U.S. Department of the Interior

U.S. Geological Survey

A One-Dimensional Model of Solid-Earth Electrical 

Resistivity beneath Florida

By Cletus Blum, Jeffrey J. Love, Kolby Pedrie, Paul A. Bedrosian, and E. Joshua Rigler

Open-File Report 2015–1185
ii

U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

 

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Blum, Cletus, Love, J.J., Pedrie, Kolby, Bedrosian, P.A., and Rigler, E.J., 2015, A one-dimensional model of 

solid-Earth electrical resistivity beneath Florida: U.S. Geological Survey Open-File Report 2015–1185, 16 p., 

http://dx.doi.org/10.3133/ofr20151185. 

ISSN 2331-1258 (online)
iii

Contents

Abstract ...................................................................................................................................................................... 1

Introduction................................................................................................................................................................. 1

Method Summary ....................................................................................................................................................... 1

Tectonic and Geologic Overview ................................................................................................................................ 2

Northern and Southern Layered-Earth Structures ...................................................................................................... 3

Resistivity versus Depth ............................................................................................................................................. 4

Transfer Functions...................................................................................................................................................... 6

Discussion and Conclusions..................................................................................................................................... 10

Acknowledgments..................................................................................................................................................... 11

References Cited...................................................................................................................................................... 12

Appendix................................................................................................................................................................... 14

Figures

1. Paleogeographic map depicting the final assembly of the Florida basement rocks...................................... 2

2. Geologic cross section showing the generalized basement structure of the Florida peninsula..................... 3

3. Graph showing resistivity-depth model for the Florida peninsula.................................................................. 4

4. Chart showing layer thickness profile with resistivity values for all three models up to a 100-kilometer 

depth............................................................................................................................................................. 5

5. Graph showing surface impedance magnitude versus frequency for the upper bound, lower bound, and 

geometric mean............................................................................................................................................ 7

6. Graph showing Schmucker-Weidelt transfer function magnitude versus frequency for the upper bound, 

lower bound, and geometric mean................................................................................................................ 8

7. Graph showing apparent resistivity versus frequency for the upper bound, lower bound, and geometric 

mean............................................................................................................................................................. 9

8. Graph showing phase angle versus frequency for the upper bound, lower bound, and geometric mean... 10
iv

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

mile (mi) 1.609 kilometer (km)

foot (ft) 0.3048 meter (m)

inch (in.) 2.54 centimeter (cm)

Resistivity (ρ) in ohm ·meter (Ω•m) may be converted to conductivity (σ) in siemens per meter (S/m) as ρ = 1

σ

. 

Abbreviations Used in this Report

1D one dimensional

3D three dimensional

FERC Federal Energy Regulatory Commission

Hz hertz

km kilometer

m meter

Ma mega-annum

NERC North American Electric Reliability Corporation

nT nanotesla (magnetic field strength, one-billionth of a tesla) 

S siemens

S/m siemens per meter

USGS United States Geological Survey

mV millivolts

Symbols

ρa apparent resistivity

ω angular frequency

σ conductivity

> greater than

< less than

M Ω megohm

Ω•m ohm meter
1 

A One-Dimensional Model of Solid-Earth Electrical 

Resistivity beneath Florida

By Cletus Blum, Jeffrey J. Love, Kolby Pedrie, Paul A. Bedrosian, and E. Joshua Rigler

Abstract

An estimated one-dimensional layered model of electrical resistivity beneath Florida was

developed from published geological and geophysical information. The resistivity of each layer is 

represented by plausible upper and lower bounds as well as a geometric mean resistivity. Corresponding 

impedance transfer functions, Schmucker-Weidelt transfer functions, apparent resistivity, and phase 

responses are calculated for inducing geomagnetic frequencies ranging from 10−5 to 100 hertz. The 

resulting one-dimensional model and response functions can be used to make general estimates of timevarying electric fields associated with geomagnetic storms such as might represent induction hazards for 

electric-power grid operation. The plausible upper- and lower-bound resistivity structures show the 

uncertainty, giving a wide range of plausible time-varying electric fields. 

