Executive Summary Introduction Study Area & Objective Methods >Results Summary & Recommendations Acknowledgements & References Statement of Work Appendix A Appendix B PDF Version

Since 1993, the USGS (in cooperation with Florida water management districts) has been developing and redesigning marine seismic acquisition and processing techniques to be used in restricted freshwater lake and canal environments (Kindinger and others, 1997, 1999, 2000). Acquiring continuous, high-resolution, single-channel seismic profiles from shallow freshwater lakes and canals has had varying degrees of success. In most cases, it has been very useful in combination with core descriptions, gamma logs and other data in describing the shallow geology (0 to 400 m, 0 to 1300 ft). Geologic characterization is in turn used to identify conduits that may provide a mechanism for surficial and aquifer waters to mix. The technique has also had limited success in identifying subsurface structural anomalies, such as fractures and subsidence features that may inhibit or provide differential ground-water flow.

Success of these techniques is dependent on the lithology beneath each lake or canal, and availability of described cores. The core descriptions used in this report were from cores that were collected adjacent to the canals, thus projected a short distance directly into the canals where the seismic data was collected. The effective resolution of these data is approximately one meter (3.28 ft) but varies due to acoustic velocity differences with changes in lithology and depth within the profile. Canal structure (such as width and depth) along with the presence of 'biogenic gas' (from organic-rich sediment) also effects the quality and penetration of the acoustic signal.

Acquisition results from this survey had varying degrees of success (Table 1). The canals with good to moderate quality profiles were C-9 (Fig. 10, 11, 12), C-6 (Fig. 13, 14, 15), ECPL canal (Fig. 16, 17, 18, 19, 20, 21, 22), and Black Creek Canal (Fig. 23). Data from the Wellfield Recharge Canal (Fig. 24) was moderate to poor with the rest of the canals being poor. Considering all seismic data approximately 80 per cent (84 km, 52 mi) of the canals provided usable data. The canals that did not have usable data include the canals along Tamiami Trail (C-4), Bird Drive, and the Florida Turnpike (Snapper Creek Canal extension) (Fig.8). Typically during acquisition there were several profiles collected within each canal sometimes due to obstructions such as control structures or low bridges for roads. Profiles from each canal have been combined to provide continuous interpretations of each canal and to facilitate discussion and figure production. Depth of interpretations was limited by the acoustic penetration. Depth of acoustic penetration varied greatly from 3 to 80 m (10 to 260 ft), but generally there was usable data to 30 m (100 ft). Hard carbonate contacts (identified from core descriptions, USACE Ser. 20 (1953) and Switanek (in press) are prevalent in the upper section of many profiles causing multiples and acoustic ringing that masked the underlying data signal. Good interpretations were commonly made to >0.02 sec TWT (50 ft at velocity of 6560ft/sec). Less confident picks were made to depths of 0.05 to 0.06 sec (150 to 200 ft at velocity of 6560ft/sec). Multiples and acoustic noise prohibited clear recognition of deeper geologic features in much of the data. In figures, profiles may have been reversed from the original direction of collection to provide a continuous image of the canal.

In all canals surveyed the Holocene sediment and Pleistocene Miami Limestone were removed during canal construction. The bottom of most canals has an acoustically transparent layer of 1 to 3m (3 to 10 ft) thick of undifferentiated organic-rich sediment. The first solid reflection (green in all figures) is the original surface left from canal construction within the Fort Thompson Formation (i.e. Fig. 11) that is typically composed of marine limestone, minor gastropod-rich freshwater limestone, or sandy limestone (Fig. 6). The Fort Thompson Formation is a vuggy, good to poor induration (hard to weak) limestone and with horizontally moderate variation. The rock-fabric facies within the Fort Thompson Formation stratigraphic cycles is moderately variable, but is conformable around much of the Lake Belt study area. Thickness of this unit varies from a ~1 to ~25 m (~3 to ~80 ft) (Fig. 5). Beneath the Fort Thompson Formation is irregular alternating layers of sand, silt and limestone of the Tamiami Formation (Fig. 11).

The seismic profiles shown in this report have identifiable reflective horizons that may or may not be recognized in corresponding core descriptions. This is controlled by several factors: (1) inexact velocity predictions for specific units (2000m/sec or 6560ft/sec TWT, was used across the section or profile), (2) vicinity of borehole location to seismic line, (3) seismic reflections on profiles are created by density changes not always represented by lithologic changes, or other variations that affect correlation, and (4) the extent of inferred fractures or subsidence features (collapse structures) are not detected in borehole surveys. Reflections indicated on the profiles by color highlights represent inferred features such as solution pipes, vuggy limestone, fractures or displacement structures (possibly collapse). These profile sections are best used to indicate trends and changes in the geology associated with porosity, and rock hardness that may affect water flow within the surficial aquifer.

