0

0

0

-10

-10

1

2.2 km

Shipboard Surveys Track Magnetic Sources in Marine Sediments—

Geophysical Studies of the Stono and North Edisto Inlets near Charleston, South Carolina

By Anjana K. Shah1and M. Scott Harris2

U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Open-File Report 2012–1112

0 10 20 30 40

−4

0

4

8

Frequency

Sum

Log amplitude

0 0.1 0.2

20

24

28

Windowed 

profile 

Anomaly (nT)

Distance (km)

Sample anomaly profile (nT)

0 2 4 6 8 10 12 14 16

20

40

60

0 2 4 6 8 10 12 14 16

0

200

400

short -λ

amplitude

Distance (km)

Summed Frequency 

spectrum

1. Abstract

Magnetic field data are traditionally used to analyze igneous and metamorphic rocks, but recent efforts have shown that magnetic sources 

within sediments may be detectable, suggesting new applications for 

high-resolution magnetic field data. Candidates for such sources include heavy mineral sands transported from upland sources, biogenic 

magnetite, and certain forms of glauconite. Magnetic field surveys can 

be used to map distributions of these sediments with much denser and 

widespread coverage than possible by sampling, providing constraints 

on the composition, deposition, and transport of local sediments. However, as sedimentary sources are likely to be much weaker than their 

crystalline counterparts, mapping requires the sensor to be relatively 

close to the source, and filtering approaches are needed to distinguish 

signals from both system noise and deeper basement features. 

Marine geophysical surveys conducted in July, 2010, over the Stono and

North Edisto River inlets and their riverine inputs south of Charleston, 

S.C., used a total field cesium magnetometer and 900 kilohertz (kHz) 

sidescan sonar. In these tidally influenced estuarine environments, 

unconsolidated Quaternary sediments of 0–10 meters (m) thickness 

overlie an indurated Tertiary substrate. Calm survey conditions allowed 

the towing of sensors 2–20 m above the seabed with background magnetometer variations of less than 0.5 nanotesla (nT) and a measurement 

spacing of less than 0.4 m. Isolated anomalies associated with metallic 

objects such as crab pots or other anthropogenic debris were filtered in

part by comparisons to the high-resolution sonar. 

500 0 500 1,000 1,500

(meters)

-16 -10 -4 -3 -2 -1 -1 -0 1

Seafloor Depth (m)

32°36'

-80°16' -80°14'

500 0 500 1,000 1,500

(meters)

-16 -10 -4 -3 -2 -1 -1 -0 1

Seafloor Depth (m)

32°34'

-80°12'

2. Titanium in stream sediments 

Stream sediments collected and analyzed as part of the National Uranium Resource Evaluation (NURE) program show areas of enhanced Ti 

that are aligned with the metamorphic Blue Ridge and Piedmont provinces, outlined in black dashed lines (defined by Fenneman and Johnson, 

1946). Titanium-rich sediments most likely originate from these provinces 

and have been transported and reworked throughout Coastal Plain sediments. Data compiled by Smith (1997).

4. Shipboard geophysical surveys Shipboard sidescan sonar and magnetic surveys were conducted in tidal rivers and inlets near Charleston, S.C. Yellow 

lines (section 4) delineate ship tracks in areas A, B, and C. Filled circles (section 4, far left) denote stream sediment 

samples (see section 2).

Short-wavelength magnetic anomalies can be highlighted using spectral filtering (see section 8). Peaks in shortwavelength spectral amplitudes, shown as gray-filled profiles in section 5, represent concentrations of shallow 

magnetic sources. Anthropogenic sources typically produce single isolated anomalies, which are often associated 

with objects that can be seen in the sidescan data. 

Clusters of short-wavelength magnetic anomalies are concentrated in shallow areas usually at river bends where 

there is confluence of waters that originate upland. Sediments with stronger magnetic properties are usually composed of Fe- and Ti-rich minerals that are denser than their silicon (Si)- and calcium (Ca)-rich counterparts and can 

thus become concentrated through sorting, forming what are essentially small placer deposits. The insides of river

bends often represent depositional environments, but placer formation usually involves entrainment sorting, where 

lighter or less dense sediments are transported elsewhere. The presence of anomalies near the insides of river bends

suggests that these areas may undergo a balance between deposition and entrainment that works to concentrate heavy 

minerals.

