Reconnaissance Borehole Geophysical, Geological, and 

Hydrological Data from the Proposed Hydrodynamic 

Compartments of the Culpeper Basin in Loudoun, Prince 

William, Culpeper, Orange, and Fairfax Counties, Virginia 

[Version 1.0] 

By Michael P. Ryan, Herbert A. Pierce, Carole D. Johnson, David M. Sutphin, David L. Daniels, 

Joseph P. Smoot, John K. Costain, Cahit Çoruh, and George E. Harlow 

Open–File Report 2006-1203 

U.S. Department of the Interior 

U.S. Geological Survey 
Reconnaissance Borehole Geophysical, Geological, and 

Hydrological Data from the Proposed Hydrodynamic 

Compartments of the Culpeper Basin in Loudoun, Prince 

William, Culpeper, Orange, and Fairfax Counties, Virginia 

[Version 1.0]

By Michael P. Ryan, Herbert A. Pierce, Carole D. Johnson, David M. Sutphin, David L. Daniels, 

Joseph P. Smoot, John K. Costain, Cahit Çoruh, and George E. Harlow

Open-File Report 2006–1203

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Meyers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Ryan, Michael P., Pierce, Herbert A., Johnson, Carole D., Sutphin, David M., Daniels, David L., Smoot, Joseph P., 

Costain, John K., Çoruh, Cahit, and Harlow, George E., 2007, Reconnaissance borehole geophysical, geological and 

hydrological data from the proposed hydrodynamic compartments of the Culpeper Basin in Loudoun, Prince William, 

Culpeper, Orange and Fairfax Counties, Virginia. [Version 1.0]: U.S. Geological Survey Open File Report 2006-1203, 

43p.

2001051109

ISBN 978-1-4113-2031-4
Contents

Abstract and Summary of Select Structural Results Within a Culpeper Basin Context....................1

Introduction.....................................................................................................................................................2

Motivation, Context and Purpose................................................................................................................2

Organization....................................................................................................................................................2

Geophysical and Geological Setting ..........................................................................................................3

Digital Topography of the Culpeper Basin ........................................................................................3

Aeromagnetic Anomalies Originating From Within and Beneath

the Culpeper Basin..................................................................................................................3

Long-wavelength Anomalies and the Basin Substructure...................................................3

Short-wavelength Anomalies and Diabase Intrusions..........................................................7

Regional Seismic Reflection Structure of Culpeper Basin Sediments

and the Basin-Basement Contact ........................................................................................7

Proposed Compartmentalization of the Culpeper Basin .........................................................................9

Basin Compartmentalization by Intrusive Complexes: Diabase Septa

and their Function ...................................................................................................................9

A Concise Summary of Relationships Suggesting Basin

Compartmentalization...........................................................................................................10

Scientific Objectives and Borehole Field Experiment Strategy ...........................................................11

The Proposed Dulles Compartment..........................................................................................................14

Balls Bluff Member of the Bull Run Formation ..............................................................................14

Geophysical Logging in the Proposed Dulles Compartment .......................................................15

The Proposed Gainesville Compartment .................................................................................................15

Direct Drilling into the Gainesville Compartment ..........................................................................15

Geophysical and Geological Logging Through the Proposed Gainesville

Compartment Margins and into the Compartment Interior............................................18

Structure of the Proposed Gainesville Compartment...................................................................22

The Proposed Rapidan Compartment: Deep Structure of the Rapidan Diabase Lopolith...............22

Geophysical Logging in the Proposed Rapidan Compartment....................................................23

The U.S. Geological Survey National Center and the Structural Eastern

Margin of the Culpeper Basin ......................................................................................................23

Summary and Conclusions.........................................................................................................................33

Acknowledgments.......................................................................................................................................33

Appendix I .....................................................................................................................................................34

Culpeper Basin, Virginia Borehole Tools ........................................................................................34

Appendix II....................................................................................................................................................35

Locations of Culpeper Basin Wells for the Geophysical Borehole Survey...............................35

Appendix III...................................................................................................................................................36

Culpeper Basin Borehole Data Files................................................................................................36

Appendix IV...................................................................................................................................................36

Culpeper Basin, Virginia Borehole Location Index Maps and

Table: Site Access and Well Location Logistics ..............................................................36

Appendix V....................................................................................................................................................36

A Glossary of Selected Terms...........................................................................................................37

References Cited..........................................................................................................................................41

iii
Figures

1. Digital terrain map of the Culpeper and Barboursville basins

of Virginia and Maryland. ............................................................................................................4

2. Digital terrain map of Loudoun County, Virginia..............................................................................5

3. Aeromagnetic map of the Culpeper and Barboursville basins,

showing magnetic anomalies and major geologic and

physiographic provinces .............................................................................................................6

3A. Short wavelength aeromagnetic anomalies in the Herndon region

of the Culpeper Basin...................................................................................................................8

3B. Short wavelength aeromagnetic anomalies in the Centreville region

of the Culpeper basin ...................................................................................................................9

4. Synthesized synoptic structural cross-section across the Culpeper Basin,

based on reprocessed seismic reflection .....................................................................10

5A and 5B Borehole geophysical survey locations within the northern (A)

and southern (B) Culpeper basin superposed on principal geologic units.................12-13

6. Geophysical logs from Dulles well No. 2 at the Washington Dulles

International Airport...................................................................................................................16

7. Geophysical logs from Dulles well No. 3 at the Washington Dulles

International Airport...................................................................................................................17

8. The America On-Line (AOL) Gainesville, Virginia well showing diabase

in the upper portion of the well and hornfels in the lowest portion...................................19

8A. WellCAD image of the 164 to 174 m interval in the AOL well

at Gainesville, Virginia ...............................................................................................................20

8B. WellCAD image of the 207 to 217m depth interval for the AOL well

at Gainesville, Virginia ...............................................................................................................21

9. Geophysical logs from Coffeewood Observation well No. 1.......................................................24

10. Geophysical logs from Coffeewood Observation well No. 2.......................................................25

11. Geophysical logs from Coffeewood Observation well No. 3.......................................................26

12. Geophysical logs from Coffeewood Observation well No. 4.......................................................27

13. Geophysical logs from Coffeewood Observation well No. 5.......................................................28

14. Geophysical logs from the extreme eastern edge of the Culpeper basin

at the U.S. Geological Survey National Center observation well .......................................30

15. WellCAD logs from the USGS well 52V-2D on the U.S. Geological Survey

National Center campus near Sunrise Valley Drive in Reston, Virginia............................31

16. Heat pulse flow meter logs for USGS well 52V-2D at the National Center...............................32

Tables

Table AII-1. Locations and drilling data for Culpeper basin and wells .............................................36

iv
Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

square foot (ft

2

) 929.0 square centimeter (cm

2

)

square foot (ft

2

) 0.09290 square meter (m

2

)

square inch (in

2

) 6.452 square centimeter (cm

2

)

square mile (mi

2

) 259.0 hectare (ha)

square mile (mi

2

) 2.590 square kilometer (km

2

)

Volume

ounce, fluid (fl. oz) 0.02957 liter (L)

pint (pt) 0.4732 liter (L)

quart (qt) 0.9464 liter (L) 

gallon (gal) 3.785 liter (L)

gallon (gal) 0.003785 cubic meter (m

3

)

gallon (gal) 3.785 cubic decimeter (dm

3

)

cubic inch (in

3

) 16.39 cubic centimeter (cm

3

)

cubic inch (in

3

) 0.01639 cubic decimeter (dm

3

)

cubic inch (in

3

) 0.01639 liter (L)

cubic foot (ft

3

) 28.32 cubic decimeter (dm

3

)

cubic foot (ft

3

) 0.02832 cubic meter (m

3

)

cubic yard (yd

3

) 0.7646 cubic meter (m

3

)

cubic mile (mi

3

) 4.168 cubic kilometer (km

3

)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

foot per minute (ft/min) 0.3048 meter per minute (m/min)

foot per hour (ft/hr) 0.3048 meter per hour (m/hr)

foot per day (ft/d) 0.3048 meter per day (m/d)

foot per year (ft/yr) 0.3048 meter per year (m/yr)

cubic foot per second (ft

3

/s) 0.02832 cubic meter per second (m

3

/s)

cubic foot per day (ft

3

/d) 0.02832 cubic meter per day (m

3

/d)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m

3

/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m

3

/s)

Mass

ounce, avoirdupois (oz) 28.35 gram (g)

pound, avoirdupois (lb) 0.4536 kilogram (kg)

Pressure

atmosphere, standard (atm) 101.3 kilopascal (kPa)

bar 100 kilopascal (kPa)

inch of mercury at 60oF (in. Hg) 3.377 kilopascal (kPa)

pound-force per square inch

(lbf/in

2

)

6.895 kilopascal (kPa)

pound-force per square foot

(lbf/ft

2

)

0.04788 kilopascal (kPa)

Density

pound per cubic foot (lb/ft

3

) 16.02 kilogram per cubic meter (kg/m

3

)

pound per cubic foot (lb/ft

3

) 0.01602 gram per cubic centimeter (g/cm

3

)

Energy

kilowatthour (kWh) 3,600,000 joule (J)

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L)

v
Specific capacity

gallon per minute per foot

[(gal/min)/ft)]

0.2070 liter per second per meter [(L/s)/m]

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)

Application rate

Leakance

foot per day per foot [(ft/d)/ft] 1 meter per day per meter

inch per year per foot [(in/yr)/ft] 83.33 millimeter per year per meter

[(mm/yr)/m]

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F–32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).”

Altitude, as used in this report, refers to distance above the vertical datum.

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

vi
Reconnaissance Borehole Geophysical, Geological, and 

Hydrological Data from the Proposed Hydrodynamic 

Compartments of the Culpeper Basin in Loudoun, Prince 

William, Culpeper, Orange, and Fairfax Counties, Virginia 

[Version 1.0]

By Michael P. Ryan, Herbert A. Pierce, Carole D. Johnson, David M. Sutphin, David L. Daniels, 

Joseph P. Smoot, John K. Costain, Cahit Çoruh, and George E. Harlow

Abstract and Summary of Select 

Structural Results Within a Culpeper 

Basin Context

A range of geophysical borehole tools have allowed 

the measurements of both fluid and petrophysical properties 

of fluid-saturated sedimentary and igneous rocks within an 

eastern North American rift zone basin. These measurements 

include: the fluid temperature-depth function (T − Z ) and the 

first spatial derivative of temperature, ( DT ); the formation DZ

electrical conductivity (s formation ) and resistivity (r formation ); the 

fluid conductivity (s fluid ) and resistivity (r fluid ); the natural 

gamma count (G) and the variations in borehole diameter 

(Dr) via caliper data. In addition, Optical TeleViewer (OTV) 

and Acoustic TeleViewer (ATV) logging, as well as heat pulse 

and E-M flowmeter logging has also been conducted. Not all 

boreholes discussed in this report were logged with the entire 

suite of tools.

The resulting program of borehole geophysical surveys has been conducted in the Mesozoic Culpeper basin of 

Virginia. Ryan and others, (2007a, b), present the primary 

evidence that the Culpeper basin is hydrodynamically compartmentalized. The results of this report are both consistent 

with the compartmentalization concept, and offer further 

elaboration of how compartments are internally structured. 

While complimenting the reports cited above, these results 

do not, however, provide definitive evidence for the proposed 

compartmentalization concept. The principal results of individual surveys are organized on a structural basis. The Dulles 

compartment. Two wells have been logged, and both are sited 

holly within the Balls Bluff Siltstone. In Dulles Well No. 2 

total depth = 289 m) thirty three spikes in the first derivative DT

w

(

of temperature with respect to depth, DZ , are consistent with 

the inflow of groundwater from a multiplicity of fractures 

that intersect the borehole. The spikes are unevenly distributed with depth, however they are broadly consistent with 

the spacing between bedding plane fractures within the Balls 

Bluff Member of the Bull Run Formation. The Gainesville 

compartment. Direct drilling has been conducted through the 

Gainesville diabase sheet into the subjacent hornfels (thermally metamorphosed after Balls Bluff Member siltstone). 

The diabase-hornfels interface (at a depth of 183 m) represents a basal flow regime where the hornfels comprises the 

permeable top of the Gainesville compartment confined by 

the diabase above. Total hole depth is 226 m. Optical TeleViewer (OTV) logging reveals that the transition from diabase 

to hornfels is characterized by moderate fracturing within the 

diabase and a massively fractured hornfels beneath. Driller’s 

pumped flow rates within the diabase are ~2 gallons per 

minute (gpm), whereas flow rates within the hornfels at the 

compartment top are ~110+ gpm. These results offer further 

evidence supporting the compartmentalization concept. In this 

context, basal flow occurs within the hornfels that immediately underlies the less permeable diabase confining unit. The 

Rapidan compartment. Five boreholes at the Coffeewood 

Correctional Center near Mitchells, Virginia have been logged 

to determine the relationships between the hornfels (thermally 

metamorphosed after Balls Bluff Member siltstone) and the 

diabase beneath. The transition from hornfels to diabase (the 

structure of the compartment base) is particularly notable in 

the natural gamma counts where, in well OBS–5, a complete 

disappearance of gamma counts occurs at a depth of 107.5 m. 