Canal C-9

Canal C-9 is a west to east canal in the northern most portion of the study area (Fig. 2, 8). Figure 10 is a composite of four seismic profiles (Lines 01ASR01-01b01, -01b02, -01b03, and 01ASR02 01b05). There are several continuous reflections in the upper 15 m (50 ft) of the section that may represent the first contact of native rock below the 'muck' acoustically transparent organic-rich sediment (green) and a contact between a good and moderate induration limestone within the Fort Thompson Formation (Fig. 10, 11, 12). These interpretations are best used as inferred features (i.e. Inferred Collapse?) to indicate trends and changes in the geology associated with porosity, and rock hardness. Numerous features that may be shallow solution pipes or vugs (blue highlights) that infrequently penetrate through more than one horizon and may be conduits for vertical water flow through the formation. In Figures 11 and 12 Cores CB-xx-4, -5, and -3, respectively, are overlain on seismic profiles 01ASR01 01b01 at Shots 3500 to 5100 and 7300 to 9000, showing the upper limestone unit (Fort Thompson Formation) underlain by variable sand, silt and limestone lithology (Tamiami Formation). The top of the Tamiami Formation as identified from core descriptions is found between 16 to 29 m (55 to 95 ft) (Fig. 10). There are numerous features (red highlight) that may represent solution pipes and collapse structures found in the upper 30 m (100 ft) primarily in the limestone units. The seismic character at Line 01b01 Shot 6500 (Fig. 10) represents the most prevalent of what may be a collapse structure that penetrates to more than 45 m (150 ft). Throughout much of the canal the hard limestone (good induration) has reflected the acoustic signal causing multiples in much of the deeper section. The core data agree with the seismic data in that they reflect significant lateral variability in lithology of the study area (Fig. 11).

Figures 11 and 12 include closely spaced cores (CB-xx-4 and -5, -3, respectively) overlain on the seismic profile. These core descriptions clearly indicate the lateral and vertical variability of the geology within the study area. In the upper portion of the section continuous reflective horizons seem to generally correlate lithologic changes in the section. The contact between the Fort Thompson and Tamiami Formations identified in core descriptions is masked by multiples. The upper section of the profile demonstrates a consistent seismic character showing what appear to be numerous inferred dissolution features that may be fractured or displaced material, solution pipes or vugs. These produce similar seismic characteristics in profile that are indistinguishable. These acoustic patterns are commonly seen in similar karst environments (Kindinger and others, 1999, 2000).

Figure 11. Seismic profile overlain with two core descriptions (CB-xx-4 and CB-xx-5). In the upper section, continuous reflective horizons generally correlate with changes in lithology. There is an apparent dissolution feature between Shots 8100 to 8700, that may represent vugs or displacement caused by collapse. Acoustic multiples (artifact of acquisition) mask the Fort Thompson and Tamiami Formations contact identified from core descriptions. Dashed line indicates contact between formations as identified from core descriptions. Core descriptions were provided by SFWMD (see Appendix B). See Figure 10 for relative placement on seismic section and Figure 8 for map location. [larger image]

Figure 12. Seismic profile overlain with one core description (CB-xx-3). Continuous reflective horizons generally correlate with changes in lithology or induration. There is an apparent dissolution feature between Shots 4000 to 4600, that may represent vugs or displacement caused by collapse. Acoustic multiples (artifact of acquisition) mask the Fort Thompson and Tamiami Formations contact identified from core descriptions. Dashed line indicates possible contact between formations as identified from core descriptions. Core descriptions were provided by SFWMD (see Appendix B). See Figure 10 for relative placement on seismic section and Figure 8 for map location. [larger image]

Canal C-6 (Miami Canal)