The Kiawah River exhibits very few grouped magnetic anomalies. It is much younger than the Stono and North Edisto 

inlets, aligned coast-parallel and bounded on either side by oyster reefs. The Kiawah River is thus likely to receive a 

much lower concentration of Fe- and Ti-rich sediments from upland sources.

5 0 5

(kilometers)

32°40' 32°50'

-80°30' -80°

Charleston

A B

-16 -10 -4 -3 -2 -1 -1 -0 1

Seafloor Depth (m)

500 0 500 1,000 1,500

(meters)

32°38'

-80°4' -80°2' -80°

C

Kiawah River

Stono

8. Magnetic filtering via spectral amplitudes

Anomalies associated with shallow magnetic sources can be visualized by applying spectral filtering methods to individual shiptrack profiles. These methods 

highlight areas where short-wavelength or “narrow” anomalies, which represent 

shallow magnetic sources, are present regardless of longer wavelength features. 

The approach is described in detail by Shah and others (2012). Briefly, for each 

point along a ship track, the profile is windowed around that point and the frequency spectrum calculated for the corresponding sub-segment (windowing 

helps to prevent “leaking” from longer wavelength anomalies). Amplitudes corresponding to shorter wavelengths are then summed and the value assigned to that 

point. This sum is highest where short-wavelength anomalies are present. 

32°36'

-80°16' -80°14'

32°34'

-80°12'

32°38'

-80°4' -80°2' -80°

North 

North Edisto

Edisto

< 5,000

 5,000 - 10,000

10,000- 20,000

> 20,000

100 0 100 200

(kilometers)

1,700 3,800 5,500 8,900 24,600

Ti (PPM)

33° 34° 35° 36° 37° 38° 39°

-81° -80° -79° -78° -77° -76° -75°245

242 242

241 241

240 240 239

237 237

235 235

234 234

229 229

228 228

227 227

226 226 225 225

224

220

219

214 214

213 213

211 211

207

206 206 205 205

204 204

203 203

202 202

201 201

200 200

199 199 198 198 197 197

196 196195 195

178 179

178

177 177

176 176 175 175

174 174

173 173

172 172

159 159

158 158

157 157

156 156

153 153

152 152

146 146

143

140 140

138 138

136 136

135 135

134 134 133 133

129 129

121 121 120

119 119

109 109 108 108

95 95 94

86 85 85 84 84 83 83

SC

NC

VA

WV MD

DE

Piedmont Blue Ridge

Coastal 

Plain

Valley and 

Ridge

Study 

area

5. Magnetics—short-wavelength spectral amplitudes over seafloor depth

6. Sidescan sonar—900 kHz

-80.26 -80.24 -80.22

0

1,000

2,000

0

50

100

150

-80.26 -80.24 -80.22

Magnetic anomaly (nT) Summed short-wavelength 

spectral amplitudes

Example from the North Edisto River (Area A)

For this study, filtering parameters were set to highlight variations of about 

14-m width or less.

9. Models

Models can be used to estimate the thickness of 

magnetic source layers. If magnetic grains are 

not aligned, induced magnetization will cause a 

canonical 3-m thick layer to generate a 5 nT 

anomaly for source properties listed. Slight 

alignment of magnetic grains (presumably in the 

direction of Earth's field) generates remanent 

magnetization, potentially reducing the needed 

layer thickness to about 1 m. 

