USGS Home - www.usgs.gov
Coastal and Marine Geology Program
Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

Subsurface Characterization of Selected Water Bodies in the St Johns River Water Management District, Northeast Florida

Northeast Florida Lakes and Rivers Home
North East Florida Atlas
About the Atlas:
Introduction
Methods You are at the introduciton to the NE Florida Atlas
Regional Geology
Karst Development & Characterization
Identification of Karst Features from Seismic Patterns
Summary
Acknowledgements
References
Printable Version of the Atlas
Contacts
About the Atlas

Methods

Seismic Profiling | Geophysical Well Logs | Map Generation

General stratigraphic column for north central Florida. The natural gamma log profiles on the left side of the figure are examples of "signatures" from wells and their regional correlation potential.
Figure 4: General stratigraphic column for north central Florida. The natural gamma log profiles on the left side of the figure are examples of "signatures" from wells and their regional correlation potential. Modified from Scott (1988) and Miller (1986). Click on the image for a larger version.
Natural gamma ray logs (for example, Index Map A, A-A') were used for correlating geologic units to the seismic reflection data. Logs used in this report are part of the SJRWMD geophysical log data base and accessed through GeoSys/4G software version 1.1 developed by Dr. Robert Lindquist and Dr. Daniel Arrington of Gainesville, Florida. Sources of the gamma logs include wells logged using SJRWMD equipment and logs digitized from various agency files or private consultant reports submitted to the SJRWMD.

Gamma logs are scaled in counts per second (cps), which provide a relative indication of gamma ray intensity. Relative gamma ray intensity can be used to identify boundaries between geologic units. The contact between the Miocene Hawthorn Group sediments and the Eocene carbonates is generally identified by low cps (<50) in the Eocene carbonates and higher cps (>50) in the Miocene and younger sediments. Additionally, the Miocene sediments are highly variable and units within the section could vary from 20 to >1000 cps. Many factors influence the absolute values that are recorded (borehole diameter, size of the probe crystal) but a characteristic "signature" can usually be identified. Pliocene and Pleistocene sediments that overlie the Miocene sediments may be identified by a reduction in cps. These sediments are generally sands and sandy clays, commonly reworked Miocene sediments are present and may greatly increase the cps.

Cross sections of the gamma logs available near the surveyed lakes are provided to show the contacts of geologic units that could be readily identified (Fig 4). Elevation of the geologic contacts were interpolated to the sites that were profiled and converted to two-way travel time and correlated to reflections that may represent the contacts. Since the lakes generally represent areas of increased stratigraphic disturbance, the elevation of the contacts are highly variable. Reflections from the least disturbed area within a site were used to correlate to the gamma logs. In some cases, the contact could not be identified but a strong reflection within a geologic unit is identified to show subsurface structure.

Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

FirstGov.gov U. S. Department of the Interior | U.S. Geological Survey
Coastal and Marine Geology Program

email Feedback | USGS privacy statement | Disclaimer | Accessibility

This page is http://coastal.er.usgs.gov/publications/ofr/00-180/intro/methods-welllogs.html
Updated December 05, 2016 @ 11:25 AM (JSS)