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Lake Belt Study Area: High-Resolution Seismic Reflection Survey, Miami-Dade County Florida
Seismic reflection data is used to image and map sedimentary and structural features of the seafloor and subsurface. These data are useful in mapping the extent of the subsurface structure, sediment thickness, and depths to various stratigraphic horizons, as well as in assessing other submarine and subsurface geologic characteristics and features. These data were collected as part of the CERP project done in cooperation with the South Florida Water Management District. Seismic reflection profiles are acquired by means of an acoustic source (usually generated electronically) and a hydrophone or hydrophone array. Both elements are typically towed in the water behind a survey vessel (Fig. 9). The sound source emits a short acoustic pulse, which propagates through the water and sediment columns. The acoustic energy is reflected at density boundaries (such as the seafloor or sediment layers beneath the seafloor) and detected at the hydrophone. This process is repeated at intervals ranging between 100 milliseconds (ms) and 1 second (s) depending on the source type. In this way, a two-dimensional image of the geologic structure beneath the ship track is constructed.
Boomer and Navigational Data Acquisition
To collect the seismic profiles for this study a TritonElics Delph High-Resolution Seismic Profile System (HRS) was used with proprietary hardware and software running in real time on a BSI Portable PC, Win98 OS. Digital data were stored on internal hard disk and transferred to compact disk (CD-ROM). The acoustic source was an electromechanical device, a GeoPulse Model 5420A Power Supply firing an Applied Acoustics AA300 Boomer Plate mounted on a catamaran sled (Fig. 9).
Each field excursion was given a unique field activity number that included a two digit year identifier (02ASR01), a three digit activity, project, or program identifier (02ASR01), and a two digit 'Cruise Leg' number (02ASR01). Under each activity are individual geophysical line numbers including a two digit year identifier (02b02), acquisition tool (b for Boomer, 02b02), and two digit line number (02b02).
Field Activity 01ASR01, the seismic source employed consisted of a boomer transducer providing 100 joules per shot. The reflected energy was received by the NexGen hydrophone streamer and recorded by PC-based TritonElics Delph Seismic acquisition software. The streamer contains 10 hydrophones evenly spaced every 2 m. Only data received by elements 7 and 8 were summed for line 01b01 and for line 01b02 through shot number 2,819 (Fig. 8 for locations). Afterward, only data received by elements 8 and 9 were summed. The streamer was positioned parallel to the boomer sled and laterally separated from it by approximately 3 m (Fig. 9). The sled was towed approximately 5 m behind the GPS antenna. The sample frequency of the data was 12 kHz and the total record length was 100 ms. The Boomer firing rate was every 0.5 sec, which resulted in a shot spacing of about 0.64m.
Field Activity 01ASR02, the seismic source employed consisted of a boomer transducer providing 280 joules per shot. Only data received by elements 8 and 9 where summed for line 01b01 through shot number 8,903 (Fig. 8 for locations). Afterward, data received by element 10 was also summed. The streamer was positioned parallel to the boomer sled and laterally separated from it by approximately 3 m. The sled was towed approximately 5 m behind the GPS antenna.
Field Activity 02ASR01, the seismic source employed consisted of a boomer transducer providing 280 joules per shot. Only data received by elements 8, 9, and 10 were summed for line 02b01 and for line 02b02 through shot number 1,748. Only data received by elements 5, 6, and 7 were summed for line 02b02 between shot numbers 1,750 and 2,828. For the rest of line 02b02 and for all other lines, only data received by elements 4, 5, and 6 were summed. The streamer was positioned parallel to the boomer sled and laterally separated from it by approximately 3 m. The sled was towed approximately 5 m behind the GPS antenna. The sample frequency of the data was 12 kHz for line 02b01 and 24 kHz for all other lines.
Field Activity 02ASR02, the seismic source employed consisted of a boomer transducer providing 280 joules per shot. Only data received by elements 3 and 4 were summed. This resulted in a higher signal to noise ratio for the data. The streamer was positioned parallel to the boomer sled and laterally separated from it by approximately 3.5 m. The sled was towed approximately 5.5 m behind the GPS antenna through shot number 8,230 of line 02b01, and approximately 7.5 m behind the antenna for the rest of the line. The sample frequency of the data was 24 KHz and the total record length was 100 ms.
The raw SEG-Y data was processed using Seismic Unix (SU) to produce the GIF formatted seismic profiles included in this report. A representative data processing sequence consisted of: 1) bandpass filter: 300-500-2500-3000 Hz, 2) automatic gain control, 3) postscript display at 15 ms/in and 215 shots/in., and 4) convert postscripts to GIF format. These data are included (inset) as Appendix A.
The TritonElics Delph Geophysical system measures and displays two-way travel time (TWT) of the acoustical pulse in milliseconds (ms). Amplitude and velocity of the signal are affected by variations in lithology of the underlying strata. Laterally consistent amplitude changes (lithologic contacts or correlations of acoustic impedance in similar lithologies) are displayed as continuous reflections on the seismic profiles. Depth to reflection is determined from the TWT, adjusted to the subsurface velocity of the signal. Carbonates have a wide range of velocities such as those reported by refraction studies conducted in areas within Alachua County, Florida (Weiner, 1982) yielded velocities of 1707 to 4939 m/s (5599 to 16,200 ft/s) for the Hawthorn Group sediments. Weiner (1982) reported lower velocities for the sand and clay sediments and higher velocities for the carbonate sediments. Suggested compressional velocities for Hawthorn Group sediments for the Florida Platform range from 1500 to 1800 meters per second - m/s (4920 to 5904 feet per second - ft/s; Tihansky, pers. comm.; Sacks and others, 1991, Kindinger and others, 1997, 1999). Due to the vertical variability of the Lake Belt Area geology (with alternating layers of carbonates, sand and silt) within this report (Cunningham. pers. comm.), we will use a mid-range velocity TWT of 2000 m/s (6560 ft/s) that is an average velocity for the Key Largo Li (Anselmetti and others, 1997).
More than 110 line-km (68 line-mi) of data were collected from 8 major canals plus the canal adjacent to the ECPL (Fig. 8). Quality of profile data varied between good to moderate and poor depending on numerous variations in canal structure and lithology (Table 1). These data were integrated with information from reports, published and unpublished core sections, original core descriptions from the SFWMD files, and personal communication with other researchers familiar with the study area.
Time to Depth Conversions
Time to depth conversions is a two-step process. Step 1 involves the conversion of the canal water column from time to a datum (depth in m/ft below sea level). A standard datum is necessary for the comparison of seismic profiles to cores. This standardization was accomplished using a velocity of 1500 m/s (4920 ft/s) as a general speed of sound through sea water (due to the resolution of the boomer data [1 m, 3.3 ft], higher resolution frequencies fall below the resolution of the data). This provided a scale by which to measure water depth, then subtracting 1.52 m (5 ft) for a standardized datum to sea level. For example in Figure 10, the standardization shows that the bottom of the C-9 canal is 2.1 m (7 ft) below sea level. Beginning at the canal bottom a velocity of 2000 m/s (6560 ft/sec) is used (general velocity of sound through variable carbonate units, see discussion above).
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 15 January, 2013 @ 12:43 PM (KP)