Anomaly (nT) 3

4

5

6

5.5 6.0 6.5 7.0

Distance (m) 

Depth (m) 6

8

10

Peak-totrough = 

3.4 nT

Magnetic source

Longitude (degrees)

0 2 4 6 8

Layer thickness (m)

Anomaly (nT)

0

2.5

5

7.5

10

2m

4m

6m

Source 

Depth

0 2 4 6 8

Layer thickness (m)

2m

4m

6m

Source 

Depth

Induced magnetization Remanent magnetization

Source = 1x10-3 SI Source = 0.05 A/m

(Basalt = 1–10 A/m)

5 0 5

(kilometers)

9

7

5

10

4

6

8

36.1

36 36 37

42

5

12

6.1 6.1

5.1

4.1

2.5

4.5

5.5

3.5

3

5

10

15

4

13.2 13.2

2.2

20

Edisto 

Island

A

B

C

Kiawah and

 Seabrook Islands

32°35' 32°40' 32°45'

-80°10' -80°

–900

–200

–100

–20

100

2,000

(nT)

Regional magnetic data were collected between 1958 and 

1978 using airborne methods from altitudes of about 150–450 m, 

with line spacing ranging from 800 m to 4,800 m (compiled by 

Daniels, 2005). Because magnetic anomalies attenuate with 

distance from the source, magnetic data collected at these 

altitudes typically represent sources with stronger magnetic 

properties such as igneous and metamorphic rocks that 

comprise the crystalline basement. Near Charleston, the 

“magnetic basement” represents Jurassic basalts at roughly 

1-kilometer (km) depth (Behrendt and others, 1983; Daniels 

and others, 1983). Weaker signals generated by sedimentary 

sources are not apparent in the aeromagnetic data.

White lines represent shipboard magnetic surveys; black 

lines delineate the coastline and waterways.

3. Regional magnetic field

1

2

3

0 1 -

0

0

3

1.7 km

0

0

0

-10

2

1.65 km

T?

Stream sediment Ti 

(ppm)

Spectral processing to highlight short-wavelength magnetic anomalies 

revealed 10- to 40-m-wide, 1- to 6-nT magnetic anomalies associated 

with shallow, sand-covered seabed as indicated by the sidescan sonar 

data. These anomalies are distinct from system noise but are too 

narrow to represent basement features. The anomalies are observed 

mostly in shallow areas where river sediments originating from upland 

areas enter the inlets. In contrast, the anomalies are not observed near 

or within the younger, coast-parallel Kiawah River, where sedimentary 

inputs are primarily from closer surrounding areas. 

Previous studies do not indicate biogenic magnetite or glauconite 

within the strata underlying the survey area, but rivers and reworked 

Pleistocene barrier island sands supplying the Charleston area inlets 

originate from within the metamorphic Piedmont and Blue Ridge terranes. Grab samples from the North Edisto River contain trace amounts 

of hematite, maghemite, ilmenite, and magnetite, as well as garnet, 

epidote, zircon, and rutile. If the sands are sorted by prevailing currents,

greater concentrations of heavy mineral sands are likely to reside a 

few centimeters below the seafloor and could generate the observed 

magnetic anomalies. The presence of anomalies in inlets connected to 

upland areas and the distribution of stream sediment titanium (Ti) over 

the Atlantic Coastal Plain suggest that the magnetic anomalies are 

generated by Ti- and iron (Fe)-rich heavy mineral sands ultimately originating from the Piedmont and Blue Ridge, which are then reworked 

and concentrated by tidal currents. The distribution of these sands 

within each of the inlets reflects the local hydrodynamic environment. 

10. References

Behrendt, J.C., Hamilton, R.M., Ackerman, H.D., Henry, V.J., and Bayer, K.C., 1983, Marine multichannel seismicreflection evidence for Cenozoic faulting and deep crustal structure near Charleston, South Carolina: U.S. Geological 

Survey Professional Paper 1313–J, 29 p.

Daniels, D.L., 2005, South Carolina aeromagnetic and gravity maps and data: A web site for distribution of data: U.S. 

Geological Survey Open-File Report 2005–1022. 

Daniels, D.L., Zietz, Isidore, and Popnoe, Peter, 1983, Distribution of subsurface lower Mesozoic rocks in the southeastern United States, as interpreted from regional aeromagnetic and gravity maps: U.S. Geological Survey Professional 

Paper 1313–K, 24 p.

