquisition, the analogue signals from the acoustic sensors need to be handled in real time, so a seismic system acquiring 24 channels would need to have 24 separate A-to-D circuits operating simultaneously and independently – converting the signal from each sensor into a stream of numeric values. 

The analogue signal is converted to a sequence of numeric sample values.  A. Cairns  

Numeric Formats

With each seismic sample being represented by a number, an important aspect to understand is that of the different “numeric formats” that can be used and why. In an ideal world there would perhaps be a single numeric format that satisfies all the needs of acquiring, storing, processing and interpreting seismic data.
However, there are several factors that impact why different numeric formats exist and why they may be used.
These include:

  • The dynamic range that is desired (the ratio of largest to smallest amplitude that can be recorded)
  • The precision that is desired (the smallest increment
  • The number of bits/bytes available (e.g. perhaps limited by recording media such as magnetic tape)
  • The desire to reduce dataset size for more efficient use of storage media and/or faster analysis.
  • Limitations on the recording equipment for real-time A-to-D conversion
  • Industry standards
Some of the types of numeric formats that have been used for seismic data include the following:
  • Integer, 2’s complement – precision is defined by number of bits
  • Integer, sign and magnitude – precision is defined by number of bits
  • Amplitude and Gain – an instrument gain factor is provided along with A-to-D value
  • Floating Point – precision is defined by number of bits in mantissa; dynamic range by exponent.
Modern computers are heavily byte-oriented (8 bits per byte) and so many modern numeric formats tend to be related to an integer number of bytes (or possibly even “nibbles” – i.e. 4 bits – in some cases). 
However, some of the historical systems using 21track tape drives or 7-track tape drives tended to use numeric formats that suited the media – i.e. 18 bits and 6 bits respectively. 
Early digital acquisition systems typically applied a stepped analog pre-amplifier to the signal before passing this to the A-to-D circuit, in order to handle real-time digitizing while maximizing the dynamic range supported by the system. 
Hence the use of an Amplitude and Gain format was appropriate for recording data from these systems – recording both the pre-amplifier gain setting and the output of the A-to-D converter. 
The official formats SEG-A and SEG-B store sample data in this manner, along with many of the earlier proprietary acquisition formats.
The Floating Point format is most commonly used for acquiring and processing modern seismic data. 

There are two commonly used variants
  • IBM 32-bit Floating Point, part of the original 1971 SEG-C acquisition format and 1975 SEG-Y exchange standard and hence still widely used today
  • IEEE Floating Point (either 32-bit or 64-bit) which is natively used in many modern computers and was formally added to the SEG-Y standard in 2002 (32-bit) and 2017 (64-bit), and to the SEG-D standard in 1994 (32-bit), 2009 (64-bit) and 2015 (little-endian versions of each).
  • The use of IEEE Floating Point allows for faster processing of seismic data as there is no need to do conversions between the IBM format if used in SEG-Y traces and the IEEE format that is used for computations in most modern computing systems.

Later acquisition systems were able to generate full Floating Point samples in real-time, and this was adopted in the SEG-C format (IBM 32-bit FP) and SEG-D format (IEEE variants in sub-formats 8058, 8080, 9058 and 9080)

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Multi Channel Recording

Seismic multi-channel recording involves using multiple sensors (geophones or hydrophones) to capture seismic waves across various locations simultaneously. This technique enhances subsurface imaging by improving signal clarity, resolution, and depth penetration.

Each channel records seismic responses from different angles, allowing for better data processing and interpretation. However, challenges include high equipment costs, complex data management, and noise interference from environmental and human sources. Synchronization between channels is critical, requiring precise timing mechanisms.

Additionally, field deployment in harsh or remote environments poses logistical difficulties. Despite these challenges, multi-channel recording remains essential for oil exploration, earthquake monitoring, and geotechnical investigations.

 

De-multiplexed Formats

The multiplexed arrangement of samples as described above is not very suitable for subsequent manipulation of the seismic trace data – i.e. processing, display and interpretation. 

One of the first steps undertaken when processing a magnetic tape containing multiplexed seismic data is to “de-multiplex” the data – this involves re-ordering the samples such that all the samples for a given sensor/channel are together and in time order.

In the case of a 24 channel seismic system recording 5 second of data sampled at 2mS, this would result in all ~2,500 samples for channel 1 being followed by all ~2,500 samples for channel 2, etc. 

As dynamic storage technologies developed, acquisition techniques allowed for the data to be de-multiplexed effectively in real time, such that the data could be written to tape (with a slight delay) in a demultiplexed format rather than a multiplexed one. 

The SEG  published the SEG-X format as a demultiplexed exchange format in 1967, although this had little industry uptake – unlike the SEG-Y format published in 1975. 

 

Multiplexed Formats

In the early years of seismic acquisition using digital techniques, dynamic digital storage (e.g. RAM and disk memory as used in modern computers) was not yet available, and so the digitised samples coming from the sensors and A-to-D circuits had to be written to a physical storage media (e.g. magnetic tape) in real time.

In the case of a 24 channel seismic system, this means that as the recording starts (an event typically known as “time zero”) there would be 24 separate numeric values being generated at each sampling interval. 
In the example of a 4ms sampling interval, this would produce 24 numeric values at time zero (one for each sensor/channel), another 24 at 4ms, another 24 at 8ms, etc.

This method of arranging the numeric values is known as “multiplexed”, and one set of samples for a given time value is typically known as a “scan”.  In the case of a 5 second record sampled at 2ms, this would result in ~2,500 scans.

The early digital recording formats for multi-channel seismic acquisition almost invariably use a multiplexed format.

 

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Proprietary versus Industry Standard Formats

The seismic industry has a long history of continuous technical developments in acquisition and processing techniques.  A number of different companies were not only involved in seismic acquisition and processing, but were also instrumental in developing these new techniques and the associated hardware and tools to deploy them.

These companies often provided both services – i.e. they would acquire the data in the field, and then process the data in their own offices, and so it was commonplace for each company to develop their own internal (proprietary) format for recording the data on magnetic tape.  

This was not ideal for the industry, with Operators preferring to have common formats that would allow them to choose different providers for the acquisition and processing services if they so desired.

The body that has had most influence over creating such standards has been the SEG (The Society of Exploration Geophysicists in the USA) which has created a number of different seismic formats, starting with SEG-A/B/X in 1967.  The SEG-D and SEG-Y formats remain ubiquitous within the industry.

The creation of these standard formats does not mean that proprietary formats are no longer used.  There are several reasons why many seismic companies still create and use non-standard, proprietary formats:

Most processing software packages have their own internal formats that are designed to provide greater flexibility and efficiency than the standard formats (e.g. SEG-Y) can provide.  These formats are often tailored to exist both on disk (during processing activities) and on tape (for intermediate or long term storage).

Historically the acquisition companies have pushed past the limits of any existing standards of the time, and some of the standards have actually been developed to formalize methods that were already in use.  As an example, prior to the SEG-A and SEG-B standards being published in 1967, a number of companies were already recording data in similar – but slightly different – formats.

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