The first step in constructing a geoacoustic model is to assemble information on the physical and elastic properties and thickness of the sediments and rocks in the study area. Rarely is enough information available, so recourse is made to predictive methods, generally those of Hamilton (e.g., 1980). These techniques are discussed in appendix A.
There are no drill holes in Catalina Basin, so what is known and believed about the geology is based on samples of surface materials (sediment cores and rock dredge hauls), studies of Catalina and San Clemente Islands, and seismic reflection profiles. Seismic reflection profiling uses a high-energy, low-frequency sound source to image the sea floor and buried reflecting horizons. Figure 2 shows a seismic reflection record from Catalina Basin. Depth to the sea floor and buried reflectors is measured in terms of two-way sound travel (reflection) time.
Basin-fill consists of soft and semiconsolidated turbidite sediment over sedimentary rock (mudstone and shale). The contact between the turbidites and sedimentary rock is an unconformity (a surface representing a period of nondeposition or erosion). The island ridges are sedimentary, volcanic, and intrusive igneous rock (e.g., granite). The ridges and their flanks are either barren of sediment or have a thin veneer of unconsolidated, relatively coarse material. Figure 3 is a surface geologic map of the Catalina Basin area.
A gridding approach was adopted for organizing the data. The area bounded by 32_50iN, 33_35iN, 118_W, and 119_W was divided into rectangular grid cells, each cell being 15 arc-
seconds on a side. The cells are centered on 7.5, 22.5, 37.5, and 52.5 seconds of latitude and longitude. This results in a grid of 240 cells in the east-west direction by 180 cells in the north-south direction. This became the framework for a geographic database containing cell indices, true water depth, sediment thickness, basement rock type, sea floor rock type (if present), sea floor sediment name (when used to estimate sediment properties), and surface sediment mean grain size. The resulting database of water depth, sediment thickness, and surface geology is used along with generic geoacoustic models discussed below to construct a model specific to each grid cell.
The organization of the geographic database is discussed in appendix B. Sediment thickness is discussed in appendix C, water depth in appendix D, and mean grain size in appendix E. Appendix F gives literature data on sediment and rock samples. Appendix G gives temperature, salinity, and sound speed data useful for constructing a geoacoustic model.
Figure 1. Catalina Basin and environs. Depth contours in meters from U.S. Coast and Geodetic Survey chart 1206N-15. Latitude limits of the database and figure are 32_50iN and 33_35iN; longitude limits are 118_W and 119_W.
Figure 2. Seismic reflection profile running northwest (to the left) through Catalina Basin. From Moore, 1969, Plate 5. The interpretation is Moore's. "A" points to
older, folded sedimentary rocks. "B" points to slightly folded sediments. "C"
points to recent sediment. "C" also marks a buried submarine channel, as does
(probably) "D". The vertical scales are seconds of two-way sound travel time and
fathoms (assuming a sea water sound speed of 4800 feet per second).
Figure 3. Geologic map of the Catalina Basin, generalized from Greene and
Kennedy (1986). In the explanation above, rock units are arranged from youn-
gest at the top to oldest at the bottom.
The functions for fine-grained sediment are valid to 950 m below the sea floor. This is equivalent to a two-way sound travel time of 1 s, the greatest thickness of unlithified sediment encountered in the basin. Coarse-grained sediment functions are valid to about 50 m at least; for the present they must be extrapolated to accommodate anomalous areas of thick sand.
where Vp is in m/s and Z is depth below the sea floor in m (Hamilton, 1985, table II). Vp(0) is the product of the sound speed ratio R and sound speed in the bottom water. R is computed using mean grain size from the database. The utility of, and rational for, the ratio R is discussed in appendix A.
where rho is in g/cm3, Z is depth below the sea floor in m, and Mz is mean grain size in phi units. Equation 3 is from Bachman (1985, table I), and equation 4 is from Hamilton (1976a, table 5).
In these equations (derived from Hamilton, 1979, figures 1 and 2), Z is depth below the sea floor in m, Vp is sound speed at Z, and Vs is shear speed in m/s.
where kp is in dB/m/kHz, and Z is in m. This is the mean of the curves in Mitchell and Focke (1980, figure 11). To obtain attenuation in dB per meter of travel, multiply kp by frequency in kHz.
where ks is in dB/kHz/m, kp is compressional attenuation, and 17.3 dB/m/kHz is a shear attenuation value for mud published by Warrick (1974). The method follows Hamilton (1980, p. 1331- 1332). To obtain attenuation in dB per meter of travel, multiply ks by frequency in kHz.
Z is depth in the sediment in meters. Vp(0) is computed as above (equation 1 and text). The constant K is evaluated (equation 14) by assuming that a surface sediment sound speed is measured at a depth of 0.05 m (see Hamilton, 1975, p. 24).
(see the remarks for equation 2 above).
Z is depth in the sediment in meters. The constant K is evaluated (equation 17) by again assuming that surface sediment shear speed is at a depth of 0.05 m. Shear speed at the sea floor (equation 18) follows Hamilton (1979, table II).
The variation of sound attenuation with depth is
Equations 19 through 22 are from Hamilton (1980, figure 19, p. 1330).
1. Obtain the water depth, surface sediment grain size, and sediment thickness for the desired location from the database. If the sea floor is rock, then select the appropriate model above and skip the remainder of this section.
2. Using water depth, interpolate the appropriate table in appendix G to find the bottom water sound speed and density.
3. Compute sound speed ratio (R) from mean grain size using equation 1.
4. Determine Vp(0) as the product of bottom water sound speed and the sound speed ratio R.
5. Knowing the velocity-depth function (equation 2 or 13) and sediment thickness in seconds of two-way travel time, integrate to find sediment thickness in meters.
6. Compute acoustic properties below the sea floor.
For example, assume that a winter-season model is required for 33_12i37.5iiN and 118_35i 07.5iiW. From the database, the depth at this location is 1303 m, sediment thickness is 0.20 s, and mean grain size is 6.39 phi.
Linearly interpolating the winter profile of appendix G, we obtain 1485.0 m/s and 1.0335 g/cm3 for seawater sound speed and density at this depth.
The sound speed ratio (R) computed using equation 1 is 1.028. Multiplying R by 1485.0 m/s yields 1526 m/s as sediment sound speed at the sea floor. The sound speed depth function (equation 2) is
Integrating until two-way travel time is 0.20 s yields a thickness of 162 m. Equations 3 through 12 yield density, shear speed, and attenuations. Table 1 is an example geoacoustic model for the situation described above.
Table 1. Example geoacoustic model.
NOTES:
Z = depth below sea floor, m
Vp = sound speed, m/s
Vs = shear wave speed, m/s
kp = compressional wave attenuation factor, dB/m/kHz
ks = shear wave attenuation factor, dB/m/kHz
rho = density, g/cm3
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