Introduction

Geoelectric fields, induced in the Earth’s conducting interior during magnetic storms, represent a 

natural hazard for the operation of electric-power grids (for example, Bolduc, 2002; Boteler, 2003). In 

response to the possible vulnerability of U.S. power grids, in May 2013, the Federal Energy Regulatory 

Commission of the Department of Energy issued FERC Order No. 779 directing the North American 

Electric Reliability Corporation (NERC-DOE, 2010) to develop power-grid standards to help mitigate 

the deleterious impact of storm-induced geoelectric fields. In support of these developments, the 

Geomagnetism Program of the U.S. Geological Survey (USGS) has assembled a simplified model of 

electrical resistivity beneath Florida, a part of the United States that was not specifically covered by an 

earlier effort undertaken by Fernberg (2012). This resistivity model for Florida contributes to a larger 

USGS and interagency U.S. Government project for evaluating induction hazards associated with 

geomagnetic storms (Love and others, 2014). 

Method Summary

To develop a one-dimensional (1D), depth-dependent model of electrical resistivity for Florida, 

we first assemble a layered-earth structure sequence for the lithosphere (crust and upper mantle) beneath 

the tectonically distinct northern and southern parts of Florida. For a given rock type, electrical 

resistivity can differ by orders of magnitude depending on mineralogy, water content, chemistry, and 

degree of weathering. Therefore, for each of the northern and southern Florida layered-earth sequences, 

we assigned a range of resistivity values based on rock type and published results from geological and 

geophysical surveys and from direct laboratory measurements of standard rock types for each layer. A 

1D model, applicable to all of Florida, has a set of layers and resistivity values that encompass the 
2 

separate northern and southern Florida layered-earth structures; a reference resistivity model is obtained 

by calculating the geometric mean from the resistivity range for each stratigraphic layer. Using the 1D 

model, we calculate impedance transfer functions, Schmucker-Weidelt transfer functions, and apparent 

resistivity and phase responses that describe the frequency-dependent relation between the induced 

geoelectric fields and the inducing geomagnetic field variation.

Tectonic and Geologic Overview

We begin by summarizing the complex tectonic and geological history of Florida (for review,

see Lane, 1994). The basement rocks of Florida originate from the Gondwana supercontinent (570–180 

Ma), a mosaic of continental blocks that is the ancestor of South America and Africa, and were

amalgamated in the Early Cambrian period through several orogonies. During the Paleozoic and early 

Mesozoic tectonic assembly of Pangea (510–180 Ma), multiple collisions between Gondwana and 

Laurentia, the ancestor of North America, formed a mountain chain complex that included what is now 

the Appalachian Mountains. With the assembly of Pangea, the Suwannee Basin and Florida-Bahama 

blocks detached from Gondwana and attached to Laurentia (Hine, 2013), as shown in figure 1. The 

Suwannee block accumulated terrane fragments from Laurentia forming a composite-terrane block 

(Mueller and others, 2014). Subsequently, with the Mesozoic opening of the Atlantic (~170–70 Ma), 

extensional forces moved and stretched the Florida-Bahama block, creating a series of basins and arches 

accompanied by localized igneous intrusions. The divide between the older Suwanee block to the north 

and the younger Florida-Bahama block to the south is known as the Bahama fracture zone. 

Figure 1. Paleogeographic map depicting the final assembly of the Florida basement rocks. Modified from Hine, 

2013, figure 3.4. 
3 

Figure 2. Geologic cross section showing the generalized basement structure of the Florida peninsula, modified 

from Barnett, 1975, figure 4, based on firsthand examination of subsurface samples and supplemented by well logs 

and literature. N, north; S, south; km, kilometer.

In the Late Jurassic (~160 Ma), substantial carbonate deposition began across the blocks of the 

Florida peninsula and continued almost uninterrupted into the late Paleogene (~30 Ma) (Hine, 2013); 

see figure 2. During this interval, Florida was beneath a warm, shallow ocean with carbonate deposition 

atop the basement accompanying fluctuations in sea level, as shown in figure 2. From the late Paleogene 

through the Neogene (~10 Ma), siliciclastic sediments shed from a second uplift of the Appalachian 

Mountains intermittently interrupted carbonate deposition (Hine, 2013). Both carbonate and siliciclastic 

sediments were deposited on the Florida peninsula from the Neogene to the Quaternary (Scott, 2001). 

