Continuous seismic-reflection profiling (CSP), also known as Subbottom Profiling, transmits seismic energy from a transducer located just beneath the water surface. In bridge scour evaluations, the method can provide an essentially continuous image of the stream channel and the sub-bottom sediment, including the in-filled scour features.
Basic Concept: In the CSP method, a transducer element is used to generate seismic energy that passes through the water column and into the subbottom. Where a change in acoustic impedance occurs, such as at the water/bottom interface, part of the seismic energy is reflected back to the water surface. The remainder of the seismic energy is transmitted further into the material. The amount of energy that is reflected by an interface is determined by the reflection coefficient of that interface, which is dependent upon the acoustic impedance of the material above and below the interface. Acoustic impedance is defined as the product of the density of a material and the velocity of sound through the material.
Figure 59 shows the acoustic waves and their travel paths for two geophones. An acoustic impedance contrast exists between layers 1 (water) and 2 (bedrock), and between layers 2 and 3. At the boundary between layers 1 and 2, part of the seismic energy is reflected and part is transmitted and continues through layer 2 until it reaches the impedance contrast between layers 2 and 3. At this interface, part of the wave is again reflected and part is transmitted and continues into layer 3 (not shown). The two traces on the right hand side of the layer picture show the signals arriving at the two geophones. In conventional land seismic reflection surveys, many geophones are used that simultaneously record the reflected seismic waves. In addition, many shots are fired as the survey progresses along a line, thus building a series of seismic records that are later processed and assembled to form a seismic section.
A similar procedure is used for a CSP survey on water. However, in this case, the energy source automatically produces "shots" at regular intervals while being towed by a boat at a constant speed. The transducer that produces the "shots" is positioned just beneath the water surface. Single or multiple hydrophones record the reflected data. The energy passes through the water column and into the subbottom and is reflected at interfaces with an impedance contrast, the first of which is the water bottom.
The frequency of the seismic signal used by the CSP technique determines the maximum penetration depth and resolution. A high-frequency signal has a short wavelength and is attenuated by subbottom materials but provides high resolution of subbottom interfaces. A low-frequency signal has a longer wavelength and is attenuated less by subbottom materials but provides lower resolution. The fixed-frequency CSP technique uses a narrow bandwidth fixed-frequency signal usually centered at 3.5, 7.0, or 14 kHz. The CSP technique using a 14 kHz signal can be used in water as shallow as 1.2m, can penetrate up to 6 m into the subbottom in certain materials, and can detect fill material as thin as 30 cm in a scour hole. The CSP technique using a 3.5 kHz signal can be used in water as shallow as 2 m, can penetrate up to 30m into the subbottom in certain materials, and can detect fill material as thin as 75 cm in a scour hole. Swept-frequency (chirp) CSP techniques that sweep from 2 to 16 kHz can be used in water as shallow as 30 cm, and can penetrate 60 m into the subbottom in silts and clays. Such signals can sometimes detect fill material as thin as 8 cm in a scour hole.
The frequency of the seismic signal used also determines the size and weight of the transducer required to generate the signal. Generally, transducers that generate low-frequency signals are larger, heavier, and have wider beam angles than transducers that generate high-frequency signals. The beam angle of a transducer or antenna is the area beneath the device that contains most of the energy of the signal. A wide beam angle allows energy to be reflected by objects alongside as well as under the transducer such as the shoreline or the sides of piers. Echoes from the shoreline or from piers (side echoes) can interfere with the data.
Another source of interference in geophysical data is caused by multiple reflections. When a signal travels from the transducer to the water bottom, it is reflected by the bottom. When the signal returns to the surface it can be reflected at the air/water interface and transmitted back to the bottom. It is reflected by the bottom again and is received at the transducer a second time. This is called a second arrival or a water-bottom multiple. The energy may continue to ring in the water column causing more than one multiple reflection. Water-bottom multiples are most evident when the water-bottom reflection coefficient is large, as in rivers with hard bottoms.
Data Acquisition: CSP surveys are conducted over water using a boat with the equipment attached as described earlier. There are two types of CSP systems. One is a fixed-frequency system, and the other is a swept-frequency system. Generally, a minimum water depth is needed to reduce reverberations from the water bottom "ringing" to acceptable levels. Often the water depth needs to be greater than a few feet deep. This interface is a strong reflector and produces high amplitude reflections. GPS can be used to position the data.
Data Processing: Often, unprocessed data contain sufficient information to satisfy the goals of many surveys. However, digital signal processing can be done and frequently improves the data making it more interpretable. Most of the processing routines are very similar to those utilized in the reflection seismic method used in hydrocarbon exploration. Processing can involve position data to correct for different speeds of the boat and to produce a geophysical profile with a horizontally corrected scale. Bandpass filtering can be applied along with deconvolution processes. Migration algorithms can be applied to the data correcting the geometry of the bottom of sinkholes and other features.
Data Interpretation: Interpretation of CSP data is mostly visual, looking for features of interest and locating reflections from known features. Two CSP sections are presented, one from a single- frequency system (figure 60) and the other from a swept-frequency system (figure 61).
Figure 60. Continuous Seismic Profiling data recorded with a 14.4- kHz transducer. Data have been filtered, spatially corrected, and deconvolved. (Placzek, et al. 1995, USGS Report 95-4009)
Figure 61. Continuous Seismic Profiling swept frequency data using 2-16 kHz transducer.(Placzek, et al. 1995, USGS Report 95-4009)
Limitations: The water bottom interface is sometimes a very strong reflector and can limit the amount of energy being transmitted deeper. In addition, the reflected signal from the water bottom travels back to the water surface where it is reflected back to the water bottom again. This process may be repeated several times creating reverberations, often called multiples, which make the seismic record more difficult to interpret.
A very strong water bottom reflector makes the deeper reflectors faint and possibly unobservable, since little energy is available to be reflected from these deeper layers. Seismic signals are scattered by air bubbles, and the CSP method is not used in settings with gassing organic materials.
Generally, water bottom materials that are coarse-grained have a greater reflection coefficient than fine-grained sediments. Because of this, the depth of penetration of the method is less when coarse-grained sediments form the water bottom surface.