C-6 Canal, also known as the Miami Canal, is in the northwestern part of the study area and cuts across from northwest to southeast (Fig. 2, 8). Figure 13 is a composite of two seismic profiles (Lines 01ASR01-01b04 and -01b05). Unlike C-9, the seismic profiles do not show flat continuous reflections in the upper 15 m (50 ft) of the section, instead there is an irregular but fairly continuous reflection (dark blue highlight). Evidence from core descriptions indicates this reflection is produced by a lithologic contact within the Fort Thompson Formation between a weak limestone (poor induration) above and a moderate to hard limestone (good to moderate induration) below (Fig. 13, 14, 15). Above the contact there are numerous discontinuous reflections and features that may be shallow solution pipes or vugs (blue highlight). The dashed line may represent the top of the Tamiami Formation a sand facies below the limestone units of the Fort Thompson Formation (Fig 14). The contact between the two formations cannot be clearly distinguished in the seismic profile and is inferred. At Line 01ASR01 01b04 Shots 4900, 8200, and 13000 to 13500 are examples of possible collapse or dissolution features, all penetrate deeper than 38 m (125 ft) (Fig. 13, 15). In the deeper portions of the seismic section acoustic noise has masked any coherent signal.

The seismic character of the rock units beneath the C-6 Canal are very different from the C-9 Canal. The C-6 has the irregular reflection (dark blue line) representing a much different lithology, possibly allowing better penetration and return of the acoustic signal. More lateral reflective features are seen in this section that probably represent rock surfaces while in the C-9 the seismic character potentially represents dissolution features.

C-6 West
Figure 13. Seismic profiles from Canal C-6 collected from the intersection of the L-30 and L-33 ECPL Canal to the west continuing to east of the Florida Highway 27. Uninterpretated profile is shown above and profile with interpretations shown below. Outlined sections are shown in detail as Figures 14 and 15. These profile sections are best used to indicate trends and changes in the geology associated with porosity, and rock hardness. Colors highlight high-amplitude reflections that may indicate horizons or surfaces that are acoustically different from the surrounding rock material and are not intended to be consistent in all figures, except for the top green highlighted reflection as the first contact of canal bottom bedrock. There is a very irregular reflective horizon present in this profile (dark blue highlight). It is inferred that the contact between the limestones with poor induration and the moderate to good induration below produces this reflection. The red highlights may represent contacts (reflections) between the sand, silt, and clay units. Acoustic multiples (artifact of acquisition) partially mask the Fort Thompson and Tamiami Formations contact in some areas. Dashed line indicates the possible contact between formations as identified from core descriptions. Core descriptions were provided by SFWMD (see Appendix B). See Figure 8 for map location of profiles and cores. [click on images above for larger version]

Figure 14. Seismic profile overlain with a core description (CB-S-4). There is a very irregular reflective horizon present in this profile (green highlight). It is inferred that the contact between the limestones with poor induration above and moderate induration below produces this reflection. The red highights may represent contacts (reflections) between the sand, silt, and clay units. Acoustic multiples (artifact of acquisition) partially mask the Ft. Thompson and Tamiami formation contact. Dashed line indicates the possible contact between formations as identified from core descriptions. Core descriptions were provided by SFWMD (see Appendix B). See Figure 13 for relative placement on seismic section and Figure 8 for map location. [larger image]

Figure 15. Seismic profile overlain with core description (CB-S-5). It is inferred that the contact (continuous irregular green highlight) between the limestones with poor induration above and moderate induration below produces this reflection. The red highights may represent contacts (reflections) between the sand, silt, and clay units. Acoustic multiples (artifact of acquisition) partially mask the Fort Thompson and Tamiami Formations contact. Dashed line indicates possible contact between formations as identified from core descriptions Core descriptions were provided by SFWMD (see Appendix B). See Figure 13 for relative placement on seismic section and Figure 8 for map location. [larger image]

East Coast Protective Levee (ECPL) Canal

The ECPL Canal is a northerly trending canal along the western boundary of the study area and is comprised of three leveesL-31, L-30, and L-33 with each having an adjacent canal (Fig. 2, 8). Figure 16 is a composite of six seismic profiles (Lines 01ASR02-01b01, 01b02, 01b06, -01b09, -01b011 (partial); 02ASR02 01b01) and is shown in three panels (Fig. 16, 19, 22). To the south half of the canal, the geology represented in these profiles has many of the same seismic characteristics as profiles from C-9, but more features and deeper signal penetration (greater than 61 m - 200 ft). There are several continuous reflections in the upper 15 m (50 ft) of the section (e.g. green and purple highlights). Numerous features that may be shallow solution pipes or vugs (blue highlight) that penetrate through more than one horizon, these may be conduits for vertical flow through the formation. An example can be found in Line 01ASR02-01b09 Shot 3200 (profile from the southern portion of the study area) showing reflections that may be displacement features with possible collapse below. Other examples are in the next profile north (specifically 01ASR02 01b11 at Shots 4000 and 6500). Also cores DLBS-3 and -9 (Fig. 17, 18) show varying types of rock units including shelly-limestone and sandy units. In this case the dipping beds in Figure 18 are associated with alternating units.

