Continuous subbottom reflection profiling is a development of echosounding technology. Research using recording echosounders had indicated that some penetration of sediments was occurring, even at relatively high sound frequencies (e.g. Rust, 1935; Stocks, 1935; Mortimer and Worthington, 1940 (see section 2.2.2 above)) and the value of subbottom profiling equipment was therefore beginning to be recognised:-
'The echosounding machine provides a means of measuring the result of subaqueous deposition over long periods of time' (Mortimer and Worthington, 1940).
Experimentation with lower frequency sub-surface profilers was carried out during the 1950s, one of the earliest applications of the technique being the tracing of a widespread ash layer in the eastern Pacific with a 12kHz echosounder (Worzel, 1959). Hersey (1963) carried out pioneering research with subbottom profilers and the technique became widely used due to its essential simplicity, adaptability and ease of interpretation (Leenhardt, 1969). A range of subbottom profilers has been produced including pingers, sparkers, boomers and air-guns allowing profiles to be obtained in all subaqueous environments. Low frequency echosounders, such as the Kelvin Hughes MS26, operating at 26kHz have achieved penetration of up to 6m into the sediments of Loch Benachally, Perthshire, in late spring(Al-Bayati and McManus, 1984).
Further adaptations to the general principle continue to be made; the Atlas Parasound system is a combination of a narrow beam deep sea survey echosounder with a subbottom profiling capability. It utilises two signals at adjacent frequencies to produce a low frequency sound pulse with a very narrow beam virtually free of side lobes. This results in greatly improved vertical resolution of up to 30cm even in deep sea sediments (see below, section 3.2.2).
3.2.2 Principles of continuous subbottom profiling
Continuous subbottom reflection profiling provides a relatively cheap and rapid graphic display of sub-surface acoustic impedance interfaces, analogous to a geological section of the area surveyed (Reynolds, 1990). The technique utilises lower frequency sound (in the range 2kHz to 300Hz, depending on the system) than echosounders resulting in the penetration of the underlying substrate.
A pulse of sound is initiated that moves through the water column as a compressional wave and is reflected at contrasts in acoustic impedance (see section 2.2.2 above). The main pulse splits when it reaches the first interface of contrasting acoustic impedance and part of the pulse is reflected directly to the transducer. The remainder is transmitted through the substrate and continues to be reflected at further impedance
contrasts, resulting in a progressively weakening reflected signal. The amount of penetration achieved is dependent upon the frequency of sound emitted by the profiler. The rate at which the sound is attenuated within the substrate depends on the 'Quality Factor' (Q) which describes the quality of a rock as a sound transmitter (McQuillin and Ardus, 1977) (see below, this section). Q is inversely proportional to the absorption (Z). The relationship between these two parameters is formulated:
27.3 Z =---dB
Q where Z = the absorption of sound (dB)
Q = the rock quality factor
Figure 3.1 shows the relationship between the frequency of the sound source and the depth of penetration. Higher frequency systems such as Pinger (3.5 kHz) provide shallow penetration (< 30m) high resolution output. Low frequency systems including boomers operate at frequencies of 14-300Hz, penetrating to 100m, but with reduced resolution. Thus it is especially important to determine the type of information required by a research programme prior to survey commencement.
Resolution of all records is dependent on the outgoing pulse repetition rate, the speed of the vessel, and the depth of the reflecting surface. This is due to the received reflected pulse representing only a small part of the area covered by the acoustic pulse. This area is termed the first Fresnel zone. A high pulse repetition rate and slow vessel speed increase the overlap of Fresnel zones and consequently the resolution of the record. Vertical resolution is also dependent on the pulse repetition rate. If the pulse generated is long, the incoming signals from depth will be masked by the next outward pulse. Therefore, to identify fine stratification in the underlying sediment, a high pulse rate must be used. The maximum vertical resolution that can be obtained is where the distance between the upper
and lower bounding surfaces of a unit is greater than or equal to one eighth of the central frequency of the acoustic pulse (Widess, 1973). This thickness is known as the critical resolution thickness.
Propagation velocities down the profile vary according to several factors that cause contrasts in acoustic impedance; sediment porosity, degree of compaction, grain size, grain orientation, degree of cementation, nature of grain to grain contact and moisture content (Palmer, 1967; Van Overeem, 1978). Typical propagation velocities of selected rock types are given in Table 3.1. This illustrates that dense, more rigid, substrates transmit compressional wave energy most efficiently. Minor changes in porosity significantly change the propagation velocity of a rock.
Table 3.1
Propagation velocities of selected rock types Propagation velocities ms-1 Recent estuarine sands and silts 1530- 1600 (D'Olier, 1979)
Glacial moraine 1600-2700 ( McQuillin and Ardus, 1977) Limestone 3500-6500 ( McQuillin and Ardus, 1977) Granite 4600-7000 (McQuillin and Ardus, 1977)
Occasionally penetration by the acoustic pulse is not achieved resulting in 'acoustic blanking' or turbidity. Research involving the coring of acoustically turbid sediments has revealed that this phenomenon is often caused by high levels of gas produced during the decay of buried organic material (Schubel and Scheimer, 1973; Muller, 1976; Jones et al., 1986). Gas usually occurs as small bubbles within the sediment which causes attenuation of sound waves and prevents further penetration (Van Overeem, 1978). The production of methane hydrates during the decay of organic matter can be the cause of reflection surfaces unrelated to sedimentary bedding planes (Manley and Flood, 1989). Such reflectors may be identified by their parallelism with the surface reflector and tendency to cut across bedding planes.
Seismic records may also contain side reflections produced when reflections are received from a target outside the Fresnel zone. This occurs if the target is orientated directly towards the transducer and appears as a hyperbola as signals are received before, during and after the transducer has passed. Hyperbolae are also produced on a smaller scale within the sediment by reflections from uneven surfaces or individual boulders (point reflectors).
Multiple reflections occur when pail of the reflected wave passes the receiving transducer, is reflected by the water surface to the bottom and back to the transducer. This results in a second reflection appearing at twice the depth of the first, and at twice the amplitude. Multiples can have the effect of masking detail at depth on the profile, and in very shallow water may occur up to six times. The amplified detail on these reflections can be of value. Removal of multiple reflections can be achieved by the employment of seismic systems that record data on magnetic tape which can then be replayed and the signal filtered.
Verification of the presence and nature of reflection surfaces indicated on a seismogram is thus important. This may be carried out in several ways. Cores may be taken from the survey area and samples of material analysed by ultrasonic impulse whereby a high frequency sound pulse is passed through a sample to measure the attenuation of sound (Mayer, 1979). The bulk density and gas content of core samples are also measured. Problems associated with this method are the disturbance caused during coring, extraction and sampling, especially in recent, unconsolidated sediments. Wire line logs of seismic velocities can be taken in consolidated sediments, or a seismic survey line can be made to coincide with a previously cored site.
Pinger seismic subbottom reflection profiling apparatus comprises a hull mounted or towed, single channel transducer from which low frequency pulses of sound are emitted. This transducer also receives the incoming pulse. Other systems require the towing of hydrophone arrays for signal reception. The reflected signal is transmitted to a recorder where time variable gain is applied to the weaker subbottom signals. Time variable gain intensifies the signals received from the most distant parts of the Pinger range to enable
accurate comparison between signal intensities across the record. The data are displayed on electrosensitive paper by the passing of an electric current through the paper via a continuously moving electrode band. Varying signal intensity is indicated through a proportional 'darkening' or 'lightening' on the chart record.
3.3 SEISMIC PROFILE INTERPRETATION: METHODOLOGY AND