Paleoflow inferences include the marked increases that occur 

in calcite vein frequency between 72 and 104 m. Calcite veins 

die out, however, dramatically above 70 m suggesting that the 

lower 32 m of hornfels immediately above the diabase experienced intense hydrothermal circulation during the cooling 

of the lopolith. Pre-logging pumping tests conducted in well 

OBS–4 proximal to the underlying Rapidan diabase lopolith 

induced exceptional drawdown suggesting the presence of a 
low permeabilty barrier (diabase). These results are entirely 

consistent with—and can be best explained by—the compartmentalization of Rapidan fluids. The structural eastern 

margin of the Culpeper basin. The eastern edge of the basin 

displays an on-lap relationship between the Poolesville Member of the Manassas Sandstone and the Proterozoic basement 

schists. The basin-basement contact and the overlying sediments dip about 10 degrees westward. Interbedded sandstones 

and siltstones comprise the Upper Triassic Manassas Sandstone, and these grade downwards into the conglomerate of the 

Reston Conglomerate (forming the base below the Manassas 

Sandstone). The Reston Conglomerate lies unconformably on 

the partially weathered Proterozoic schists of the underlying 

basement. In well 52V–2D, pumping tests induced inflow to 

the well bore from three separate depth levels resulting in a net 

upward flow polarity for the well. Sites of inflow have been 

correlated with bedding plane fractures within the Manassas 

Sandstone. This well has a transmissivity, T, of 44 x 10-4 to 47 

x 10-5 ft2/day and a storativity, S, of 1 x 10-4 to 6 x 10-5.

Introduction

The Culpeper basin is part of a much larger system of 

ancient depressions or troughs, that lie inboard of the Atlantic Coastal Plain, and largely within the Applachian Piedmont Geologic Province of eastern North America, and the 

transition region with the neighboring Blue Ridge Geologic 

Province. This basin system formed during an abortive attempt 

to make a great ocean basin during the Late Triassic and Early 

Jurassic, and the eroded remnants of the basins record major 

episodes of sedimentation, igneous intrusion and eruption, and 

pervasive contact metamorphism. Altogether, some twenty 

nine basins formed between what is now Nova Scotia and 

Georgia. Many of these basins are discontinuous along their 

strike, and have therefore recorded isolated environments for 

fluvial and lacustrine sedimentation.

Several basins (including the Culpeper, Gettysburg, and 

Newark basins) are fault-bounded on the west, and Mesozoic 

crustal stretching has produced assymetrical patterns of basin 

subsidence resulting in a progressive basin deepening to the 

west, and a virtual onlap relationship with the pre-basin Proterozoic rocks to the east. A result of such a pattern of basin 

deepening is the development of sequences of sandstones and 

siltstones that systemmatically increase in dip towards the 

accomodating western border faults. A second major structural theme in several of the major Mesozoic basins (including 

the Culpeper) concerns the geometry of igneous intrusion, as 

discussed below. Froelich (1982, 1985) and Lee and Froelich 

(1989) discuss the general geology of the Culpeper basin, and 

Smoot (1989) discusses the sedimentation environments and 

sedimentary facies of the Mesozoic with respect to fluvial and 

shallow lacustrine deposition in the Culpeper basin. Ryan and 

others, 2007a, b, discuss the role of diabase-induced compartmentalization in the Culpeper basin (and other Mesozoic 

basins), and illustrate (using alteration mineral suites within 

the diabase and adjacent hornfels, among other evidence) how 

this process has played a role in organizing the paleo- and contemporary-flow of crustal fluids at local and regional scales. 

Within this report, the Newark Supergroup nomenclature of 

Weems and Olsen (1997) is adopted.

Motivation, Context and Purpose

We report here on the initial results from borehole geophysical surveys within the Culpeper basin. This is the first 

phase of a larger effort to compare and contrast the regionalto-local structural and lithologic features of Culpeper basin 

rocks that promote (or inhibit) the storage and transmission of 

groundwater. The selection of well locations was guided by 

these factors: (i) position within geologic units representing 

mayor sedimentary and/or structural features within the basin; 

(ii) appreciable depth of well, such that a significant sampling 

interval was possible; (iii) reasonable geographic distributions 

of wells; and (iv) boreholes within a significant ‘basin-structural’ and/or ‘cultural’ context. In this last category, the wells 

at the U.S. Geological Survey National Center campus sample 

the extreme eastern edge structure of the basin, whereas wells 

within the Dulles International Airport are positioned within a 

central region of the airport and the proposed Dulles hydrodynamic compartment. The purpose of the report is to develop a 

geological, petrophysical and meter-scale hydrological picture 

of the sandstone and siltstone contents of the basin’s hydrodynamic compartments, the hornfels that comprises the compartment margins, and the diabase that comprises the compartment 

seals.

The purpose of the report is not, however, to establish 

the basis for the proposed compartmentalization concept. 

That basis has been presented in abstracts (Ryan and others, 

1997, 2000, 2002) and is presented in two related companion 

papers by Ryan and others (2007a, b), among other reports, 

although we do briefly summarize certain aspects of that basis 

here. Several of the borehole results of this report, however, 

are very consistent with—and are optimally explained by the 

compartmentalization mechanism. Therefore our results are 

placed within the compartmentalization context, within which, 

we think, they can be better understood. It is expected that the 

reports of Ryan and others, 2007a, b may be read in conjunction with this report. In addition, the report of Ryan and others, 

(2007c) presents magnetotelluric (MT) survey results at basinscale that describe the cross-sectional geometry of the basin 

and intra-basin compartments defined by diabase bodies.

Organization

We have tried to prepare a report that will be of use to 

those with backgrounds in geology, geophysics and hydrology. 

So that the report can be more accessable to this larger community, results are first prefaced by introductory summaries on 

aspects of regional geology and geophysics. These summaries 

 Reconnaissance Borehole Geophysical, Geological, and Hydrological Data, Culpeper Basin, Virginia
include the areal geology of the Culpeper basin, the digital 

topography, regional aeromagnetic anomaly distributions and 

the limited structural inferences drawn from seismic reflection surveys. In these summaries, we attempt to paint a picture 

of our emerging image of the three-dimensional structure 

of the Culpeper basin. A brief summary of the proposed 

compartmentalization concept next illustrates our model of 

how and why fluids are stored and migrate in the subsurface, 

highlighting some of the principal evidence for the concept, 

as summarized from Ryan and others (2007a, 2007b). Next, 

the report discusses the borehole logging data on a compartment-by-compartment basis, including the proposed Dulles-, 

Gainesville- and Rapidan-compartments. Then, a summary of 

logging options is included in appendix I. Tables and graphs 

of borehole data for each locale are presented in appendix II. 

Culpeper basin borehole data file downloading instructions 

are given in appendix III. Access maps for each of those wells 

logged are included in Appendix IV on field logistics. Finally, 

appendix V provides a brief glossary of selected terms. 

Geophysical and Geological Setting

Digital Topography of the Culpeper Basin.

The physiographic grain of eastern North America is 

dominated by the Appalachian Mountains, the Blue Ridge 

Geologic Province, the Piedmont Geologic Province, and their 

relationships with the Atlantic Coastal Plain and the Atlantic 

Ocean coastline. Figure 1 illustrates several of these relationships, and sets the Culpeper and Barboursville basins within 

the topography of the surrounding Piedmont. The grid for 

this view was assembled from 144 USGS 7.5 minute Digital 

Elevation Model (DEM) files. Terrain values were spaced at 

30 meters. Oasis Montaj software by GeoSoft was used to 

create the grid and the display of the terrain and the related 

aeromagnetic map of this report. Terrain colors were assigned 

by a histogram equalization distribution function in which 

all color intervals receive approximately equal display area. 

Boundary lines for the Culpeper and Barboursville Early 

Mesozoic basins are shown as white lines (the center of fig. 1) 

and for the Atlantic Coastal Plain (the eastern part of the figure). The Culpeper basin is clearly lower in elevation than the 

surrounding Piedmont crystalline terrain. The highest topography within the basin is mostly along the western border 

fault. Elevations range from 370 to 60 meters above sea level 

across the basins. The surface and near-surface hydrology is 

influenced by flow down—or within—a regional hydraulic 

gradient that starts in the highlands of the bordering western 

mountains, and migrates towards the east and north (Laczniak 

and Zenone, 1985). Superposed on this regional trend are the 

strong influences of recharge and local and sub-regional drainage. Frequently the local groundwater divides are topographic 

highs supported by the resistant diabase bodies that form 

low permeability compartment walls at depth. The surface 

drainage from the Culpeper basin follows at least five major 

pathways eastward. The fine texture of parallel lines in the 

west-central part of the Culpeper basin (see fig. 1) is an area 

of low parallel ridges produced by the differential erosion 

of the upturned edges of sediments laid down in ancient, but 

now west-dipping lake beds, a pattern well-exposed along the 

Dulles Greenway between Dulles International Airport and 

the town of Leesburg, Virginia. The pattern is truncated by the 

smoother terrain over exposed diabase. The prominent lineament in the southwest corner of the map, southeast of these 

two basins, is the expression of the Mountain Run Fault.

The digital topography of the Loudoun County portion of 

the Culpeper basin is presented in figure 2. Figure 2 illustrates 

the contrasts in topography between the western highlands and 

the lowland regions of the basin, highlighting the distribution 

of diabase bodies, and the dendritic stream networks that drain 

the interior of the northern part of the basin.

Aeromagnetic Anomalies Originating from 

Within and Beneath the Culpeper Basin

Long-wavelength Anomalies and the Basin 

Substructure

Figure 3 illustrates the aeromagnetic anomalies of the 

Culpeper and Barboursville basins and surroundings, from the 

Blue Ridge Geologic Province eastward through the basins to, 

and sometimes beyond, the western margins of the Atlantic 

Coastal Plain. The grid of aeromagnetic data for the Culpeper 

basin map is composed of information from 20 separate 

airborne surveys acquired by the USGS and Virginia Division 

of Mineral Resources, and flown between 1948 and 1989 with 

widely varying flight line spacing.

Magnetic anomalies within the basin boundaries originate 

from two primary rock sources:

1) Intra-basin early Mesozoic diabase intrusions, with 

additional anomalies produced by associated thermally-metamorphosed sediments (hornfels) adjacent to the diabase, and 

by basalt flows of the same age; and

2) Paleozoic-to-Precambrian metamorphic rocks that 

form the crystalline basement beneath the basin.

Many of the basin-basement anomalies are continuous 

with those anomalies produced by exposed rock units outside 

the basin, thus permitting the source rocks to be identified. 

Several of these rock units produce anomalies with much 

larger amplitudes than the intra-basin anomalies. Where the 

two sets of anomalies are superimposed, separation may be 

possible based on wavelength. Long-wavelength anomalies are 

typically linked to deeper regional features in the Proterozoic 

basement, such as the Catoctin Formation, the Mather Gorge 

Formation and the Sykesville Formation.

Prominent anomalies are derived from low-oxidation 

state, magnetite-bearing metabasalts of the Catoctin Formation 

that are exposed in both limbs of the Blue Ridge anticlinorium. 

More oxidized, non-magnetic, hematite-bearing metabasalts 

Geophysical and Geologial Setting 
Figure 1. Digital terrain map of the Culpeper and Barboursville 

basins of Virginia and Maryland. The basin boundaries are outlined in white, with the relatively large Culpeper basin to the north 

of the more diminutive Barboursville. The position of the Atlantic 

Coastal Plain is the sutured-appearing white line in the southeast 

quadrant. The Blue Ridge Mountains dominate the landscape to 

the west and northwest. Major drainages are shown as deep blue 

dendritic patterns, and the Potomac River passes from northwest 

to southeast through the northern end of the basin. Borehole 

geophysical survey locations are indicated by yellow dots.

 Reconnaissance Borehole Geophysical, Geological, and Hydrological Data, Culpeper Basin, Virginia
Figure 2. Digital terrain map of Loudoun County, Virginia. 

Outlines of the northern margin of the Culpeper basin are 

given in blue, whereas the outlines of the principal diabase 

bodies (central and eastern side of basin) are given in red 

(db). The diabase bodies are relatively low in permeability and 

effectively compartmentalize their surrounding sediments and 

metasediments with respect to fluid flow and storage. The 

sinusoidal contours of the northern margins of Loudoun County 

are defined by (and coincident with) the Potomac River (not 

labeled). Dendritic stream and river patterns in green illustrate 

the regional flow from west to east as well as into the Potomac 

River. The maroon spine of the northern Bull Run Mountains and 

Catoctin Mountain separates the Culpeper Basin in the eastern 

half of the figure from the Proterozoic geology in the west. Overall, 

relief spans the range 370 to 60 meters above mean sea level. 

Yellow dots denote the locations of borehole geophysical surveys 

at Dulles International Airport (western group of two locations) 

(D2, D3) and the U.S. Geological Survey National Center (NC2) 

(eastern-most location near the eastern margins of the Herndon 

diabase lopolith).