In the southern portion of the EPCL Canal profiles the contact between the Fort Thompson and Tamiami Formation (as identified from core descriptions) is clearly discernible (Fig. 16, 17, 18, 19). This reflection produces weak multiples in some areas to the south. Going north passed Shot 7500 (Line 02ASR02 02b01) increasing hardness of the units above produce strong multiples that mask the Fort Thompson/Tamiami Formations contact (Fig. 16, 18). As seen in Figure 18, the lithology has changed with the presence of a wackstone unit not found in the core description approximately 3.0 km (1.8 mi) south (Fig. 17). Within the Tamiami Formation, core descriptions indicate several alternating rock units each having the potential to produce acoustic reflections. In this specific area with weak or no multiples there are many possible reflections that represent contacts between units or internal structures within the units.

Proceeding north in this canal the data quality decreases as the upper limestone unit increases in hardness. Seismic character of the upper most portion of the section (~7.6 m, ~25 ft) indicates many inferred solution pipes or vuggy features (blue highlights) penetrating the top of the limestone. Figures 20 and 21 show noisy seismic profiles with strong multiples and less discernible units below the upper hard limestone. Core descriptions from this area indicate that packstone and wackstone units of the Fort Thompson Formation overlie shell and sand layers of the Tamiami Formation (Fig. 20, 21). Hard lithologic units in the upper section have produced strong multiples in the profiles of Figure 19. Similar features and structures are seen in Figure 22 along with noisy chaotic signal returns. Throughout this report interpretations are best used to indicate trends and changes in the geology associated with porosity, and rock hardness.

South L-31
Figure 16. Seismic profiles from the L-31 Levee portion of the ECPL Canals collected from the southern control structure and to the control structure just south of the Black Creek Canal intersection. Uninterpretated profile is shown above and profile with interpretations shown below. Outlined sections are shown in detail as Figures 17 and 18. These profile sections are best used to indicate trends and changes in the geology associated with porosity, and rock hardness. Colors highlight high-amplitude reflections that may indicate horizons or surfaces that are acoustically different from the surrounding rock material and are not intended to be consistent in all figures, except for the top green highlighted reflection as the first contact of canal bottom bedrock. It is inferred that the blue and red highlights indicate fractures or vugs associated with internal secondary porosity structures or from angular rock created by collapse. Acoustic multiples (artifact of equation) partially mask the Fort Thompson and Tamiami Formations contact in some areas. Dashed line indicates the possible contact between formations as identified from core descriptions. Core descriptions from Switanek (in press). See Figure 8 for map location of profiles and cores. [click on images above for larger version]
Figure 17. Seismic profile overlain with core description (DLBS-3). Continuous reflective horizons (color highlighted) generally correlate with changes in lithology. Dashed lines indicate possible contact between formations as identified from core descriptions. Core descriptions from Switanek (in press). See Figure 16 for relative placement on seismic section and Figure 8 for location. [larger image]
Figure 18. Seismic profile overlain with core description. Dashed line indicates possible contact between formations and alternating beds of shell and limestone as identified from core descriptions. Core descriptions from Switanek (in press). See Figure 16 for relative placement on seismic section and Figure 8 for location. [larger image]
South L-30
Figure 19. Seismic profiles from L-30 Levee portion of the ECPL Canals collected from the south control structure and to the north control structure (just north of the C-6 Canal intersection. Uninterpretated profile is shown above and profile with interpretations shown below. Outlined sections are shown in detail as Figures 20 and 21. These profile sections are best used to indicate trends and changes in the geology associated with porosity, and rock hardness. Colors highlight high-amplitude reflections that may indicate horizons or surfaces that are acoustically different from the surrounding rock material and are not intended to be consistent in all figures, except for the top green highlighted reflection as the first contact of canal bottom bedrock. It is inferred that the blue and red highlights indicate fractures or vugs associated with internal secondary porosity structures or from angles created by collapse structures. Acoustic multiples (artifact of equation) partially mask the Fort Thompson and Tamiami Formations contact in some areas. Dashed line indicates the possible contact between formations as identified from core descriptions. Core descriptions from Switanek (in press). See Figure 8 for map location of profiles and cores. [click on images above for larger version]

Black Creek Canal

Black Creek Canal in the southern part of the study area (Fig. 8) and is connected with the ECPL canal. Figure 23 is a composite of two seismic profiles (Lines 01ASR02 01b011 (partial), 01ASR01 01b10). The subbottom of this canal has very similar seismic character and penetration as the ECPL canal (Fig. 16) and can be described in similar fashion with continuous reflections in the upper 15 m (50 ft) of the section. Numerous inferred features that may be shallow solution pipes or large vugs. Several examples of collapse may be seen at Shot 1500 - Line 01ASR02 01b11 and 5000 - Line 01ASR01 01b10. No core descriptions were available for correlation to these profiles.

Wellfield Recharge Canal

The Wellfield Recharge Canal is in the northern part of the study area (Fig. 8) and is connected with the ECPL canal through a water-control structure (Fig. 19). Figure 24 is a composite of two seismic profiles (Lines 01ASR02-01b003 and -01b4). Data collection in this canal was a challenge due to the presence of multiple construction boomers across the canal. There are areas of continuous and non-continuous reflections in this canal with inferred solution and possible collapse features. Signal penetration was limited along much of the canal, but near the mid-portion of Line 01ASR02 01b04 better penetration was acquired to below 61 m (200 ft). Core descriptions of DLBS-4 indicate that the upper limestone in this area has moderate to poor induration indicating a weak limestone (Fig. 24, 25). This weak limestone may allow a better signal to noise ratio providing a 'window' of better data.

Figure 20. Seismic profile overlain with core description (DLDS-10). Dashed line indicates possible contact between formations as identified from core descriptions. This portion of the profile has been masked by very strong multiples. Core descriptions from Switanek (in press). See Figure 19 for relative placement on seismic section and Figure 8 for location. [larger image]

Figure 21. Seismic profile overlain with core description (DLBS-6). Dashed line indicates possible contact between formations as identified from core descriptions. This portion of the profile has been masked by strong multiples and a chaotic/noisy? acoustic signal. Core descriptions from Switanek (in press). See Figure 19 for relative placement on seismic section and Figure 8 for location. [larger image]

South L-33

Figure 22. Seismic profiles from L-33 Levee portion of the ECPL Canals collected from the south control structure and to the north approximately 3 km (1.9 mi) beyond the C-9 Canal intersection. Uninterpretated profile is shown above and profile with interpretations shown below. These profile sections are best used to indicate trends and changes in the geology associated with porosity, and rock hardness. Colors highlight high-amplitude reflections that may indicate horizons or surfaces that are acoustically different from the surrounding rock material and are not intended to be consistent in all figures, except for the top green highlighted reflection as the first contact of canal bottom bedrock. It is inferred that the blue and red highlights indicate fractures or vugs associated with internal secondary porosity structures or from angles created by collapse structures. Acoustic noise (artifact of acquisition) partially mask the Fort Thompson and Tamiami Formations contact in some areas. Dashed line indicates the possible contact between formations as identified from pump test stratigraphy. See Figure 8 for map location of profiles and pump site test. [click on images above for larger version]

Black Creek Canal

Figure 23. Seismic profiles from Black Creek Canal collected from ECPL L-33 Canal to south end of Black Creek Canal. Uninterpretated profile is shown above and profile with interpretations shown below. There are no core samples available to verify this section. Colors highlight high-amplitude reflections that may indicate horizons or surfaces that are acoustically different from the surrounding rock material and are not intended to be consistent in all figures, except for the top green highlighted reflection as the first contact of canal bottom bedrock. See Figure 8 for location. [click on images above for larger version]

Recharge Canal North

Figure 24. Seismic profiles from Wellfield Recharge Canal collected from south of the L-30 EPCL control structure and to the south end of quarries. Uninterpretated profile is shown above and profile with interpretations shown below. Outlined section is shown in detail as Figure 25. These profile sections are best used to indicate trends and changes in the geology associated with porosity, and rock hardness. Colors highlight high-amplitude reflections that may indicate horizons or surfaces that are acoustically different from the surrounding rock material and are not intended to be consistent in all figures, except for the top green highlighted reflection as the first contact of canal bottom bedrock. Acoustic multiples (artifact of acquisition) mask the Fort Thompson and Tamiami Formations contact identified from core descriptions. Dashed line indicates possible contact between formations as identified from core descriptions. Core descriptions from Switanek (in press). Figure 8 for map location. [click on images above for larger version]

Figure 25. Seismic profile with core description. Continuous reflective horizons seem to correlate generally with changes in lithology. Core descriptions were provided by SFWMD. See Figure 24 for relative placement on seismic section and Figure 8 for location. [larger image]

< Go back to Methods | Go ahead to Summary and Recommendations >