Towing Depth Research Articles

Seismic response characteristics of marine reflection profiling systems

Summary. Factors influencing the seismic response characteristics of marine profiling systems are reviewed. The single frequency case is used to illustrate the influence of different frequencies on the response, as well as the towing depths of the source and receiver, and the geometry of a linear receiving array. The more realistic case of band-limited source waveforms is considered, using frequency spectra calculated from theoretically derived airgun signals. The results show that the number and shape of sidelobes of the profiling system response, as well as the filtering characteristics for reflections arising from reflectors in the vertical plane perpendicular to the axis of the receiver array are determined by the depths of the source and receiver and the relative amplitudes of the frequencies in the source waveform. These factors, along with the configuration of the hydrophone elements in the receiver array, determine the frequency and amplitude attenuation of reflections in the vertical plane containing the receiver array. The filtering characteristics of the system both in and out of the vertical plane containing the receiver array are discussed, with implications for discriminating between off-axis and in-plane reflections. A plan view of the response of the system is constructed in the time domain for various profiling configurations and sources of different frequency content at a given time. This example shows how useful the resulting pictures are for optimizing acquisition parameters in profiling experiments.

Upper crustal structure beneath the continental slope off the joban Coast, Honshu, Japan.

An airgun-OBS (ocean bottom seismometer) refraction profiling was performed at the continental slope off the Joban coast, Honshu, Japan. Three identical profiles were shot at 200-300 m water depth to see the effects of airgun signal change mainly due to airgun shot depth and reproducibility of record sections. It was found that an airgun towing depth of more than 10 m for a 4.9 liter chamber with about 90 kg/cm2 pressure shot yields better signal penetration than do shallower depths. Picked arrivals from the three profiles matched within 0.1 s scatter.P wavespeed structure down to about 8 km depth was obtained from this experiment. Constraints from the data of two OBSs require a laterally varying model. Our model consists of layers with 1.7, 2.9, 4.3, 4.7, and 5.5 km/s wavespeed. The top four layers have about 1.2 km thicknesses. The layer distinction is not clear below 4.3 km/s layer, suggesting gradual increase. However, the wavespeed does not increase as high as 6 km/s, which is characteristic of the upper crust beneath northern Japan. This may be due to the shortness of the refraction profiles.

The influence of wind-induced bubbles on echo integration surveys

An investigation of attenuation of acoustic energy caused by gas bubbles in the surface layers has been carried out. This was done primarily to study the effect on echo integration of fish abundance when using hull-mounted transducers. Two different approaches have been used. The first examines the variation of the echo intensity from an acoustically stable bottom layer and the second measures the total volume reverberation as a function of depths. The bubble density, size distribution, and the attenuation caused by the bubbles is estimated from the measurements done under different weather conditions. The results show that the acoustic attenuation caused by wind-induced gas bubbles in the surface layers appear at a lower wind force and at a greater magnitude than earlier reported and expected. The attenuation is found to increase rapidly with increasing frequency. The results are also used to find the minimum towing depths of a transducer as a function of the wind speed necessary in order to keep the attenuation due to the bubbles below a given number.

Simulation studies of the response of deeply towed vehicle to various towing ship maneuvers

The behavior of a long cable towed at slow speeds through the ocean depends in a complex fashion on the path followed by the towing ship relative to the water. A cable simulation program was used to characterize the response of the cable by using idealized towing ship maneuvers as input to the program. The response of the cable was noted and it was found that the behavior of the cable was strongly dependent on the fundamental period of the towing vessel maneuvers. Sinusoidal deviations of the towing ship from a straight towing track resulted in delayed and reduced excursions of the towed vehicle from the tract; the estimated response ratio varied from 0·002 to 0·800, depending both on the period of the deviations (periods ranged from 5·5 to 4·0 hr) and on the towing depth (2 or 6 km). The ship's speed was 3 km/hr. The time lag between ship motion and vehicle response was approximately 0·5 hr for the shallow case and 1·3 hr for the deep case. Simulations runs of a low dragk (faired) cable showed that the behavior of the vehicle when towed at a depth of 6 km was similar to that obtained with a conventional cable at 2 km depth. The response of the towed vehicle to a right-angle turn of the towing ship was investigated and a generalized model of the response developed. The effects of a controllable side force on the towed vehicle were also simulated and it was noted that a deviation (2-hr period) of the towed vehicle from a straight-line track could be reduced from 40 to 2 m by impressing a side force on the vehicle with an average magnitude of 150 newtons (30 lb).

Over/under acquisition and data processing: the next quantum leap in seismic technology?

David Hill, Leendert Combee, and John Bacon, WesternGeco, introduce a new configuration for towed-streamer seismic data acquisition. They argue that the ‘over/under’ technique is a major advance on conventional techniques providing previously unattained signal bandwidth, where the low-frequency content gives deeper penetration, and therefore, improved imaging beneath basalt, salt, and other highly absorptive overburdens. In a conventional towed-streamer marine acquisition configuration, shallow sources and shallow cables increase the high-frequency content of the seismic data needed for resolution. However, shallow sources and shallow cables attenuate the low frequencies, which are necessary for stratigraphic and structural inversion, and for imaging deep objectives. Towing shallow also makes the data more susceptible to environmental noise. In contrast deep sources and deep cables enhance the low frequencies, but attenuate the high frequencies. In addition, the data recorded via a deep tow have a higher signal-to-ambient-noise ratio due to the more benign towing environment. A conventional towed-streamer survey design therefore, attempts to balance these conflicting aspects to arrive at a tow depth for the sources and cables that optimizes the bandwidth and signal-to-noise ratio of the data for a specific target depth or two-way travel time, often at the expense of other shallower or deeper objectives. An over/under, towed-streamer configuration is a method of acquiring seismic data where cables are typically towed in pairs at two different cable depths, with one cable vertically above the other. The depths of these paired cables are typically significantly deeper than would be used for a conventional towed-streamer configuration. In conjunction with these paired cables, it is possible to acquire data with paired sources at two differing source depths. Again, the depths of these paired sources are typically significantly deeper than would be used for a conventional towed-streamer configuration. The seismic data recorded by the over/under towed-streamer configuration are combined in data processing into a single dataset that has the high-frequency characteristics of conventional data recorded at a shallow towing depth and the lowfrequency characteristics of conventional data recorded at a deeper towing depth. This combination process is commonly referred to in the geophysical literature as deghosting. The current benefits of over/under data compared with conventional data can be summarized as: - A significantly broader signal bandwidth, where the lowfrequency content gives deeper penetration, and therefore, improved imaging beneath basalt, salt, and other highly absorptive overburdens. Moreover, the bandwidth extension to lower frequencies makes seismic inversion less dependent upon model-based methods - A simpler signal wavelet with the bandwidth extension to higher frequencies giving enhanced resolving power, allowing for a more detailed stratigraphic interpretation - Higher signal-to-ambient-noise ratio as a consequence of the deeper towed-cable pairs - An extended weather window enabled by the deeper towed-cable pairs.

An opening and closing mechanism for the high-speed plankton sampler HAI

A simple opening and closing device is described for the HAI, a modified Gulf III High-speed Plankton Sampler. To open the sample at a selected towing depth, a messenger-weight actuates a mechanism which permits a lid to be drawn away from the mouth of the sampler by a rubber spring. To close the sampler, a second messenger causes the mechanism to let the lid swing back down over the mouth.