Geophysical and Geological Setting 
 Reconnaissance Borehole Geophysical, Geological, and Hydrological Data, Culpeper Basin, Virginia
Figure 3. Aeromagnetic map of the Culpeper and 

Barboursville basins, showing the relationships between 

the magnetic anomalies and the major geologic and 

physiographic provinces of the region. Long wavelength 

anomalies include the 250 to 500 nT signature of the Catoctin 

metabasalt, that passes obliquely through the basement 

of the Culpeper basin, and lies just to the southeast of the 

Barboursville basin at those latitudes. An additional long 

wavelength anomaly is produced by the Mather Gorge 

Formation, which strikes NE-SW, and passes through the 

Culpeper basin basement in its east-central sector. The 

boundaries of the District of Columbia are given by the 

diamond-shaped box near the middle of the right-hand side 

of the figure. Yellow dots denote the locations of borehole 

geophysical survey locations. Boxes labled “A” and “B” 

denote the limits of figures 3A and 3B.

may make up more than half of the formation, depending on 

the area. Gravity is high over both classes of the metabasalt. 

The southeastern limb of this fold system lies west of the basin 

in the northern part of the area but crosses completely beneath 

the southwestern part of the Culpeper basin and re-appears on 

the southeast side of the basin.

Large-amplitude magnetic anomalies along the eastern 

edge of the basin reflect a broad band of quartz-rich metasediments in the Mather Gorge Formation and in the associated 

mélange of the Sykesville Formation; both of which may be 

Paleozoic in age (S. Southworth, pers. Comm.). The high 

magnetite content of these metasedimentary rocks is highly 

unusual and perhaps implies a unique chemistry. Large lenticular ultramafic bodies that are variably magnetic, are associated with both units. The lithic fragments within the mélange 

unit include Mather Gorge rocks, metavolcanic schists, 

metagabbro, and ultramafics (Drake and Lee, 1989). These 

formations have had a complex structural and metamorphic 

history. This complexity in history is reflected in the pattern 

of magnetic anomalies in the Maryland and Northern Virginia 

Piedmont.

Short-wavelength Anomalies and Diabase 

Intrusions

The diabase intrusions that compartmentalize subsurface 

flow regimes have tight spatial signatures on aeromagnetic 

anomaly maps. Typically, the short-wavelength intra-basin 

anomalies are associated with diabase lopoliths, major diabase 

dikes, and prominent sheet-like diabase bodies. Examples are 

the anomalies associated with the Herndon and Centreville 

diabase lopoliths illustrated in figures 3A and 3B, respectively. In these figures, they appear as great horseshoe-shaped 

anomalies that ‘open’ toward the north. Both the Herndon and 

Centreville bodies have a ‘concave-up’ geometry and are suggested to compartmentalize subsurface flow in three ways: by 

providing a low premeability floor to their sedimentary in-fill; 

by providing a low permeability ‘ceiling’ to the sediments 

beneath, thus promoting basal flow, and by providing a low 

permeability barrier to regional horizontal flow, thus bounding 

compartments that are several kilometers in scale. The ‘Dulles 

compartment’ is one such example. It measures about 15 km 

(N-S) and is bounded by the Herndon lopolith on the north and 

the Centreville lopolith on the south (Pierce and Ryan, 2003; 

Ryan and others, 2007a).

Regional Seismic Reflection Structure of 

Culpeper Basin Sediments and the BasinBasement Contact

Two Virginia Tech seismic reflection lines (VT–1, the 

eastern line; and VT–2, the western line) have sampled the 

Culpeper basin’s lithology and structure and have been combined to produce an (incomplete) overall transect that cuts 

much of the basin mid-section along a roughly WNW-ESE 

azimuth. Details of instrumentation and data processing may 

be found in Ryan and others, 2007a, however we summarize 

the results within the context of the present borehole geophysics survey. These two reflection lines have provided a structural context for understanding the variably-dipping sedimentary structures that comprise the fluid storage compartments 

for the Culpeper basin. The wells discussed in this report 

penetrate the upper strata of these deep compartments.

The two 2-D seismic reflection lines (VT–1 and VT–2) 

have been combined by projecting them onto a plane that is 

parallel to, but intermediately-positioned between them. When 

so projected, there is a 5 km gap between the west end of Line 

1 and the east end of Line 2. This composite image is presented in figure 4, where the gap between the lines is labeled 

‘magma ascent and diabase emplacement interval’. The basincenter ends of both VT–1 and VT–2 were terminated due to 

the proximity of the vertically-oriented energy-adsorbing diabase bodies that core the basin’s central axis. Within the basin, 

the progressive and systematic changes in the dips of sedimentary rocks is clearly evident: dips on the order of 10 degrees in 

the east (right side) transition to dips of as much as 32 degrees 

in the west (left side). The systematic east-to-west progressive 

increase in dip has been confirmed by direct field measurements in exposed sections. The basin-basement contact is also 

clearly evident, and in the west it is broadly consistent with a 

listric-like continuity of the basin basement reflector with the 

western border fault and, in the east, with gentle slopes that 

are compatible with an “on-lap” relationship between the basin 

lithologies and the surrounding and underlying Proterozoic 

rocks. In the Proterozoic rocks beneath the basin, bright reflectors have defined the layering within two broadly anticlinal 

structures, whose fold amplitudes are on the order of 1 second 

of two-way travel time, and whose wavelengths are roughly 

8 to 10 km. These correspond to the Catoctin Formation 

metavolcanics in the west, and to the broad anticlinal structure 

centered over the 17 km marker point in the east. This eastern-most antiform structure is generally coincident with the 

Mather Gorge Formation visible in the aeromagnetic map of 

Geophysical and Geological Setting 
Figure 3A. Short wavelength aeromagnetic anomalies in the 

Herndon region of the Culpeper basin. The sinuous outline of 

the eastern margin of the basin is provided by the white line, 

including the locations of faults that pass into the basin from 

the neighboring Proterozoic rocks to the east. The horseshoeshaped anomaly of the Herndon diabase lopolith (maroons 

and magenta) has a 1:1 coincidence with mapped locations of 

diabase. The anomaly itself corresponds to the upturned edges of 

the lopolith, and thus emphasizes it’s concave-up geometry. The 

lopolith cradles a lens of the Poolesville Member of the Manassas 

Sandstone. Extending southwestward from the southern end of 

the lopolith is a ridge-like `basement` anomaly, a fraction of which 

corresponds to a narrow mapped diabase dike. In the northwest 

lies the ENE-WSW striking diabase dike that crops out along Rte. 

606 (Old Ox Road), north of Dulles International Airport. South of 

that dike, lies the Dulles compartment, comprised of Balls Bluff 

Member siltstone (shown here in blue and purple). Borehole 

geophysical survey locations at the Airport are given by the pair of 

yellow dots.

figure 3. Indeed, the Catoctin Formation metabasalt anomaly 

and the Mather Gorge Formation anomaly are two of the three 

long-wavelength anomalies that move beneath the Culpeper 

basin from northeast to southwest and then to the south and 

southeast, as illustrated earlier in figure 3. These two structures are among three broad antiform subsurface structures 

that may be traced for about 400 km from Maryland to northern Georgia (Çoruh, et al., 1988). The third long-wavelength 

anomaly is the Sykesville Formation, east of, but parallel to 

the Mather Gorge Formation, as remarked above.

 Reconnaissance Borehole Geophysical, Geological, and Hydrological Data, Culpeper Basin, Virginia
Figure 3B.—Short wavelength aeromagnetic anomalies in the 

Centreville region of the Culpeper basin. The eastern basin margin 

is given by the sinuous white line. The Centreville diabase lopolith 

anomaly is the horseshoe-shaped feature that dominates the north 

(maroons), and cradles a lens-shaped inlier of Balls Bluff Member 

siltstone in its concave-up interior. The `basement anomaly` that 

extends to the southwest from the lopolith, is strikingly similar to 

its counterpart in the Herndon area (fig. 3A). The Piney Branch 

Complex is an anomaly within the Proterozoic crystalline rocks 

that lie just outside the basin.

Proposed Compartmentalization of the 

Culpeper Basin

Basin Compartmentalization by Intrusive 

Complexes: Diabase Septa and their Function.

The Culpeper basin is approximately 132 km in length 

and about 34 km at maximum width. The basin is cored along 

its central axis with a great complex of diabase dikes, sheets, 

lopoliths and irregularly-shaped mafic bodies. The relatively 

low fluid permeability of diabase is typically in the range 10-17

< k < 10-21 m2

, and this low permeability is broadly correlated 

with relatively high electrical resistivities. Permeability values 

Proposed Compartmentalization of the Culpeper Basin 
Figure 4. Synthesized synoptic structural cross-section across 

the Culpeper basin, based on reprocessed seismic reflection lines 

supervised by John Costain and Cahit Çoruh of the Virginia Tech 

Regional Geophysics Laboratory (after Ryan, et al., 2007a). VT–L1 

and VT–L2 refer to two seismic lines that approached each other 

across the basin. The gentle westward dips of the sedimentary 

rocks on the eastern side of the basin contrast strongly with the 

pronounced steeply westward dipping attitude of sediments in 

the western portion of the basin. The sediments are massively 

fractured in ways that promote fluid storage and migration, 

and they comprise the core regions of the Culpeper basin’s 

groundwater compartments. The seismic section is from west 

(left) to east (right) and spans the distance from Aldie to Fairfax, 

Virginia. Shallow bright reflectors denote the progressivelywestward dip of intra-basin sediments, as well as the gently 

antiformal-configured Catoctin metavolcanics in the west and 

their Proterozoic basement counterparts beneath the basin in 

the east. The locus of diabase magma ascent and emplacement 

occupies the central portion of the cross-section. Two-way travel 

times of 3 seconds for rocks of these velocities correspond, very 

broadly, to about 9 km depth.

of 10-17 m2 and lower are considered to be virtually impermeable (for example, Bear, 1972, 1979). The permeability of 

the surrounding sandstones, siltstones, and conglomerates 

is typically in the range 10-12 < k < 10-14 m2 (with correlative 

lower electrical resistivities). There are therefore at least three 

orders of magnitude—and potentially upwards of nine orders 

of magnitude—difference in permeability between the diabase 

units and their sedimentary surroundings. This implies that 

bodies of diabase are expected to deflect, impede, impound, 

and otherwise strongly interfere with the regional flow of 

groundwater in the basin. Where complexly arranged in vertical stacks of sheets and/or horizontal boxworks of intrusives, 

these bodies are suggested to compartmentalize the basin 

flow—both horizontally and vertically. Predicted (but not yet 

tested) implications of such compartmentalization, if real, 

include the potential for: (a) somewhat stagnant flow at depth 

within a compartment, and relatively freely-flowing fluids at 

shallow levels; (b) isotopically old groundwater deep within 

a compartment, and progressively younger waters at shallow 

levels; and (c) higher amounts of total dissolved solids in compartmentalized waters, and lower values in the near-surface. 

One can thus think of the Culpeper basin as a great ‘supertanker’, where the diabase bodies act as bulkheads within 

this conceptual vessel. Ryan and Yang (1999), and Ryan and 

others (1997, 2000, 2002, 2007d) discuss compartmentalization in low and high temperature systems, including the roles 

of fractures on hydrothermal convection, and Laczniak and 

Zenone (1985) outline the roles of pumping test scenarios on 

the hydrology of sections of the Culpeper basin. Ryan et al., 

2007a, 2007b, and 2007c illustrate how diabase bodies deflect, 

impound and compartmentalize local and regional flow. These 

authors also show how diabase bodies comprise groundwater 

divides—systematically complicating the regional flow patterns with the superposition of local topographic influences.

A Concise Summary of Relationships Suggesting 

Basin Compartmentalization

Several lines of evidence suggest that the Culpeper 

basin is hydrodynamically compartmentalized today and was 

similarly compartmentalized during the Mesozoic when Lower 

Jurassic intrusions and eruptions completed the three-dimensional internal structure of the paleo-basin and excited hydro10 Reconnaissance Borehole Geophysical, Geological, and Hydrological Data, Culpeper Basin, Virginia
thermal convection. The evidence for compartmentalization 

is presented and discussed in Ryan and others, (2007a, 2007b 

and 2007c), and in Pierce and Ryan, (2003), and includes:

1. The relatively low permeability of diabase relative 

to surrounding hornfels and sandstone/siltstone 

sequences.

2. The typically relatively low pumped groundwater 

yields from wells sited within diabase vis-à-vis wells 

sited within neighboring sandstones or siltstones.

3. The ubiquitous nature of diabase within the basin, its 

wide three-dimensional distribution and volumetric 

signficance.

4. Frequently (but not universally) observed high values for groundwater heads measured in wells on the 

recharge side of diabase bodies vis-à-vis measurements collected on the discharge side.

5. Contact springs (single and multiple) located on 

the recharge side of diabase bodies and the evident 

absence of systematic contact spring development on 

the discharge side.

6. Abnormalities in the pumped drawdown curves for 

wells proximal to diabase contacts consistent with 

the perturbation of the developing cone of depression 

by a no-flow or low-flow boundary coincident with 

(i.e., identical to) the diabase body.

7. Order of magnitude differences in the electrical 

conductivity of diabase and neighboring sandstone/

siltstone sequences as revealed by magnetotelluric 

(MT) and audiomagnetotelluric (AMT) measurements. Diabase resistivities are typically ~1,000 to 

~4,000 ohm-m, whereas sandstone/siltstone resistivities range over ~60 to ~400 ohm-m.