Northern and Southern Layered-Earth Structures

The preceding geological summary motivates our assembly of two Florida-based, layered-earth 

structural sequences on either side of the northwest-southeast trending Bahama fracture zone. Within the 

deep lithosphere and asthenosphere, both structural sequences are taken to be identical; they include the 

lower mantle, which for this project extends from 670- to 1,000-kilometers (km) depth, the mantle 

transition zone from 410- to 670-km depth, and an upper mantle from 40- to 410-km depth. An 

interpreted crustal thickness of 40 km is based on the Florida-to-Edmonton Broadband Seismometer 

Experiment (French and others, 2009). North of the Bahama fracture zone, late Precambrian igneous 

and metamorphic rocks (mostly granite) of the Suwannee block compose the basement and come to 

within 1 km of the surface (Barnett, 1975). South of the Bahama fracture zone, igneous and 

metamorphic rocks (granite with basaltic intrusions) of the Florida-Bahama block are known to 

characterize the basement deeper than 6 km (Smith, 1982). We assume that both the north and south 

basements are homogenous down to the base of the crust at 40-km depth. In the north, an early 

Paleozoic sedimentary sequence overlays igneous basement rocks (Barnett, 1975), while to the south, 

limestone and other carbonate rocks directly overlay the igneous and metamorphic basement (Hine, 

2013). 
4 

Figure 3. Resistivity-depth model for the Florida peninsula. Upper and lower resistivity bounds are shown with 

dashed and dotted black lines, respectively. The solid green line shows the geometric mean.

Resistivity versus Depth

In figure 3, we show the resistivity-depth model, including the upper bound, lower bound, and 

corresponding geometric mean resistivities versus depth; numerical values are given in tables 1–3 of the 

appendix and references therein. The upper, lower, and mean depth versus resistivity models are shown 

in figure 4. The layer structure of the upper model corresponds with the northern layer structure, and the 

layer structure of the lower model corresponds with the layer structure of the southern layer structure. 

Taking into consideration the northern and southern layered-earth structures and the upper and lower 

bounds of their resistivity values, the end members were chosen for the upper and lower bounds. In 

practice, given the northern and southern structures, the upper resistivity bound uses the resistivity 

values for each layer of the northern structure and the lower resistivity bound uses the lowest resistivity 

values of the southern structure. The geometric mean is calculated from the layer thicknesses and 

resistivity values of the upper and lower bounds. 

0.1

1.0

10.0

100.0

1,000.0

0.1 1.0 10.0 100.0 1,000.0 10,000.0

Depth, in kilometers 

Resistivity, in ohm meters

Upper bound

Lower bound

Geometric mean
5 

Figure 4. Layer thickness profile with resistivity values for all three models up to a 100-kilometer depth. Note 

depth values are not to scale. Ω•m, ohm meter; km, kilometer. 

Sedimentary strata: Resistivity values are commonly reported corresponding to rock age as well 

as rock type. Published sedimentary rock resistivity values of similar type rocks and ages found in 

Florida range from 5 to 900 Ω•m (Keller and Frischknecht, 1966; Jones, 1999). 

Crustal basement (<40-km depth): Resistivity values for early Paleozoic intrusive rocks typically 

range from 1,000 to 5,000 Ω•m, while Mesozoic extrusive rocks have resistivities ranging from 20 to 

500 Ω•m (Keller, 1966). Specific resistivity values for rock types have been published, giving granite 

(intrusive rock) a resistivity value of 4,300 Ω•m and basalt (extrusive rock) of 800 Ω•m (Palacky, 

1988). Generally speaking, published resistivity values for the upper crust range from 800 to 30,000 

Ω•m, and lower-crustal resistivity values range from 8 to 200 Ω•m (Jones, 1999). Given the lithology of 

the basement rocks in Florida, a resistivity value of 10,000 Ω•m is used for the upper bound. For the 

lower bound, a low published resistivity value of 8 Ω•m (Keller, 1966) is used.

Upper mantle (40- to 410-km depth): Resistivity values for the upper mantle can be constrained 

from magnetotelluric surveys, with a wide range of values reported from different regions. Globally, the 

continental upper mantle has resistivity values ranging from 50 to 200 Ω•m, while oceanic upper mantle 

has significantly higher resistivity, on the order of 105 Ω•m (Jones, 1999). Two magnetotelluric surveys 

with transects across the Appalachian Mountains through the coastal plains (for example, Mareschal and 

others, 1983; Ogawa and others, 1996) report upper mantle resistivity ranging from 1,000 to 3,000 Ω•m. 

The upper-bound resistivity value is taken from these two surveys, whereas the lower-bound resistivity 

value of 50 Ω•m is based upon Jones 1999. 
6 

Lower mantle and mantle transition zone (410- to >1,000-km depth): The resistivity values for 

the mantle (100- to 1,000-km depth) are taken from a coarse global resistivity model by Kelbert and 

others (2009), which are based upon magnetic observatory data. The station closest to Florida that is 

used in Kelbert’s analysis is the Stennis magnetic observatory, which is located at the John C. Stennis 

Space Center in Mississippi. 

Transfer Functions

The preceding 1D resistivity model can be used to estimate the amplitude and direction of the 

geoelectric field induced at the Earth’s surface by geomagnetic activity. This can be accomplished by 

calculating a frequency domain transfer function relating the induced geoelectric field and the inducing 

geomagnetic field (for example, Simpson and Bahr, 2005; Lordan, 2013). 

The impedance tensor, Z, specifies the relation between the geoelectric field, E, in millivolts per 

kilometer (mV/km), and the magnetic intensity, B/μ, in amps per meter (A/m); here, μ is the magnetic 

permeability, assumed to be identical to the free space permeability. In the frequency domain, 

E = Z·B/μ. 

The Schmucker-Weidelt transfer function (Simpson and Bahr, 2005) C specifies the relation 

between the geoelectric field E and the time rate of change of the magnetic field, dB/dt, in nanoteslas 

per second (nT/s). In the frequency domain, 

E = C· iωB, 

where ω is angular frequency, measured in radians per second, and i is the square root of -1. For 

a 1D depth-dependent resistivity model (figs. 5 and 6), a frequency-dependent apparent resistivity, ρa, 

(fig. 7) can be calculated; it is related to the magnitude of Z as

ρa= |Z(ω)|2 /(μω). 

Impedance phase (∅), usually plotted along with apparent resistivity (fig. 8), can be calculated as

∅ = tan−1(

Ex

By

). 
7 

Figure 5. Surface impedance magnitude versus frequency for the upper bound, lower bound, and geometric 

mean. 

0.00001

0.0001

0.001

0.01

0.1

1

100 10 1 0.1 0.01 0.001 0.0001 0.00001

Surface impedance magnitude, in ohms

Frequency, in hertz

Upper bound

Lower bound

Geometric mean
8 

Figure 6. Schmucker-Weidelt transfer function magnitude versus frequency for the upper bound, lower bound, 

and geometric mean. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

100 10 1 0.1 0.01 0.001 0.0001 0.00001

C

ω, in millivolt seconds per nanotesla (mV*s/nT)

Frequency, in hertz

Upper bound

Lower bound

Geometric mean
9 

Figure 7. Apparent resistivity versus frequency for the upper bound, lower bound, and geometric mean. 

1

10

100

1000

10000

100 10 1 0.1 0.01 0.001 0.0001 0.00001

Apparent resistivty, in ohm meters

Frequency, in hertz

Upper bound

Lower bound

Geometric mean
10 

Figure 8. Phase angle versus frequency for the upper bound, lower bound, and geometric mean. 

Discussion and Conclusions

The results of this study are limited by the lack of a detailed Florida-wide magnetotelluric 

survey. The EarthScope program of the National Science Foundation has supported a transportable array 

of magnetotelluric stations that have, so far, been deployed across the northwestern and midwestern 

United States (Schultz, 2009; http://www.earthscope.org). As part of that project, data have been 

inverted for regional three-dimensional (3D) resistivity models at lithospheric scales, revealing

significant anisotropy in resistivity (for example, Patro and Egbert, 2008; Meqbel and others, 2012; 

Bedrosian and Feucht, 2014). Given Florida’s complicated geologic and tectonic history, it is reasonable 

to expect that the crust and upper mantle have significant anisotropy in resistivity structure. 

Furthermore, the large resistivity contrast between the crust and the ocean can significantly distort the 

amplitude, direction, and phase of induced geoelectric fields. The complexity of these important effects 

is not captured using a one-dimensional (1D) resistivity model. 

The USGS Geomagnetism Program recently completed a magnetotelluric survey of the Florida 

peninsula which can be used to model the 3D resistivity of Florida. The 1D model presented in this 

report can provide general geological and geophysical context for 3D modeling. It will be of interest to 

0

10

20

30

40

50

60

70

80

90

100 10 1 0.1 0.01 0.001 0.0001 0.00001

Phase angle, in degrees 

Frequency, in hertz

Upper bound

Lower bound

Geometric mean
11 

compare the electromagnetic response functions from the 3D models with the corresponding response 

functions reported here for 1D resistivity, specifically noting directional heterogeneity. With respect to

magnetic-storm induction hazards, it will be of interest to compare hypothetical storm-time geoelectric 

time series calculated using the impedance tensors of the 1D model with more accurate geoelectric time 

series calculated from the site-specific impedance tensors obtained from direct magnetotelluric 

measurement.

Acknowledgments

We thank B.L. Burton, C.A. Finn, J. McCarthy, F.K. Rengers, and J.L. Slate for reviewing this

manuscript. 
12 

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ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20151185
14 

Appendix

Table 1. Details of upper-bound model. 

[km, kilometer; Ω•m, ohm meter; S/m, siemens per meter] 

Layer Depth

(km)

Thickness

(km)

Resistivity

(Ω•m)

Conductivity 

(S/m) Comments

1. Sedimentary 

section

1 0–1 km 1 km 2 900 0.0011 Resistivity values chosen using 

upper bound of old sediment 

range2

2. Crustal 

basement

3 1–40 km 39 km 2 10,000 0.0002 Resistivity values chosen from

Jones, 19992

3. Upper mantle 4 40–100 km 60 km 5 3,000 0.001 Resistivity value chosen from 

Mareschal and others, 1983, 

transect interpretation5

4. Upper mantle 4 100–250 km 150 km 138 4 0.005 Values correspond to John C. 

Stennis magnetic 

observatory4

5. Upper mantle 4 250–410 km 160 km 34 4 0.01 Values correspond to John C. 

Stennis magnetic 

observatory4

6. Transition zone 4 410–520 km 110 km 15.5 4 0.05 Values correspond to John C.

Stennis magnetic 

observatory4

7. Transition zone 4 520–670 km 150 km 4.2 4 0.1778 Values correspond to John C. 

Stennis magnetic 

observatory4

8. Lower mantle 4 670–900 km 230 km 1.2 6 0.631 Values correspond to John C.

Stennis magnetic 

observatory4

9. Lower mantle 4 900–1,000 km 100 km 0.87 6 1.12 Values correspond to John C.

Stennis magnetic 

observatory4

1 Barnett, 1975, table 1 

2 Jones, 1999, figure 1 

3 French and others, 2009, figure 2

4 Kelbert and others, 2009, figure 2; global and regional resistivity model, North American regional resistivity chosen

5 Mareschal and others, 1983

6 Keller, 1966, table 10
15 

Table 2. Details of lower-bound model. 

[km, kilometer; Ω•m, ohm meter; S/m, siemens per meter]

Layer Depth

(km)

Thickness

(km)

Resistivity

(Ω•m)

Conductivity

(S/m) Comments

1. Sedimentary 

basin

1 0–6 km 6 km 2 5 0.2 5 Resistivity values chosen from

Keller 

2. Crust 3 6–40 km 34 km 5 1,000 0.0001 5 Resistivity values chosen from

Keller

3. Upper mantle 4 40–100 km 60 km 1 50 0.0125 1 Resistivity values chosen from

Jones

4. Upper mantle 4 100–250 km 150 km 138 4 0.005 4 Values correspond to Stennis 

magnetic observatory

5. Upper mantle 4 250–410 km 160 km 34 4 0.01 4 Values correspond to John C. 

Stennis magnetic observatory 

6. Transition zone 4 410–520 km 110 km 15.5 4 0.05 4 Values correspond to John C. 

Stennis magnetic observatory

7. Transition zone 4 520–670 km 150 km 4.2 4 0.1778 4 Values correspond to John C. 

Stennis magnetic observatory

8. Lower mantle 4 670–900 km 230 km 1.2 4 0.631 4 Values correspond to John C. 

Stennis magnetic observatory

9. Lower mantle 4 900–1,000 km 100 km 0.87 4 1.12 4 Values correspond to John C. 

Stennis magnetic observatory

1 Barnett, 1975, table 1 

2 Jones, 1999, figure 1 

3 French and others, 2009, figure 2

4 Kelbert and others (2009), figure 2, global and regional resistivity model, North American regional resistivity chosen

5 Keller, 1966, table 10
16 

Table 3. Details of geometric mean model. 

[km, kilometer; Ω•m, ohm meter; S/m, siemens per meter]

Layer Depth

(km)

Thickness

(km)

Resistivity

(Ω•m)

Conductivity

(S/m)

1. Sedimentary basin 0–1 km 1 km 67 0.01493

2. Sedimentary basin 1–6 km 5 km 224 0.00446

3. Crust 6–40 km 34 km 3,162 0.00032

4. Upper mantle 40–100 km 60 km 387 0.00258

5. Upper mantle 100–250 km 150 km 138 0.005

6. Upper mantle 250–410 km 160 km 34 0.01

7. Transition zone 410–520 km 110 km 15.5 0.05

8. Transition zone 520–670 km 150 km 4.2 0.1778

9. Lower mantle 670–900 km 230 km 1.2 0.631

10. Lower mantle 900–1,000 km 100 km 0.87 1.12