Fenneman, N.M., and Johnson, D.W., 1946, Physiographic divisions of the conterminous U.S.: U.S. Geological Survey, 

scale 1:7,000,000, online polygon coverage available from http://water.usgs.gov/lookup/getspatial?physio.

Shah, A.K., Vogt, P.R., Rosenbaum, J.G., Newell, Wayne, Cronin, T.M., Willard, D.A., Hagen, R., Brozena, J.M. and

Hofstra, Al, 2012, Shipboard magnetic field “noise” reveals shallow heavy mineral sediment concentrations in

Chesapeake Bay: Marine Geology, v. 303–306, p. 26–41.

Smith, S.M., 1997, National Geochemical Database: Reformatted data from the National Uranium Resource Evaluation 

(NURE) Hydrogeochemical and Stream Sediment Reconnaissance (HSSR) Program, version 1.40 (2006): U.S. 

Geological Survey Open-File Report 97–492.

1U.S. Geological Survey, Denver, Colo.

2College of Charleston, Charleston, S.C.

Suggested citation: Shah, A.K., and Harris, M.S., 2012, Shipboard surveys track magnetic 

sources in marine sediments—Geophysical studies of the Stono and North Edisto Inlets near 

Charleston, South Carolina: U.S. Geological Survey Open-File Report 2012–1112.

This report is available at: http://pubs.usgs.gov/of/2012/1112. 

For more information about this publication, contact:

Center Director

USGS Crustal Geophysics and Geochemistry Science Center

Box 25046, MS 964

Denver, CO 80225

Or visit the Crustal Geophysics and geochemistry Science Center Web site at: 

http://crustal.cr.usgs.gov. 

To learn more about the USGS and its information products, 

visit http://www.usgs.gov, or call 1-888-ASK-USGS.

Acknowledgments: 

We thank Robin Bell and Frank Nitsche of the Lamont-Doherty Earth Observatory for use of their 

marine magnetometer. We also thank College of Charleston students Katie Luciano and Harris 

Pantlik for their help in the field. Dick Tripp, USGS, conducted separation and petrography of grab 

sample trace minerals. 

Seafloor depth data are from the National Geophysical Data Center Coastal Relief Model. Onshore 

topography data are from NASA’s Shuttle Radar Topography Mission. Satellite Imagery is from 

Microsoft’s Virtual Earth Server. 

This effort was funded by the USGS Minerals Program (Large Mineralized Systems Task) and the 

College of Charleston Department of Geology and Environmental Geosciences. 

Portions of this report were presented at the Fall 2010 Meeting of the American Geophysical Union.

Although this information product, for the most part, is in the public domain, it also contains copyrighted materials as noted in the text. Permission to reproduce copyrighted items for other than 

personal use must be secured from the copyright owner. Any use of trade, product, or firm names is 

for descriptive purposes only and does not imply endorsement by the U.S. Government.

Charleston

7. Anomalies and sediment type

The sidescan sonar data, when ground-truthed using grab samples from 

Area A, show dark (less reflective; lower acoustic backscatter) areas associated with finer-grained sediments; lighter (more reflective; higher acoustic 

backscatter), smooth areas associated with medium- to coarse-grained sands; 

and hatched areas in the deeper channels that are associated with very 

coarse sands and gravels. The hatches delineate ripples created by stronger 

tidal currents, and the direction of these ripples varies since some areas were 

surveyed at different points in the tidal cycle. 

Short-wavelength magnetic anomaly clusters, outlined in green (right), are 

mostly observed in areas representing medium- to coarse-sands. Exceptions 

include anomalies observed over some finer-grained sands and an absence of 

anomalies over possible Tertiary outcrop (panel 2, labeled “T”). White lines 

represent 2-m depth contours. Locations of these data are shown in section 6.

A B

C

Notes: Models assume Earth’s local field: amplitude = 48898.5 nT, declination = –7°, inclination = +61.5°. 

Source properties are based on sand samples from Chesapeake Bay (Shah and others, 2012).