8. Analytic (complex variable-based) treatments of 

flow processes adjacent to impermeable boundaries 

illustrate the profound roles of diabase in deflecting 

and impounding groundwater.

9. Finite element simulations of the effects of diabase intrusives on subsurface flow in model basin 

volumes as functions of progressive contrasts in 

permeability between the diabase and surrounding 

sandstones and siltstones. Such studies have constrained the permeability contrasts required between 

diabase bodies and their surroundings for the onset 

of compartmentalization.

We (Ryan and others, 2007b) have also evaluated the 

geometry of diabase bodies in the Gettysburg and HartfordDeerfield basins in relationship to to their potential roles 

as compartment boundaries, and find consistency with the 

relationships discovered within the Culpeper basin. By extension then, these relationships suggest also that the Gettysburg, 

Hartford, Deerfield—and likely the Newark―basins are similarly compartmentalized although the details of this compartmentalization may differ.

Throughout this report, references will be made to the 

Dulles compartment, the Gainesville compartment, and the 

Rapidan compartment, among others. These names are used 

informally, and when the term “compartmentalization” is used, 

it is understood that it should be read “proposed compartmentalization”.

Scientific Objectives and Borehole 

Field Experiment Strategy

Our borehole geophysical and geological logging program was initiated in 2000, and is part of a broader program of 

geophysical and geological research aimed at understanding 

the contemporary—and paleo-fracture flow hydrology of the 

major Mesozoic basins of eastern North America. We seek to 

understand the three-dimensional structure of the Culpeper 

basin and the geometric interrelations between primary basin 

rock types and their bearing on subsurface flow. Of particular 

interest are permeable rocks that store significant volumes 

of groundwater, which are transmissive, and that therefore 

comprise important aquifers. Fracture flow and fracture-based 

storativity is of particular interest in this context. Examples of 

such high storage capacity rocks are the Balls Bluff Member 

of the Bull Run Formation and the Poolesville Member of the 

Manassas Sandstone within the Culpeper basin of Virginia and 

Maryland (see the geologic reference maps of figures 5A and 

5B). However equally important hydrodynamically are the 

relatively impermeable rocks since, if widely and systemmatically arranged within the basin, impermeable rocks have the 

potential of acting as internal ‘no-flow’ boundaries, thus contoling the rate, direction and extent of fluid migration, as well 

as several aspects of the basin’s internal groundwater storage. 

The prime example of such impermeable rocks are the massive 

diabase bodies that occur throughout the Culpeper basin (Ryan 

and others, 2007a, 2007b).

Our experiments have been designed to determine aspects 

of the lithologic, structural and hydrologic properties of several Culpeper basin rock types. Parameters of interest include 

the formation electrical resistivity structure, bedding plane 

fractures, cross-bedding fractures, electrical conductivity of 

the borehole fluids, the temperature-depth distribution in the 

well bore fluid, the first derivative of the temperature-depth 

distribution ( DT ), and mineral lineations and small-scale litho- DZ

logic variations from millimeter-to-meter scale range. Lithologic variations of interest include the relative clay contents, 

mineral layering, and vein distributions and their geometry. 

We would like to relate these lithologic and structural variations to the broad structure of the groundwater compartment 

within which it is found, or alternately, to the transition from 

the compartment-bounding diabase intrusion to the adjacent 

hornfels. Where possible, we also wish to relate the inferred 

Scientific Objectives and Borehole Field Experiment Strategy 11
Jurassic basalt flows

Pebbly sandstone

Conglomerate of metamorphic rocks

Conglomerate of limestone

Diabase

Normal fault

Possible fault

Thermally altered sandstone

Thermally altered mudstone

Thermally altered conglomerate

Thermally altered limestone

Black shale and

 siltstone

Jurassic sandstone and cyclic siltstone

Triassic sandstone and cyclic siltstone

Cyclic siltstone and shale

Siltstone and minor sandstone

Sandstone and minor siltstone

N

0 5 10

0 10 20 30 40

15 20 25

Miles

Kilometers

NORTHERN CULPEPER BASIN

 VIRGINIA

Borehole Geophysical Surveys

Well location

Dulles No. 2

Dulles No. 3

USGS National 

 Center

Figures 5A and 5B. Borehole geophysical survey locations within the northern (A)and southern (B) Culpeper basin superposed on 

principal geologic units. Principal diabase lopoliths, dikes and sheets are denoted in red, and their thermal metamorphic haloes are 

given in dark pink (surrounding red diabase). The primary sites are, from north to south, Dulles International Airport; U.S. Geological 

Survey National Center campus in Reston, Virginia; Gainesville, Virginia; and Rapidan, Virginia. Color coding on lithologic units is: light 

yellow (Poolesville Member of the Manassas Sandstone); Light blue (Balls Bluff Member of the Bull Run Formation); 

1 Reconnaissance Borehole Geophysical, Geological, and Hydrological Data, Culpeper Basin, Virginia

A

A
Jurassic basalt flows

Pebbly sandstone

Conglomerate of metamorphic rocks

Conglomerate of limestone

Diabase

Normal fault

Possible fault

Thermally altered sandstone

Thermally altered mudstone

Thermally altered conglomerate

Thermally altered limestone

Black shale and

 siltstone

Jurassic sandstone and cyclic siltstone

Triassic sandstone and cyclic siltstone

Cyclic siltstone and shale

Siltstone and minor sandstone

Sandstone and minor siltstone

N

0 5 10

0 10 20 30 40

15 20 25

Miles

Kilometers

SOUTHERN CULPEPER BASIN

 VIRGINIA

Borehole Geophysical Surveys

Well location

Coffeewood

well field

Rapidan

compartment

Gainesville

compartment

Gainesville (AOL)

5A and 5B. (Continued) Medium blue with light blue striations (alternating lacustrine and fluvial phases, of the Balls Bluff Member); 

Medium green (Turkey Run Formation); bright yellow (Catharpin Creek member); Orange (Goose Creek Member of the Catharpin 

Creek Formation (a conglomerate)); Light pale Blue in western portion of basin (Midland Formation); Brown (Sander Basalt); Brown 

(Hickory Grove Basalt); and Bright green (Cedar Mountain Member of the Bull Run Formation (a “border” conglomerate)). Proposed 

compartment names are indicated with leaders to their respective locations.

B

Scientific Objectives and Borehole Field Experiment Strategy 1

B
lithologic and structural variations seen in boreholes with their 

counterparts seen in outcrop exposures. Borehole geophysics 

and geology are critically important for guiding the continuing development of conceptual models of fracture flow, and in 

ultimately properly constraining derivative numerical models 

of subsurface fluid migration.

Four study areas within the Culpeper basin have been 

selected for reconnaissance borehole logging. These include 

locations within the proposed Dulles compartment, the 

Gainesville compartment, the Rapidan compartment, and 

the U.S. Geological Survey’s National Center campus (figs. 

5A and 5B). Each location explores different aspects of the 

geologic environment of fluid storage and migration. Within 

the Dulles compartment, we have used deep wells at Dulles 

International Airport to sample the structure of the Balls Bluff 

Member of the Bull Run Formation the dominant aquifer and 

rock type at this location, and throughout much of the central 

portion of the basin. Within the Gainesville compartment, we 

have used a relatively deep well that penetrated the compartment-sealing diabase lopolith and penetrated the subjacent 

permeable hornfels, to explore the physical nature of compartment septa, or “bulkheads”, and the establishment of basal 

flow regimes. At the Rapidan compartment, we have logged 

an array of wells that have been sited within the hornfels lens 

of metamorphosed Balls Bluff Member siltstone, cradled 

within the concave-up interior of the Rapidan lopolith. And at 

the U.S. Geological Survey’s National Center, we have used 

two monitoring and test wells to examine the nature of the 

Culpeper basin`s eastern structural margin, where Mesozoic 

basin rocks have an on-lap relationship lying unconformably 

on the adjacent Proterozoic rocks. Each of these environments 

with selected results from reconnaissance logging are discussed below.

The Proposed Dulles Compartment

Dulles International Airport rests on top of a large 

proposed subsurface groundwater compartment (see figs. 2, 

3A and fig. 5A). The proposed compartment is bounded on 

the north by a prominent diabase dike exposed along Virginia 

route 606 (Old Ox Road) in the vicinity of the Dulles Greenway, bounded on the south by the Centreville diabase lopolith, and bounded on the west by the irregular diabase body 

beneath the western margins of Dulles Airport property, in 

combination with a swarm of N-S-alligned diabase dikes. It is 

also largely bounded to the east by the western margins of the 

Herndon diabase lopolith. Contained within, is a large expanse 

of Balls Bluff Member siltstone and thermally-metamorphosed 

remnants near the diabase contacts. The inferred systematic 

closure of the Balls Bluff Member fractures with depth (due to 

progressive increases in the horizontal components of confining pressure) is inferred to effectively ‘floor’ the compartment, 

by analogy with experimental results on other—yet roughly 

similar—lithologies (Ryan, 1987). The fluids within the compartment are thought, strictly however, to be partially compartmentalized, since just east of the airport a small subsurface 

opening near the Herndon lopolith occurs. We refer to this 

portion of the Culpeper basin as the “Dulles compartment”. 

The electrical resistivity structure of the Dulles compartment, 

as well as other compartments within the Culpeper basin have 

been studied by AudioMagnetotelluric [AMT] surveys, and is 

presented in the companion report of Pierce and Ryan (2003).

Balls Bluff Member of the Bull Run Formation

Both lacustrine and fluvial phases of the Balls Bluff 

Member siltstone are exposed in the Culpeper basin (Lee 

and Froelich, 1989; Smoot, 1989, 1991; Faill, 2003, and 

J.P. Smoot, unpublished mapping). The sequence astride 

the Dulles airport access road (just west of VA Rte. 28), for 

example, contains thin beds colored a deep rust orange and 

having distinctive westward dips. Inferred paleo-dessication 

of the uppermost portions of beds occassionally has produced 

dehydration shrinkage fractures arranged in crudely polygonal 

patterns that extend vertically downward into the rock mass 

from bedding planes. The mineralization (calcite and quartzdominated) within these polygonal networks are thus remnants 

of some of the first ‘fracture flow events’ during basin evolution. Within a bed, an upward decrease in clay content and an 

increased volume fraction of sand may be consistent with correspondingly higher permeability, the development of matrix 

flow, and for subsequent epidote mineralization during thermal 

metamorphism, a common association in field exposures 

(Ryan and others, 2007a).

Fracture patterns in the Balls Bluff Member include bedding plane fractures and cross-bedding fractures. Both the bedding plane and cross-bedding fractures are consistent with origins dominantly related to decompression-induced expansion 

during erosional unroofing. Mineralized fractures corresponding to paleo flow events are typically open today, and available 

for contemporary groundwater flow. Cross-bedding fractures 

tend to be mutually orthogonal to each other and they are also 

collectively orthogonal to the bounding bedding planes. This 

overall fracture geometry produces an orthorhombic fracture 

network in three dimensions, when the Balls Bluff Member is 

considered as a whole. Thus the Balls Bluff fracture systems 

define a crudely “brick work” geometry in three-dimensions. This “brick work” or crudely orthorhombic geometry 

is suggested to be a recurring theme in the fracture-controlled 

permeability at shallow depths within much of the basin. 

Contemporary apertures of 0.1 mm correspond to approximate 

permeabilities of 8. x 10-10 m2

 [800 darcys], whereas apertures 

of 4.0 mm correlate with approximate permeabilities of 5. × 

10-6 m2

 [mega Darcy range]. Within the Balls Bluff Member 

fractures, zeolite facies metamorphism (proximal to adjacent 

diabase heat sources) have crystallized calcite, CaCO3; stellerite, Ca(Al2

Si7O18)7[H2O]; gedrite, (Mg,Fe+2)

5

Al2[Si6Al2O22]

(OH,F)2; and chabazite, CaAl2Si4O12 6[H2O] identified in hand 

samples. The thickness range for the Balls Bluff is from about 

1 Reconnaissance Borehole Geophysical, Geological, and Hydrological Data, Culpeper Basin, Virginia
80 m near the northern margins of basin, to about 1,690 m in 

the central portion of the basin. The distribution of the Balls 

Bluff Member may be seen in figures 5A and 5B.

Geophysical Logging in the Proposed Dulles 

Compartment

The lithology of the Dulles compartment as determined 

by borehole logging is almost entirely Balls Bluff Member 

of the Bull Run Formation. However outcrop inspection on 

the western portions of Airport property indicate the presence 

of minor hornfels (after Balls Bluff Member) due to thermal 

metamorphism above a diabase sheet heat source. This diabase 

sheet is actively being quarried to the east-southeast of the US 

Route 50 and VA route 606 intersection. The quarry operator 

is the Chantilly Crushed Stone Corporation. The Dulles compartment depth is not known in detail, however 2-D seismic 

reflection profiling (Ryan and others, 2007a) is able to roughly 

estimate the depth at about 1.5 to 2.0 km.

Preliminary logging on Dulles International Airport 

property was conducted in 2001. Two wells were logged. 

Well No. 2 was logged to a depth of 289 m (original depth of 

the well was 290 m), and well No. 3 was logged to 43 meters 

(the original drilled depth was 290 m; thus the lower 247 m 

has been silted-in or bridged since well completion). Both 

wells were logged for the fluid temperature-depth function 

and the first derivative of temperature with depth or ‘delta 

temperature’ ( DT

DZ ), formation electrical conductivity and 

resistivity, fluid conductivity and resistivity, natural gamma, 

and caliper data (see figs. 6 and 7). Further characterization of 

the proposed Dulles compartment would benefit from optical 

and acoustic televiewer logging, heat pulse flowmeter logging 

and electromagnetic (E-M) flowmeter logging. Based on the 

total depth estimates for the Dulles compartment as discussed 

above, the maximum well penetration from work conducted 

thus far is suggested to be about about 14 to 16 percent of the 

overall compartment depth as estimated by seismic reflection.

The WellCAD borehole geophysical logs for Dulles 

wells No. 2 and 3 are organized in a five column format (see 

figure 6): column 1 contains the three-arm caliper, casing and 

lithologic information based on cuttings; column 2 contains 

the formation resistivity (Ohm-m) and conductivity (mmho-m) 

data; column 3 contains the fluid temperature ( ° C and ° F) 

and temperature derivative as a function of depth (dimensionless) data; column 4 presents the fluid resistivity (Ohm-m) and 

fluid conductivity or specific conductance (micro-Seimens/

cm); and column 5 presents the natural gamma counts (counts 

per second) as a function of depth.

The entire section within wells No. 2 and 3 is within 

the Balls Bluff Member, and the wells are far removed from 

the lateral diabase boundaries that define the margins of the 

Dulles compartment. Thus the purpose of the Dulles logging is the characterization of the Balls Bluff Member well 

within a compartment interior. Subtle variations with depth 

in the temperature of the ambient water column may contain 

information relating to localized water inflow or outflow from 

the borehole. In the absence of flow, the borehole temperature 

reflects the geothermal gradient, where a rough rule of thumb 

indicates an increase in temperature of about 1 degree per 100 

ft of depth. Active flow into or out of the borehole, however, 

has the potential to perturb the linearity in the T-Z profile. An 

indication of differential seepage from fractures is a spike in 

the temperature derivative, DT

DZ , suggesting a local inflection 

in the T-Z profile. Within the Balls Bluff Member, at least 33 

such spikes in the temperature derivative occur in Dulles well 

No.2, suggesting the possible influx of water along fractures 

at these levels. The T-derivative spikes are unevenly distributed with depth, however field observations of the Balls Bluff 

Member in outcrop, suggest that bedding plane fractures are a 

prime contributor to bulk fracture flow.

The Proposed Gainesville 

Compartment

Direct Drilling into the Gainesville Compartment

An additional confirmation of the validity of the compartmentalization model was demonstrated during well drilling 

in July of 2001, at the America On-Line (AOL) data storage 

facility at Gainesville, Virginia. Drilling was performed by 

Mark McClellan and Duane Sturgill of Groundwater Systems, 

Inc., under contract from HIS-GeoTrans of Sterling, Virginia, 

and the Poole and Kent Corporation of Baltimore, Maryland. 

The AOL facility was sited on the Gainesville diabase sheet, 

just south of the intersections of routes US Interstate 66 and 

Virginia Route 29 (see site location in figs. 5A, B). The very 

low permeability of the diabase yielded correspondingly low 

flow rates: about 2 gal./min. Five holes drilled into the diabase 

sheet continued to produce negligible (low flow rate) results. 

The last hole, however, was drilled into and just through the 

diabase sheet, to a depth of about 225.5 meters (740 ft.), and 

completed or terminated in the highly fractured and permeable hornfels of the compartment’s margins. The flow rate 

(within the hornfels) was about 100 to 110 gal./min. (Cohen, 

2001)-sufficient for AOL to use as cooling water for their data 

storage center. The overall flow rate for the entire well—incorporating the low productivity of the diabase—was 90 gpm 

based on air-lift testing conducted by Groundwater Systems, 

Incorporated.

Flow testing based on pumping at 100-110 gpm has been 

used to develop time-drawdown curves for the production well 

and evaluate the transmissivity (T), storage coefficient (S), and 

leakance parameter (B), using the Theis equation and the Hantush leaky aquifer solution (Cohen, 2001). These tests characterize the relatively fractured and thermally- metamorphosed 

Balls Bluff Member of the Bull Run Formation (hornfels) in 

the transition region between the hornfels and the overlying 

The Proposed Gainesville Compartment 1
Well Name: Dulles # 2

Temperature FT

15 Deg C 21

Delta Temperature FT

-0.1 Deg C 0.1

Temperature IT

59 Deg F 69.8

Temperature

Specific Conductivity

0 uS/cm 3500

Fluid Resistivity

0 Ohm-m 35

Fluid

Natural Gamma IT

50 API-GR 200

Natural Gamma FT

50 API-GR 200

Natural Gamma

Conductivity IT

0.3 mmho/m 300

Apparent conductivity IT

0.3 mmho/m 300

Resistivity IT

0.3 Ohm-m 300

Rock

Depth

meters

1:1750

Lithology

Caliper (three-arm)

0 cm 55

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

290

Casing

Balls Bluff 

siltstone

Figure . Geophysical logs from Dulles well No. 2 at the Washington Dulles International Airport.

1 Reconnaissance Borehole Geophysical, Geological, and Hydrological Data, Culpeper Basin, Virginia
Figure 7 Geophysical logs from Dulles well No. 3 at the Washington Dulles International Airport.

The Proposed Gainesville Compartment 1
diabase. For the Theis simulations, transmissivities and storativities were 2.909 x 10-2 ft2

/min and 5.145 x 10-2, respectively, whereas for the Hantush leaky aquifer simulations, the 

transmissivities were 1.902x10-2 ft2/min., with a storativity of 

1.267x10-1, and a leakance parameter of 2.889x10-2.

Geophysical and Geological Logging Through 

the Proposed Gainesville Compartment Margins 

and into the Compartment Interior

The WellCAD logging data for the Gainesville borehole 

is organized into four primary columns (fig. 8): Column 1 

contains the optical televiewer (OTV) data. The orientation scheme begins with the N-E-S-W-N sequence from the 

left hand margin, through a sequence of ruled vertical lines, 

to the right hand margin. Thus the viewer moves progressively clockwise around the borehole’s inside circumference 

in looking at the data from left-to-right on the plot. These 

cardinal directions correspond, in turn, to the compass azimuth 

directions familiar to field geologists: 90, 180, 270, and 360 

degrees, respectively. The OTV data have been analyzed with 

WellCAD (4.2) software and several picks of geologic features 

have been made. The features have the following color codes: 

black: fractures showing no relative lateral displacement 

(joints); dark blue: fractures showing some lateral displacement (faults); light blue: mineral veins; green: bedding-conformable mineral layering; pink: highly-inclined to near-vertical fractures. Mineral layering within the diabase tends to be 

pyroxene-rich, whereas mineral layering within the hornfels 

tends to be clay-rich. Column 2 contains the virtual core. This 

image is highly pixelated and serves schematic purposes only. 

The ‘ledges’ of the core may correspond to the locations of 

fractures. Column 3 contains the three-arm caliper plot and the 

inferred lithologic log. Finally, column 4 contains the temperature-depth plot.

Within the full and extended version of WellCAD (4.2) or 

the free WellCad reader (4.2) the virtual core may be manipulated to reveal or enhance lithologic and/or structural features. 

These manipulations include core rotation, axial core compression, axial core extension, slicing, or along-axis “halving”, and 

multiple (split screen) presentations of several cores simultaneously. A “northside marker” along the core axis denotes the 

0-degree position, which is magnetic north. This marker will 

appear as a thin colored line along the core length.

The ‘unwrapped’ core reveals the same structures that the 

virtual core depicts, and usually at much higher resolution, due 

to the pixilation requirements of quality virtual core construction. In extended WellCAD (4.2), the unwrapped presentation 

may be compressed or extended as desired, and changes in the 

resolution and visability of features will progressively change 

in the process. Such changes in scaling may be done without 

“loosing” the top or bottom of the core. Core compression 

has the advantage of concise summary presentations of data 

for, for eample, side-by-side comparisons with core sections 

from neighboring wells or boreholes. For maximum detail and 

resolution, however, scale extension is recommended. Extension may continue to be applied until image degradation is 

noticed. Settings may then be “backed off” until resolution is 

maximized.

Planar features such as brittle elastic fractures, faults, 

veins, and mineral layering, are seen as horizontal lines or 

bands in the core sections if they have no significant dip. The 

same features will appear as sinusoidal lines or sinusoidal 

bands of various amplitudes and wavelengths if they dip. The 

larger the dip, the larger their amplitude on the unwrapped 

core section. Sinusoidal features with particularly high amplitudes are, therefore, high-angle or steeply-dipping features. 

Examples of such features may be seen in figures 8A and 8B.

Once “unrolled” and extended so that lithologic and 

structural features can be seen with clarity, specific “picks” 

can be made using the full and extended WellCAD (4.2) 

software. In the core sections provided in this report, picks 

of fractures, veins, mineral lineations, and faults have been 

made only where there was clear indication that the feature 

was complete and untruncated by other structures/lithologies. 

Thus, only a fraction of the total population of these features 

has been selected for viewing.

The geology sampled in this borehole consists of a part 

of the Gainesville diabase sheet within the upper 183 m, and 

hornfels (after the Balls Bluff Member) in the lower 43 m. 

The diabase is heavily fractured, and the picks provided show 

a tendency for the fractures to be either slightly dipping (to 

the ENE over the 50-160 m interval) or to be relatively flatlying (the 152-200 m interval). The origin of the subvertical 

fractures in the diabase may be ascribed to both a thermal and 

a tectonic (regional stretch) component. In the OTV images, 

near-vertical fractures are rare (a sampling artifact of the 

vertical orientation of the borehole) whereas sub-horizontal 

fractures are plentiful. Ryan and others, (2007a) suggest that 

such horizontal fractures in diabase are subordinate to subvertical fractures, are the products of erosional decompression, 

and that they die out rapidly with increasing depth. This view 

is based on the prevailing occurrence of abundant subvertical 

fractures in diabase outcrops, and the distinctly subordinate 

numbers of subhorizontal fractures. Furthermore, and based in 

part on examination of three-dimensional outcrop exposures 

in combination with the borehole data, the fractures resolved 

in this OTV image seem to be relatively poorly-interconnected 

both laterally and in three-dimensions, since the pumped flow 

rates from the upper 183 m were less than 2 g.p.m. as inferred 

from air lift testing. In actively worked diabase quarries within 

the Culpeper basin, the sub-horizontal fractures die out rapidly 

with depth, leaving only the numerically-dominant verticallyoriented fractures exposed. It is important to realize, therefore, 

that a reliance on only borehole OTV data causes a distinctly 

misleading picture of the nature of the diabase fracture populations.

As may be seen in both the three-arm caliper plot as 

well as in the virtual core, the hornfels is highly fractured. 

This relative intensity of fracturing marks one of the principal 

indicators for identifying the diabase-to-hornfels transition 

1 Reconnaissance Borehole Geophysical, Geological, and Hydrological Data, Culpeper Basin, Virginia
Figure 8. The America On-Line (AOL) Gainesville, Virginia well showing diabase in the upper portion of the well and hornfels in the 

lowest portion. This image is included in the attachments as a WellCAD WCF type file for the WellCAD reader

The Proposed Gainesville Compartment 1
Figure 8A. WellCAD image of the 164 to 174 m interval in the AOL well at Gainesville, Virginia through the Gainesville diabase sheet. 

The image illustrates the running caliper record (third column with gray background and checked pattern), the virtual core (3-D image) 

and the unwrapped borehole wall with several fracture ‘picks’ shown as black and blue lines with sinusoidal geometry where fractures 

are steeply dipping, and gently ‘rolling’ geometry where fractures have negligible dips. (Left column).
Figure 8B. WellCAD image of the 207 to 217m depth interval for the America On-Line well through the Gainesville diabase sheet. 

The image shows hornfels immediately beneath the diabase, where fine green lines on the optical televiewer log represent mineral 

lineations produced by thermal metamorphism (after clay varves in the parent Balls Bluff Member siltstone). From left to right on the 

figure: Metric depth scale; Unwrapped borehole walls of the optical televiewer, illustrating the hornfels fabric with mineral lineations; 

Virtual core (note the high amplitude breakouts correlating with fracture intersections, especially near the top at 208 m depth); Caliper 

record (crenulated line with red-pink pattern). 

The Proposed Gainesville Compartment 1
zone. Others include an abrupt increase in the number of clayrich relect mineral lineations (the green WellCAD picks in 

the OTV log). The massively-fractured nature of the hornfels 

below the diabase-hornfels contact coupled with an inferred 

increase in 3-D fracture interconnectivity is linked with the 

abrupt jump in pumped flow rates (from less than 2 g.p.m. 

in diabase to about 110 g.p.m. in the adjacent hornfels). The 

abrupt deflection to lower temperatures in the temperaturedepth (T–Z) curve at the 186 m level, also suggests a possible additional groundwater influx. Although this fracture is 

in diabase, it is so close to the diabase-hornfels contact that 

inter-fracture communication with the hornfels may have been 

established. Heat pulse and E–M flow meter measurements 

have not yet been made within this diabase-hornfels sequence. 

In addition, a natural gamma log would be expected to show 

total gamma counts climbing abruptly from near-zero values 

beneath the diabase-hornfels contact—another indication that 

the transition interval had been reached.

Structure of the Proposed Gainesville 

Compartment

The results of this study suggest that the geometry 

of compartmentalization is more complex than the simple 

concept of assemblages of side-by-side vertically-oriented 

diabase septa (e.g.: dikes), containing large volumes of highly 

fractured sediments between them. In addition to that type of 

organization, compartimentalization operates vertically as well 

as horizontally. Thus, the Gainesville compartment lies, in 

part, beneath the Gainesville diabase sheet. Hydrodynamically, 

this comprises a basal flow regime, where flow is confined by 

the diabase sheet above (see, also, Ryan and others, 2007a). 

It is reasonable to suspect that other sheets and lopoliths have 

subjacent fluid-rich compartments as well. Such compartment interiors would be expected to be comprised of relatively 

heavily fractured hornfels in their immediate vicinity, grading 

downwards into non-metamorphosed but pervasively fractured 

sediments, with increasing depth. The Boyds, Belmont, Herndon, Centreville, Bealeton and Rapidan diabase bodies are all 

candidates for this type of compartment organization.

Fractures within the diabase are invariably mineralized, 

and vein systems analogous to those in the dike near Dulles 

International Airport (on Virginia Route 606) are well developed. Chlorite-mineralized veins and fracture walls are commonly dilatant, and may provide very local migration paths for 

water within the diabase. Polar plots of chlorite vein orientations show two preferred trends that intersect at high angles, 

and thus have the potential for 3-D interconnections within the 

rock mass. Similarly, polar plots of contemporary fluid flow 

fractures (from direct visual evidence of active zones of seepage) also have two preferred trends, and their 3-D interconnections of these fractures may promote this flow. The position of 

the Gainesville compartment may be seen in figures 5A, B.

The Proposed Rapidan Compartment: 

Deep Structure of the Rapidan Diabase 

Lopolith

A large intrusive body, here informally referred to as the 

“Rapidan diabase lopolith”, is named for the largest nearby 

community. It has been mapped as a great sheet-like body (Lee 

and Froelich, 1989; J.P. Smoot and D.M. Sutphin, imformal 

communications, 2002). The body is a concave-up intrusive 

that cradles in it’s interior a massive inlier of Balls Bluff 

Member siltstone. It rests in turn on the contact-metamorphosed-equivalent hornfels. The true geometry of the body is 

thus lopolithic (see fig. 5B). The results of the AudioMagnetotelluric study of Pierce and Ryan (2003) suggest that at depth 

the diabase body appears to be structurally weakly divided 

into two high resistivity lobes, however this inference rests on 

the data of one vertical audioMagnetotelluric (AMT) sounding station profile. We interpret this apparent division, if real, 

as being mainly induced by undulations in the contact zone 

topography between the diabase and the Balls Bluff Member 

at the time of intrusion. Undulations in contact topography are 

relatively common for mafic intrusions invading sediments, 

particularly if the sediments are not fully lithified, as is appropriate for the Culpeper basin. Typical examples are found in 

basalts as they invade soft sediments, and the Gulf of California spreading center near the Salton Sea geothermal region 

of California, is one such example, among other locations 

world-wide. In the AMT-based electrical resistivity cross-sections of Pierce and Ryan (2003), lobes of resistive diabase 

make contact with the mantling hornfels. These irregularities 

(high amplitude undulations) occur within the contact region 

between the diabase intrusion and the Balls Bluff Member, 

and are believed to have developed during intrusion. Multiple (pulse-like) intrusion episodes also produce this type of 

complex contact structure, and routinely incorporate lenses or 

screens of country rock between them. Multiple intrusion episodes are now known to have occured in both the Gettysburg 

basin (Ryan and others, 2007b) and Culpeper basin (Ryan and 

others, 2007a), and would be expected for other Mesozoic 

basins invaded by diabase during the Lower Jurassic. Both 

multiple intrusion pulses and upper contact surface undulations in intrusion-host rock topography are to be expected for 

the Rapidan lopolith as well as other Culpeper basin lopoliths. 

Screens of Balls Bluff Member siltstone caught up between 

diabase intrusion pulses would therefore remain today as 

hornfels screens, which are characteristically more intensely 

fractured, more susceptible to weathering, and much lower in 

bulk electrical resistivity than their diabase neighbors. Overall, 

the Rapidan diabase lopolith has a thickness of at least ~550 

meters in the vicinity of Michells, Virginia, based on the AMT 

study of Pierce and Ryan (2003). The body is believed not to 

be monolithic and planar, but to have significant three-dimensional topography in the upper contact surface and in the 

interrelationships with the hornfels above it as suggested by 

aspects of this survey.
Geophysical Logging in the Proposed Rapidan 

Compartment

We have used borehole geophysics tools to characterize 

some of the bulk petrophysical properties of the Coffeewood 

Correctional Center site at depth, including using a Mount 

Sopris Instruments* winch with an Advanced Logic Technologies (ALT)* OBI–40 optical televiewer (OTV) for imaging 

fracture geometries, and generating inventories of fracture 

densities. These tools and measurements include a Century* 

three-arm caliper, natural gamma, formation resistivity, fluid 

resistivity, single point (SP) and American Petroleum (AP) 

conductivities (via the electromagnetic induction tool), and 

temperature measurements, all as functions of depth. The 

asterisk (*) denotes a registered trademark.

The original total well field is comprised of nine wells. 

Three of the original nine wells are currently dedicated pumping wells and are centrally located. The five other boreholes 

are currently monitoring wells and surround the central group 

of three. Our field program has logged all five of the monitoring wells (see figs. 9 through 13), as part of this investigation 

with two being selected for OTV logging.

In the well numbering scheme, we have adopted the references “OBS–1”, “OBS–2”, for example, to denote the well 

of interest within the field. These correspond to the first and 

second observation wells, respectively, within the series. This 

scheme, in turn, correlates with a numbering system adopted 

by the original site engineers (MMM Design Group, Inc.) 

in collaboration with the commercial groundwater consulting company charged with site evaluation (Emery & Garrett Groundwater, Inc., 1992). The specific correlations are: 

OBS–1 = MMM–E; OBS–2 = MMM–A; OBS–3 = MMM–B; 

OBS–4 = MMM–F; and OBS–5 = MMM–4. The progression 

from OBS–1 through OBS–5 moves from west to east across 

the Coffeewood facility. See figure AIII-3, for a well access 

site map.

The bedrock geology at the Coffeewood facility (see 

figure 5B) consists of hornfels (after Balls Bluff Member siltstone) overlying the Rapidan diabase lopolith. From a regional 

perspective, the Rapidan hydrodynamic compartment is thus 

a lens-like inlier of hornfels cradled in the concave-up upper 

surface of the diabase itself. The compartment is thus unconfined above, but it is everywhere confined laterally due to the 

presence of the surrounding low permeability diabase. With 

respect to the Coffeewood site, however, the hornfels rests 

conformably on the diabase, dipping to the north, but thinning 

considerably to the south and southeast. Two of the wells at 

Coffeewood have had a history of artesian flow. The Coffeewood wells are all collared in hornfels and OBS–1,–2, and –3 

are entirely in hornfels for their total depths. Wells OBS–4 and 

–5, however, bottom within the Rapidan diabase lopolith. The 

transition from hornfels above to diabase below is particularly 

notable in the natural gamma logs. Well OBS–5, for example, 

shows a complete disappearance of natural gamma counts at 

the 107.5 m depth level. This is interpreted to mark the first 

appearance of diabase, due to the expectation of negligible 

amounts of Th-232, U-238 and K-40 in diabase. Similarly, the 

natural gamma log in well OBS–4 shows a total drop off in 

natural gamma counts at 82.6 m depth, and again this drop off 

is interpreted as the hornfels-diabase contact zone. Collateral 

support for this interpretation comes from the abundance of 

hydrothermal veins in well OBS–5, which are suggested to 

include, among other mineralogies, calcite. Marked increases 

in the calcite vein frequency occur between the 72 and 104 m 

depth levels but die out dramatically above 70 m (see the OTV 

log). We suggest that this vein distribution is due to the hydrothermal alteration patterns immediately above the diabase 

heat source. As such, the veins appear to have recorded the 

pathways of convective paleo-flow. The intersection of the diabase contact in two of the boreholes is insufficient to constrain 

a local contact dip (assuming planarity), however the contact 

with the diabase dips towards the north at about 10-15 degrees 

based on earlier studies (Emery & Garrett Groundwater, 

Inc., 1992). The horizontal separation between wells OBS–5 

and OBS–4 is about 1,828 m and, after reconcilliation of the 

differing elevations of the well casing tops, the depth of the 

diabase-hornfels contact is about 17 m deeper in well OBS–5 

than in OBS–4. We seriously doubt that a shallow intrusive 

contact such as this is actually planar, but anticipate instead, 

small-scale undulations in the upper contact topography.

In some sections, the hornfels is massively fractured, as is 

evident on the OTV and three-arm caliper logs, and these fractures are inferred to provide the 3-D fracture network required 

to move groundwater in the lower portions of the Rapidan 

compartment. Early EGGI well testing (May, 1992) as part of 

the Coffeewood site design has identified the hydrodynamic 

characteristics and groundwater-producing horizons associated 

with each well. The most telling pumping test—from a compartmentalization perspective—relates to the performance of 

well OBS–4 (MMM-F). Pumping in well MMM–2 (not one of 

our experiment wells) during May, 1992, induced significant 

drawdown in well OBS–4 (MMM–F). This is due, primarily, 

to the close proximity of well OBS–4 to the diabase—tantamount to the developing cone of depression impinging on a 

no-flow internal boundary at depth. Dipping (and thickening) 

regionally to the north, the hornfels thins considerably in the 

vicinity of well OBS–4. The permeable hornfels rock column 

above the diabase in this well is a scant 82 m in thickness, 

and thus would be expected to have a relatively limited total 

storage capacity. We regard the borehole measurements and 

observations at the Coffeewood site as very consistent with a 

compartmentalized flow relationship between the metamorphosed Balls Bluff Member siltstone and the diabase beneath.

The U.S. Geological Survey National 

Center and the Structural Eastern 

Margin of the Culpeper Basin

The U.S. Geological Survey (USGS) National Center 

rests on the eastern margin of the Culpeper basin. The 105-

acre National Center campus is bisected by the margin contact, 

The Proposed Rapidan Compartment: Deep Structure of the Rapidan Diabase Lopolith 
Figure 9. Geophysical logs from Coffeewood Observation well No. 1 in hornfels above the Rapidan diabase lopolith.
Figure 10. Geophysical logs from Coffeewood Observation well No. 2 in hornfels above the Rapidan diabase lopolith.

The U.S. Geological Survey National Center and the Structural Eastern Margin of the Culpeper Basin 
Figure 11. Geophysical logs from Coffeewood Observation well No. 3 in hornfels above the Rapidan diabase lopolith.
Figure 12. Geophysical logs from Coffeewood Observation well No. 4 in hornfels above the Rapidan diabase lopolith.

The U.S. Geological Survey National Center and the Structural Eastern Margin of the Culpeper Basin 
Figure 13. Geophysical logs from Coffeewood Observation well No. 5 in hornfels above the Rapidan diabase lopolith.
with the Triassaic siltstones, sandstones, and conglomerates 

of the basin to the west and northwest, and the late Precambrian and early Paleozoic pelitic shists to the south and east. 

Beneath the Center, the basin-basement contact and sediments 

dip about 10 degrees to the west. Interbedded sandstones and 

siltstones make up the Triassic Manassas Sandstone, and these 

grade downwards into the conglomerate of the Reston Conglomerate. The Reston Conglomerate lies unconformably on 

the partially weathered schists of the underlying Proterozoic 

basement.

Two observation wells were drilled in 1976 near Sunrise 

Valley Drive in Reston, Virginia for groundwater monitoring 

and for borehole geophysics equipment trials and tests (figure 

5A). These wells are designated #52V–1D (10 inch casing; 

northernmost well; nearest Sunrise Valley Drive) drilled to 

50.3 m and #52V–2D (8 inch casing; southernmost well; 

collared within a dedicated permanent monitoring house) and 

drilled to 62.5 m. Both wells have penetrated the basin-basement contact into Proterozoic rocks. Well #52V–1D penetrated 

the basement contact zone between 38.1 and 44.2 m depth. 

The casing of both wells extends to about 10.6 m. Open hole 

water levels are typically 2.5 to 3.65 m beneath the land surface.

We have conducted reconnaissance borehole geophysical 

surveys of well 52V–1D, using standard Century* three-arm 

caliper, natural gamma, formation resistivity, fluid resistivity, 

single point (SP) and American Petroleum (AP) conductivities (with the electromagnetic induction conductivity tool), 

and temperature monitoring as functions of depth (see figure 

14). In addition, a RAAX* BIPS optical televiewer system 

was used to examine the borehole walls for structural and 

lithologic information. The caliper logging indicates several 

zones of rapid hole widening, consistent with the encounter 

of dilatant bedding plane fractures and/or other subhorizontal 

jointing and minor faults. These fracture zones occur in both 

the sandstone-siltstone sequence of the Manassas Sandstone, 

as well as in the lower conglomerate of the Reston Conglomerate. Prior flow testing (Larson, 1978) suggests that the well 

produces at about 1 to 5 g.p.m., a flow rate sufficient for a 

single family dwelling. Prior pumping tests (Larson, 1978) 

have indicated a transmissivity of 30-38 ft2/day [230-290 

gallons/day/foot] with a storage coefficient of 3-6 x 10-6. The 

asterisk (*) denotes a registered trademark.

The characteristics of well 52V–2D include a transmissivity of 44 x 10-4 to 47 x 10-5 ft2

/day [330-350 gallons/day/

foot], and a storativity of 1 x 10-4 to 6 x 10-5. This greater 

transmissivity and storativity is consistent with the larger 

volume of Manassas Sandstone penetrated by this well, and 

attests to the important role played by the Manassas Sandstone 

bedding planes and cross-bedding joints in both moving and 

storing groundwater.

Well log data from the National Center are organized in a 

five column format (figure 15). Small differences from well to 

well correspond to the availability of either acoustic TeleViewer (ATV) data (well 52V–1D) or to heat-pulse flowmeter 

data (well 52V–2D). For well 52V–2D: In column 1, an ALT* 

OBI-40 Optical TeleViewer system was used to examine the 

borehole walls for lithologic and structural information. The 

presentation of the optical TeleViewer data follows the format 

used above for the Dulles and the Gainesville compartment 

sites. This column contains the unwrapped borehole wall 

lithology. In the unwrapped core, the coding for features is: 

(color 2-D image; orientation within core: from left-to-right 

vertical lines = N-E-S-W-N). Included are WellCAD (4.2) 

picks for the major (transmissive only) fractures (in black), 

mineral lineations and layering (in green), and veins (light 

blue); column 2 contains the virtual core. The virtual core 

(three dimensional color figure) depicts structures (typically 

fractures), mineral lineations, faults, veins, and high-angle 

fractures on a cylindrical surface constructed from the optical 

data set.; column 3 is the lithologic type (brick red = Reston 

Member of the Manassas Sandstone; (upper gray pebbly pattern = Conglomerate Member)); (lower gray pebble pattern 

= Proterozoic schist pebble conglomerate); (wavy parallel 

lines on green background = Proterozoic basement schist); 

three-arm caliper log (indentations in the lithology; and temperature (red band or red line), degrees F.); column 4 is the 

specific conductance (micro-Siemens/cm) and the temperature 

(degrees F.); column 5 contains the natural gamma counts with 

depth.

Used in conjunction with a Mount Sopris* heat pulse 

flow meter, pumping tests were conducted in well 52V–2D on 

16 March 1995. Flowmeter data were collected first without 

pumping, to identify ambient flow conditions in the borehole. 

Then flowmeter measurements were made under pumping 

conditions to identify transmissive fractures. Figure 16 presents a summary plot of ambient upflow rates as a function of 

depth. No ambient flow was measured in the borehole. Next, 

flow in the well and in the contributing fractures was stimulated by pumping at rates of about 1 g.p.m., and the heat pulse 

flow meter was used to determine the locations of significant 

additional inflow and the correlative fracture flow rates. Figure 

16 summarizes the ambient (blue curve on right-hand-side) 

and pumped flow rates induced by somewhat variable rate 

pumping (red curve on left-hand-side). The summary results 

for the 52V–2D pumping flow experiment are provided in 

figure 16, where the three depth levels contributing significant 

shifts to yet higher upflow rates are, in turn, correlated with 

influxes to the well bore from fractures at those levels.

Outcrop examination of the Manassas Sandstone, like 

the Balls Bluff Member siltstone, reveals that there are three 

primary families of joints: bedding plane fractures and a set 

of cross-bedding fractures that are crudely orthogonal to each 

other. The cross-bedding joints are, in turn, orthogonal to the 

bedding planes themselves (Ryan and others, 2007a), and the 

rock is thus orthotropic with respect to fracture permeability. 

These joints provide the primary porosity for fluid storage 

and the primary permeable pathways for fluid migration. Well 

52V–2D’s log depicts two fractures contributing to flow: one 

at about 17 m depth and another at 21 m. We tentatively interpret both of these fractures to be coincident with Manassas 

Sandstone bedding planes.

The U.S. Geological Survey National Center and the Structural Eastern Margin of the Culpeper Basin 
Figure 1. Geophysical logs from the extreme eastern edge of the Culpeper basin at the U.S. Geological Survey National Center 

observation well above the Culpeper basin basement.
Figure 1. WellCAD logs from the USGS well 52V-2D on the U.S. Geological Survey National Center campus near Sunrise Valley Drive in 

Reston, Virginia.

The U.S. Geological Survey National Center and the Structural Eastern Margin of the Culpeper Basin 1
:

p

10

20

30

40

50

Figure 1. Heat pulse flow meter logs for USGS well 52V-2D at the National Center campus for conditions of pumping at 1 g.p.m (left plot) 

and for ambient (non-pumping) conditions (right plot).
Summary and Conclusions

 The results of this report are both consistent with the 

compartmentalization concept (Ryan et al., 2007a, 2007b), and 

offer further elaboration of how compartments are internally 

structured. The principal results of individual surveys are 

organized on a structural basis. The Dulles compartment.

Two wells have been logged, and both are sited wholly within 

the Balls Bluff Member of the Bull Run Formation. In Dulles 

Well No. 2 (total depth = 289 m) thirty three spikes in the 

first derivative of temperature with respect to depth, ( ), DT

DZ

suggest the inflow of groundwater from a multiplicity of 

fractures that intersect the borehole. The spikes are unevenly 

distributed with depth, however they are broadly consistent 

with the spacing between bedding plane fractures within the 

Balls Bluff Member siltstone. The Gainesville compartment.

Direct drilling has been conducted through the Gainesville 

diabase sheet into the subjacent hornfels (thermally metamorphosed after Balls Bluff Member siltstone). The diabasehornfels interface (at a depth of 183 m) represents a basal 

flow regime where the hornfels comprises the permeable 

top of the Gainesville compartment confined by the diabase 

above. Total hole depth is 226 m. Optical TeleViewer (OTV) 

logging reveals that the transition from diabase to hornfels is 

characterized by moderate fracturing within the diabase to a 

massively fractured hornfels. Pumped flow rates within the 

diabase are ~2 gallons per minute (gpm), whereas flow rates 

within the hornfels at the compartment top are ~110+ gpm. 

These results offer further evidence supporting the (basal) 

compartmentalization concept. The Rapidan compartment.

Five boreholes at the Coffeewood Correctional Center near 

Mitchells, Virginia have been logged to determine the relationships between the hornfels (thermally metamorphosed after 

Balls Bluff Member siltstone) and the diabase beneath. The 

transition from hornfels to diabase is particularly notable in 

the natural gamma counts where, in well OBS–5, a complete 

disappearance of gamma counts occurs at a depth of 107.5 m. 

Marked increases occur in the calcite vein frequency between 

72 and 104 m, but calcite veins die out dramatically above 

70 m, suggesting that the bottom 32 m of hornfels immediately above the diabase experienced hydrothermal circulation 

during the cooling of the lopolith. Pre-logging pumping tests 

conducted in well OBS–4 proximal to the underlying Rapidan 

diabase lopolith induced exceptional drawdown suggesting 

the presence of a low permeabilty barrier (the diabase). These 

results are entirely consistent with—and can be best explained 

by—the compartmentalization of Rapidan fluids. The structural eastern margin of the Culpeper basin. The eastern 

edge of the basin displays an on-lap relationship between the 

Poolesville Member of the Manassas Sandstone and the Proterozoic basement schists. The basin-basement contact and the 

overlying sediments dip about 10 degrees westward. Interbedded sandstones and siltstones comprise the Triassic Manassas 

Sandstone, and these grade downwards into the conglomerate of the Reston Conglomerate at the base of the Manassas 

Sandstone). The Reston Conglomerate lies unconformably on 

the partially weathered Proterozoic schists of the underlying 

basement. In well 52V–2D, pumping tests induced inflow to 

the well bore from three separate depth levels resulting in a net 

upward flow polarity for the well. Sites of inflow have been 

correlated with bedding plane fractures within the Manassas 

Sandstone. This well has a transmissivity, T, of 44 x 10-4 to 47 

x 10-5 ft2/day and a storativity, S, of 1 x 10-4 to 6 x 10-5.

Acknowledgments

We would like to thank the several individuals and corporations that have given generously of their time and resources 

to make field studies, in-situ experiments and monitoring on 

their properties, and/or liason with their staff possible. We 

acknowledge Robert Cohen and Charles Faust of HIS-Geotrans Co., Sterling, Virginia; Mark McClellan and Duane 

Sturgill of Groundwater Systems, Inc.; and Vince Melchior of 

Poole & Kent Company, Baltimore, Maryland, fluid handling 

contractors for America On-line Corporation’s data storage 

center at Gainesville, Virginia.

The administrative staff and the Federal, State, county, 

and municipal governments and agencies who have facilitated various aspects of this work, including access to their 

data bases, public records, their lands, and publications: Jeff 

Crossman and Robert Leake of the Coffeewood Correctional 

Center, Virginia Department of Corrections at Mitchells, 

Virginia; Mark Waslo and Michael Walsh of the Metropolitan 

Washington Airports Authority, and the Engineering Division 

of Dulles International Airport, for gracious access to Dulles 

lands, facilities, and records. The Culpeper, Fairfax, Fauquier, 

Loudoun, Prince William, and Orange County governments of 

Virginia, are thanked for their interest, support and cooperation, as are the governments of Frederick and Montgomery 

Counties, Maryland.

Glen A. Rubis of the Loudoun County Department of 

Building and Development, Leesburg, Virginia, served as a 

guide to contractors reports and assisted materially in the compilation of groundwater data bases for Loudoun county. David 

Nelms and Gary Sperian of the Richmond District of the 

Water Resources Discipline of the U.S. Geological Survey are 

cordially thanked for generously sharing resources and advice 

at several stages of this work, as are John Williams and Alton 

Anderson of the Troy, New York office of the Water Resources 

Discipline. Kevin Knutsen of the Branch of Geophysics of the 

USGS Office of Ground Water assisted in the collection of 

data at the USGS National Center.

Rama Kotra has provided administrative, financial and 

other resource support within the Eastern Region of the 

Survey`s Geologic Discipline. Debbie Colley, Donna Johnstone, Bonnie Jackson, Marylin Lane, and Holly Wilson have 

all provided helpful administrative support.

J. Wright Horton, Jr. and David L. Daniels provided 

perceptive and helpful reviews of an earlier version of the 

Summary and Conclusions 
manuscript that lead to improvements in the presentation. 

Dave Daniel’s review was completed several months prior 

to his ultimate involvement as a coauthor. It is a pleasure to 

acknowledge our colleagues in the Water Resources Discipline 

of the U.S. Geological Survey, and we thank Carole Johnson 

and George Harlow for joining us in authorship. MPR thanks 

coauthor Carole Johnson for an exceptionally thorough and 

helpful coauthor review on an initial draft of the manuscript.

Patricia Packard (USGS) prepared the page layout and 

final text. Marci Prins (Marci Prins Professional Graphic 

Production, Honolulu, HI) applied her drafting skills to select 

figures 

Century Geophysical* of Tulsa, Oklahoma, Mt. Sopris 

Instruments* of Golden, Colorado, Oasis Montaj* of GeoSoft 

Corporation*, WellCAD*, and Visual Compulog* are trade 

names. The use of trade names is for descriptive purposes 

only and does not imply endorsement by the U.S. Geological Survey, the U.S. Department of the Interior, or the U.S. 

Government.

Appendix I

Culpeper Basin, Virginia Borehole Tools

The borehole methods used in this survey involve several 

techniques for the measurement of in-situ bulk petrophysical and fluid properties. Normally, the logging equipment 

described here is configured with a dedicated logging truck. 

However, the Century Geophysical* and Mount Sopris* 

systems (see below) are modular and self-contained, and may 

be air-lifted into remote regions for specialized borehole logging operations. Asterisks (*) throughout this section denote 

registered trademarks.

Borehole measurements have been made in several different groundwater compartments throughout the Culpeper basin. 

These include composite dike- and lopolith-bounded compartments (at Dulles International Airport); lopolith-supported 

compartments (at Rapidan, Virginia); and diabase-confined 

basal-flow compartments (as at Gainesville, Virginia). In addition, we have taken preliminary measurements at the Culpeper 

basin’s eastern margin where the juxtapostion of Mesozoic and 

Proterozoic lithologies has been logged. Additional discussion of logging approaches and data evaluation procedures are 

given in Asquith and Gibson (1983), Goldberg et al. (2002), 

Johnson et al. (2002), Keys (1989), Lee (1982), Luthi (2001), 

and Paillet and Kapucu (1989).

A combination of Century Geophysics* (Tulsa, Oklahoma) and Mount Sopris* (Golden, Colorado) tools have been 

used in the borehole data acquisition. The Century* system 

is self-contained, with the three essential components being 

the selected tool, the PC-Data Logger, and the winch. This 

system includes the Induction resistivity, natural gamma, Elog, acoustic televiewer, E-M flowmeter, and three-arm caliper 

tools. Similarly, the Mount Sopris* system is comprised of 

three primary components: the selected tool, the PC-Data Logger, and the winch. For the Mount Sopris* system, the primary 

tools are the Optical Televiewer and the heat-pulse flow meter. 

For both the Century* and the Mount Sopris* systems, tool 

calibrations are set through the PC control module, and most 

data collection and data acquisition parameters are designated 

via the PC interface. Nominal adjustments in the tool ascent/

decent rates may be made directly on the winch. Details of 

tool usage at specific sites may be seen in Table 4.

Three-arm caliper. Irregularities in borehole radius and 

cross-sectional shape are produced by a number of factors 

including man-made (drilling-related) and natural causes. The 

intersection of fractures and fracture zones with the walls of 

boreholes often leads to irregularities that may be detected by 

the caliper tool. Thus the caliper detects relatively large and 

dilatant fractures that intersect the well bore. The three-arm 

caliper is configured with three spring-loaded arms separated 

by 120 degree inter-arm angles when viewed down the tool 

axis. Bedding plane fractures, which generally penetrate the 

borehole in sub-circular or elliptical sections will be crossed 

by all three arms of the caliper and, if of sufficient aperture, 

will be expressed in the caliper record as an abrupt increase 

in the well diameter versus depth curve. Thus the detection of 

fractures for the study of fracture flow is facilitated by the use 

of the caliper tool.

Electromagnetic Induction borehole methods. The 

Century* slim-hole electromagnetic induction tool contains 

three electrical coils. One of these coils is for transmitting an 

alternating current into the surrounding formation and a second coil is designated for receiving. The transmitting coil sets 

up a ground loop comprised of eddy currents, which in turn, 

set up secondary magnetic fields that induce an electrical voltage received in the upper receiving coil. This received voltage 

is directly proportional to the electrical conductivity of the 

surrounding rocks (Keys, 1989). The radius of the measurement region (zone of influence) surrounding the wellbore is, 

variably, 25.4 to 127 cm beyond the wellbore wall. Electrical 

resistivity is the inverse of electrical conductivity.

Fluid tool. This is a ‘multiple sensor’ tool that has 

several built-in capabilities, including: fluid resistivity, specific conductance, natural gamma, and temperature sensors. 

Specific conductance can be calculated because both the fluid 

resistivity probe and the temperature probe are run simultaneously allowing corrections to 25 degrees Celsius.

Optical TeleViewer (OTV). The optical televiewer combines a fixed-focus, short-focal-length camera, a light source, 

and a conical mirror assemblage to image the entire borehole 

in a 360 degree perspective. Pixelation of the images interacts 

with the ascent/decent rate of the tool to determine the overall 

image quality and resolution (see the discussion above under 

the specific sections related to the data derived from this tool). 

During data acquisition, OTV images of the borehole are 

displayed (unwrapped) in real time on the PC monitor in the 

logging truck. Typically, the data is further processed by use 

of WellCAD (4.2) software, where the image may be further 
analyzed for lithologic, mineralogic, and structural information. The study has used a total of three optical televiewers: an 

Advanced Logic Technologies (ALT) OBI-40, a RAAX BIPS, 

and a Robertson Geologging OTV.

Acoustic TeleViewer (ATV). The acoustic televiewer is 

comprised of an ultrasonic transducer that both transmits and 

receives acoustic waves. The transmitter rotates several times 

per second and emits short bursts of ultrasonic pulses. The 

pulses transit the borehole fluid (water) and are reflected off 

the borehole wall. Returning to the tool, the acoustic pulses 

are then received by the transducer. Two measurements are 

made by the ATV: the total (two-way) travel time of the ultrasonic wave, and the amplitude of the reflected signal (Luthi, 

2001). The televiewer digitizes 256 measurements around 

the borehole at each depth (DZ) interval (0.005 meters). The 

data is referenced to true North and then displayed using 

Visual Compu-Log* software or WellCAD (4.2). Because the 

transducer is rotating at the same time the tool is being either 

lowered or raised in the borehole, the actual data acquisition pathway is spiral in geometry. For each ‘scan’ the data is 

pixilated to the depth interval. During data acquisition, images 

of the borehole’s acoustic signature are displayed (unwrapped 

and oriented) in real time on the PC logging monitor.

The acoustic reflection coefficient, R, is a function of the 

summed products and differences of the densities (ρ) and compressional wave velocities (V) of the borehole fluid (subscript 

‘1’) and the borehole walls (subscript ‘2’)

r2V2 - r1V1

R = r2

V2 + r1V1

where the product is referred to as the acoustic imped rV -

ance. The acoustic impedance of water is 1.5 and sandstone is 

9.0 Mrayl (kg⋅ m-2 ⋅ S-1). In ATV records, areas of poor energy 

return such as fractures and cavities within the well bore wall 

appear as dark lines, dark bands and dark ‘welts’, and contrast 

sharply with the surrounding bright regions of higher acoustic 

impedance such as competent wall rock. Also, impedance 

may show differences in rock types and travel times will 

reflect enlargements in the borehole. In practice, two acoustic 

calipers, continuous temperature, 3-axis magnetometers and 

2-axis accelleration sensors for deviation are also recorded 

during downhole data acquisition.

Natural gamma radiation: The Century* Slim-Hole 

induction tool. Natural gamma ray logging determines the 

total counts of Gamma emissions related to thorium, uranium 

and potassium—three naturally radioactive elements. The concentrations are typically detected through measurements using 

a NaI(Tl) scintillation counter, oriented along the borehole 

tool. The geochemistry of the naturally occurring radioactive 

elements with respect to the mineral species that they form 

common associations with include: (i) for Thorium [Th-232 

series]: Monazite; (ii) for Uranium [U-238 series]: clays, 

organic phosphates and monazite; and (iii) for Potassium 

[K-40]: orthoclase, muscovite, biotite, evaporites and illite. 

The emission of natural gamma radiation from Th, U or K in 

association with various clay minerals has lead to the general 

use of the tool as a ‘shale indicator’ (Luthi, 2001). The volume 

of clay, Vcl , is given by 

GR - GR

Vcl = c · min

GRmax - GRmax

where GR is the gamma ray reading in the depth interval of 

focus, the coefficient c accounts for the relative abundance of 

clay in shales (a percentage factor, usually about 0.6), and the 

max-min notations refer to a shale (maximum clay content) 

and a clay-free lithology (minimum clay content), respectively. 

In the context of the present study, while the inferred clay 

content of the siltstone units varied, the most salient aspect of 

the variable gamma counts are the virtually complete lack of 

emissions in diabase bodies.

Fracture flow monitoring and measurement. A Mount 

Sopris* heat pulse flow meter has been used to determine the 

flow rates from discrete fractures. In principle, the heart of 

the flow meter is a pair of thermistors that are symmetrically 

mounted along the tool axis above and below a fluid heater 

grid. A rubber diaphram near the heater grid and between the 

entry ports seal the well bore from fluid-tool bypass, insuring 

that all water moves through the tool after entering one of the 

two fluid intakes. The direction of flow may be, of course, 

either upward or downward. The closure of a heating element 

circuit thermally ‘tags’ the parcel of water in proximity to 

the heater, which then flows along the tool axis until passing 

either the upper or lower thermistors. Measuring when the parcel passes and which thermistor is passed, and knowing what 

the separations are between heater and sensor—in combination with the tool inside diameter—makes the determination of 

both the flow direction and flow rate possible.

Appendix II

Locations of Culpeper Basin Wells for the 

Geophysical Borehole Survey

In table AII-1, the wells correspond to specific compartments within the Culpeper basin. These are, in order of discussion within the text: the Dulles compartment; the Gainesville 

compartment; and the Rapidan compartment. The final 

structural region discussed is the eastern margin of the basin 

at the U.S. Geological Survey National Center. In the Table, 

the entries corresponding to these compartments are, respectively, for the Dulles compartment: “Dulles #2 and #3”; for the 

Gainesville compartment: “AOL Gainesville”; for the Rapidan 

Compartment: “Coffeewood”; and for the eastern structural 

margin of the basin: “National Center”.

Appendix II 
Table AII-1. Locations and drilling data for Culpeper basin and wells.

Well Latitude Long UTME UTMN GPS

elev.

Case

dia.

Drilled

depth

Log

depth

Water

depth Date

National Center 52V-1D 38.95125 -77.36627 294942 4313822 0.25 47.5 3.14 1/23/2001

National Center 52V-2D 38.94879 -77.3667 294900 4313550

Dulles #2 38.94235 -77.33686 287466 4312768 106.4 0.33 291 278 7.1 1/10/2001

Dulles #3 38.93194 -77.43645 288008 4311860 109.7 0.2 313.9 41.4 1/10/2001

Coffeewood #1 38.36628 -78.03282 759241 4250418 103 0.15 83.8 76.49 3.9 11/28/2000

Coffeewood #2 38.36992 -78.03177 759321 4250825 95 0.15 83.8 77 8.99 11/28/2000

Coffeewood #3 38.37278 -78.02333 760046 4251365 0.15 80 8.67 12/1/2000

Coffeewood #4 38.36831 -78.00931 761284 4250916 0.15 94.5 9.6 12/1/2000

Coffeewood #5 38.36559 -78.02879 759598 4250352 86 0.24 137.2 118 2.24 11/27/2000

AOL Gainsville 38.77644 -77.59586 274484 4295158 116 0.2 225.5 222.25 18.1 7/19/2001

Appendix III

Culpeper Basin Borehole Data Files

There are two document types that are handled by the 

WellCAD reader, a borehole document (with a .wcl file extension name) and a field document (with a .wcf file extension 

name). Both file types are included on the DVD in a directory 

called “Culpeper Basin Borehole Data.”

To view data (.wcl or .wcf) files you will need to install 

the WellCAD reader (4.2) on this DVD in the directory called 

“WellCAD Reader” or download the free WellCAD file reader 

from Advanced Logic Technologies [ALT]. [URL:http://www.

alt.lu]. The reader version of WellCAD included here allows 

you to open files, rotate the virtual core, view the logs and the 

data, scale, and print WellCAD files

Appendix IV

Culpeper Basin, Virginia Borehole Location 

Index Maps and Table:

Site Access and Well Location Logistics

Maps of use in well site access are organized in this 

appendix (figs. AIV-1 through AIV-5). The maps are based 

on standard USGS 7.5 min., 1:24,000 scale topographic base 

maps. They include the Mitchells, Virginia area (Culpeper 

County, Virginia), Dulles International Airport Authority 

(Fairfax and Loudoun Counties, Virginia), the Gainesville 

America On-Line (AOL) site (Prince William County, Virginia), and the USGS National Center site (Fairfax County, 

Virginia). With the sole exception of the U.S. Geological

Survey, all other areas that are either Federal, State of Virginia, 

or privately-owned and are generally closed to the public. Special permissions must be secured to gain access. The maps are 

presented here for completeness, and to place the wells within 

their cultural context.

Appendix V

A Glossary of Selected Terms

Definitions of hydrologic terms and porous media trans–

port parameters may be found in Bear (1972, 1979), Dullien 

(1979), Strack (1999), Polubarinova-Kochina (1962), and 

Kovacs (1981). In certain cases, terms below may not appear 

within the text but are, instead, used as part of the definition of 

other terms that do appear.

Darcy’s law: Darcy’s law expresses the relationship between 

fluid flow and the driving forces for flow in combination with 

the resistive agents that oppose or retard the flow. It is quite 

analogous to Fourier’s law of heat conduction, Fick’s law of 

diffusion, Newton’s law of viscosity, and Ohm’s law in electricity. The volumetric flow rate, Q, is:

Where A is the cross-sectional area of the reference volume, 

L is the length of the reference volume, P is the piezometric 

head of the fluid, hshear is the dynamic shear viscosity, and k is 

the permeability.

Q = –(kA/hshear)(DP/L)
Dulles International Airport Authority wells (Loudoun County, Virginia) 

The Dulles compartment

Figure AIV-1. Map showing locations of two Dulles area boreholes. The Dulles area borehole data set contains two logged wells. This 

map is an index to the original sites and their selection location projected onto a portion of the original 7.5 min. quadrangle map. The 

grid is 500 m on a side. Data for the surveys may be downloaded by clicking on: data.

Appendix V 
Gainesville, Virginia. America On-Line (AOL) well (Prince William County, Virginia)

The Gainesville compartment

Figure AIV-2. Maps showing location of the Gainesville, Virginia, area borehole (AOL-GV-740), which was completed on America OnLine’s data center site. Data for this survey may be downloaded by clicking on: data.
Mitchells, Virginia area wells (Culpeper County, Virginia) 

The Rapidan compartment

Figure AIV-3. The Mitchells area, Culpeper County, