The deepest boreholes at the levee sites were logged with geophysical probes prior to casing installation. Whereas the underlying lithology cannot be determined solely by geophysical logs, the general characteristics of lithology and water content can be summarized (Boggs, 1995). The log traces of each borehole and all geophysical data are summarized graphically on north-south and east-west cross-sections and shown by Harvey and others (2000). The standard suite of geophysical logs collected during this investigation included caliper, natural gamma ray, neutron, and lateral logs. This report mainly discusses the caliper, gamma ray, and neutron logs because these tools were the most responsive during the logging operations and indicate intervals of lithologic change.

Caliper Logging

Caliper traces of ENR wells consistently showed evidence of washouts throughout the length of the boreholes. Washouts often occur where there are heaving sands because of excessive hydraulic pressures, changes in lithology, solution holes, or a lack of sediment compaction. Washouts were most extensive at sites MP2 and MOP2, and were present in both limestone layers and well-sorted sand layers. These washouts probably indicate layers with varying degrees of cementation or compaction. The largest washout was seen at the MP3-A borehole at an elevation of -13 ft NGVD and extended 16 in from the center of the borehole. This elevation coincides with a lithologic change from sandy limestone to well-sorted sand. The caliper log from borehole S10C showed minimal evidence of washouts.

Natural Gamma Ray Logging

Natural gamma ray logs traditionally have been used in Florida to correlate formations regionally and to identify clays or formations with natural radioactivity, such as the Hawthorn Group (Scott, 1988). Recent work in Florida by Krupa (1999) and Cunningham (1998) indicates that natural gamma ray responses from sediments in the Surficial aquifer do not always coincide with the phosphatic sediments.

In the ENR, the natural gamma ray logs have peaks in layers from 0 to -25 ft NGVD at each site. However, only sites MP1-A, MOP1, MOP2, and MOP3 show natural gamma ray peaks in the interval between -25 to -40 ft NGVD. At greater depths, the logs generally are featureless, with a few smaller peaks. Further south at S10C, the natural gamma ray peaks occur below a depth of -35 ft NGVD, as seen in Harvey and others (2000, p. 118 and 121).

There is limited geophysical information surrounding WCA-2A and data are not available within WCA-2A; therefore, an available gamma ray log from well PB-00019 (Swayze, 1988) was used. This site is located 6 mi southeast of the S10C site.

Figure 9. North (left) to south (right) changes in borehole gamma counts, showing inferred layers of secondary depositional crusts (SDC) and southern zone of erosion and deposition of poorly sorted sands with medium grain size, north-central Everglades, south Florida. [larger version]

Two correlations of natural gamma ray logs were prepared. The correlation of the west - east ENR transect is shown in Harvey and others (2000, p. 120-129). The gamma ray correlations along the north-south ENR/WCA-2A transect are shown in figure 9. Whereas the natural gamma ray logs at S10C and PB-00019 show only minor inflections to a depth of -30 ft NGVD, the ENR traces show appreciable peaks on the gamma logs to a depth of about -35 ft NGVD.

The natural gamma ray peaks below -35 ft NGVD are similar in both the ENR and WCA-2A (fig. 9). The sediment analysis did not indicate a lithologic explanation for the peaks, such as clays or phosphatic deposits. Therefore, they may be SDCs or erosional surfaces, which potentially can restrict vertical and lateral ground-water flow within the Surficial aquifer. Additional research would be needed to evaluate the effect of SDCs on water quality and flow.

It is likely that SDCs were formed over thousands of years during the normal fluctuations of the ground water within the vadose zone in the sediments. Because S10C is closer to the Atlantic Ocean, it is more likely affected by tidal and nearshore processes. Thus, SDCs probably formed within both sites but subsequently were eroded away at the S10C site. After the erosion, poorly sorted sediments then were deposited over the area at S01C.

Neutron Logging

The neutron log is a hydrogen detector and measures amount of hydrogen within the sediment pore space; therefore, it is an indicator of porosity. Elevated hydrogen content above the natural background can be attributed to lower porosity within the sediments. The neutron tool utilized in this investigation had not been calibrated recently. Therefore, the neutron log traces only are used to describe general characteristics and trends within the aquifer.

The major peaks in the neutron logs occur at shallow (less than -30 ft NGVD) depths in the ENR, whereas peaks occur throughout the entire borehole at S10C (+18 to -77 ft NGVD). The neutron log activity at S10C indicates zones of reduced porosity. The neutron logs at sites MP1, MP2, MP3, MOP1, MOP2, and MOP3 all show elevated hydrogen levels at shallow depths (less than 20 ft) than at deeper depths, indicating lower porosity (table 5). At S10C, the neutron log showed a distinct deflection to the left (low hydrogen content) at an elevation of 5 to 11 ft NGVD. This deflection occurred in a layer of well-sorted sand and gravel and could indicate a layer of high porosity. The remainder of this log traces displayed elevated hydrogen levels at elevations of +2, -2.5, and -42.5 ft NGVD. These elevated levels also are in layers of well-sorted sand with gravel and indicate layers of lower porosity. These deflections correlate to deflections on the gamma ray log and also could indicate SDCs.

Electric Logs

Electric or resistivity logs measure the electrical properties of the formation such as the resistivity of fluid in interconnected pores or the effective porosity (Keys, 1990). The resistivity of the formation is affected by lithology, porosity, and water quality. The long-normal (LN) resistivity profiles sometimes can provide a view of the preferential flow paths and associated water quality (Keys, 1990). Deflections or peaks occur when formation water of different resistivity is encountered by the probe. These logs are composed of "shallow" and "deep" penetrating sondes with each sonde investigating at a different distance into the formation, from the center of the borehole. In this study, the LN and short normal (SN) were used. The LN measures 64 in. into the formation and the SN measures 16 in. into the formation.

Typical resistivity traces of the shallow aquifers in Florida show higher resistivity at or near the surface, which generally declines with depth. In the Surficial aquifer, the shallow, more resistive water is recharged from the surface. Relict seawater in the deeper portion of the Surficial aquifer has lower resistivity. Resistivity traces that indicate underflow or preferential flow have the same gentle slope with periodic areas of reduced resistivity shown by a deflection, usually to the left, in the resistivity trace. This trace generally indicates a more dormant area (higher specific conductance) of the ground water.

When the natural layering of water within an aquifer is disturbed from anthropogenic activities, mixing occurs. The resistivity logs showed that anthropogenic activities had affected the sites. The upper 20 ft of the Surficial aquifer showed freshwater on the resistivity logs, indicating that relict seawater had been replaced by freshwater recharge. This recharge is likely from surface water in the western part of the ENR. The SN and LN traces in this study are generally smooth, with resistivity steadily decreasing with depth. This result is shown at MOP3 by the three electric traces exhibiting higher resistivity in the upper sediments and smooth traces of steadily decreasing resistivity in the bottom of the borehole. In contrast, site MP3 shows deflections to the left and right at elevations between -60 and -80 ft NGVD. This well is on a levee at the northwestern corner of the ENR. The resistivity traces at ENR sites MP1 and MOP2A show the highest resistivity levels with measurements of 400 ohm-m in the top 10 ft of the borehole.

The water underlying S10C shows relatively high resistivity at depth. This high resistivity indicates deeper recharge of freshwater, likely as a result of anthropogenic activity. Results of the lateral logging suggest some preferential flow paths, indicated by the deflections in the trace seen throughout the depth of the borehole. The SN resistivity log had an increase in resistivity to the right at -50 ft NGVD indicating a possible preferential flow path or freshening of the ground water. These differences may indicate water from different recharge sources.