Artificial Offshore Seismic Sources

Synonyms

Marine seismic sources; Seismic energy sources

Introduction

Artificial Offshore Seismic Sources are instruments that are used for generating short-duration pressure pulses in the marine water column, usually with constant repetition rate. They provide the acoustic energy for marine seismic surveys. Marine seismic surveys are performed in order to image geological structure (layers and faults) beneath the seafloor and to characterize rocks and rock strength in terms of seismic wave velocity or dynamic elastic constants. Therefore, short pressure pulses are applied to the water column that travel as acoustic waves down to the sea bottom where they are converted into seismic body and interface waves (elastic waves). These seismic waves penetrate deeper into the underground and are partially reflected at geological interfaces and/or refracted back to the surface (Fig. 1, right). This returning part of the wave field is recorded either at the seafloor with seismographs or in the water column with hydroacoustic receivers. In order to allow for profiling both seismic source and receivers are towed behind a vessel carrying out repeated measurements while moving forward. The technique of marine seismic has been developed and improved continuously since the late 1940s. This entry provides an overview of the purpose and principles marine seismic sources. More detailed information can be found, for example, in Jones (1999) and Meunier (2011).

Artificial Offshore Seismic Sources, Fig. 1

Right: Principle of marine seismic data acquisition; pulse-like seismic waves travel through geological layers and are reflected back to the surface at interfaces where density or propagation velocity change; wave propagation paths indicated by rays (black lines). Left: examples of seismic sources for near-surface prospecting; from top to bottom: Airgun (GI-gun), Sparker, Boomer

General Considerations

The goal of marine seismic reflection surveys is to image geological structure down to a predefined target depth at highest possible resolution. Therefore, the pressure pulses generated by a marine seismic source are usually designed to have a sharp onset and short duration (some to some 10 ms; exception marine vibrator source) corresponding to a broadest possible frequency spectrum. The short duration, respective large bandwidth, is necessary to enable distinguishing of reflections (echoes) arriving at the receivers subsequently from interfaces at increasing depths. Seismic wave propagation is affected by absorption that leads to a notable continuous decrease of amplitudes starting from the high-frequency end of the spectrum. Therefore, penetration depth increases with decreasing signal frequency. While it propagates, the wave front widens and signal amplitudes decrease accordingly (“geometrical spreading”). This effect needs to be compensated for by increasing the initial strength of the pressure pulse in order to keep reflected signals of deep horizons detectable. Finally, in order to obtain a reliable depth section, the measured profiles have to be sampled spatially with a sufficiently high density realized by repeating the measurements while the vessel is moving. This implies that the marine seismic source needs to be reloadable within a certain time interval depending on the velocity of the ship. Typical repetition intervals are between 0.5 up to some 10 s. The target range relevant for seismic exploration – for example in earthquake engineering – may extend from near-seafloor layering down to several kilometers depth, for example, in order to detect and trace deep reaching fault systems. Correspondingly, different sorts of seismic sources are in use, each accounting for different target depth or infrastructure related situations (Table 1).

Artificial Offshore Seismic Sources, Table 1 Technical data and properties of typical marine seismic sources. The given value of structural resolution corresponds to a quarter wavelength at 1,600 m/s wave speed

Types of Marine Seismic Sources and Their Key Principles

Airguns and GI-guns

Airguns or G-(Generator)-Guns generate an acoustic impulse by releasing a high pressure air bubble underwater. The radiation pattern of the airgun source corresponds to an acoustic dipole because an image-source mirrored at the water surface has to be considered in order to account for the free-surface condition. It causes a so-called ghost reflection following the primary pulse with negative polarity and a time delay depending on source depth. While moving to the sea surface, the generated bubble oscillates around the equilibrium of air- and hydrostatic water pressure with a natural period T. T depends on the energy of the injected air E, the confining hydrostatic pressure P of the water column, and the fluid density d according to the Rayleigh-Willis formula T ≈ 1.14d^1/2 P^−5/6E^1/3. This oscillation generates an undesired lengthening of the seismic signal. In order to overcome the bubble oscillation, GI-(Generator Injector)-Guns are very common that inject a second air pulse to collapse the initial bubble before it starts to oscillate. G-Guns and GI-Guns can be mounted in arrays to enhance the signal amplitude and, when fired with time offsets, to modulate the shape of the signal.

The generated frequency content mainly depends on the volume of the airgun chamber, the assigned pressure, and the tow depth.

Water Guns

A water gun is a pneumatic seismic source operated by compressed air, too. The air is used to drive a piston pressing water from a chamber inside the gun into the water column. The seismic impulse is generated by the implosion of a cavity created behind the expelled water. It is free of bubble oscillations and contains higher frequencies than typically generated by airguns.

Sparker

The sparker source generates an acoustic impulse by a high-voltage spark discharge between two electrodes in the water. The spark consists of a high-pressure plasma whose expansion and collapse generates the acoustic peak. The high voltage needed to generate such a plasma is stored in a bank of capacitors, which is reloaded for each shot. Sparkers can be used at different energy levels, depending on the capacitor bank. Commonly sparkers are used in arrays to enhance the signal amplitude and to maintain the signal shape which usually changes because the electrodes tend to tip-off during usage. Sparkers can be applied in brackish water and seawater.

Boomer

Boomers are also based on combining electromagnetic and mechanical principles. Similar to the sparker case, electric charges are assembled in a bank of capacitors releasing a high-voltage pulse to the source. The discharge is led through a flat coil inside the source body. From below an aluminum plate is pressed against the coil by springs. The magnetic field of the coil induces eddy currents inside this plate. These currents produce magnetic fields that repel the plate from the coil (Lenz’s law). The spring then moves the plate back, leaving a collapsing cavitational volume that generates the acoustic impulse.

Pinger

Sediment echo sounders (SES) use piezoelectric “pingers” as electroacoustic converters.

Some materials (especially crystalline) tend to show electromechanical properties which means that the material is stretched when voltage is applied. Closely packed piezoelectric crystals are very effective electroacoustic converters and can be used as acoustic sources.

These pingers are applied as single point sources or – even more often – in arrays which enable, when used with time offsets, a modulation of the radiation pattern of the source. Alternative developments are based on the parametric nonlinearity effect to enhance the acoustic efficiency and the stability of the signal.

Marine Vibrator

Marine vibrator sources are basically volumetrically oscillating shells that transmit a chirp signal (“sweep”) to the water column. The chirp is linearly modulated in frequency and lasts typically several seconds. Seismic reflections can only be recognized after the records were cross-correlated with the source signal. This concept was adopted from the onshore Vibroseis^TM method (e.g., Sheriff and Geldart 1995). The shells are constructed flexible or otherwise expandable; the oscillations are driven by hydraulic, magnetostrictive, or electromagnetic devices placed inside (e.g., Tenghamn 2006; Meunier 2011). Like airguns and water guns, marine vibrators can be combined to form arrays in order to improve energy output and radiation characteristics. They work in a similar frequency range as airguns and release similar seismic energy in total. However, since they provide energy at lower rate over a much longer time interval they are causing less environmental impact than the impulsive source that release energy within some 10 ms. Since the vessel moves significantly while the source is active, Doppler effects occur in the records that need to be corrected in during data processing. For near-surface investigations, Vibroseis-type chirps can be generated through low-energy piezoelectric transducers. However, marine vibrators for deep (km-scale) sounding are still under development.

Summary

Artificial Offshore Seismic Sources are instruments emitting sound signals for marine seismic surveys that are performed for investigating offshore geological structure. Functional principles of marine seismic source depend on the required energy and frequency content of the signals, including piezoelectric and high-pressure air pulsers as low and high energy end members. The structural resolution of the seismic images of geological layering ranges from some 10 cm to some 10 m depending on the depth of the target horizons and the seismic sources applied.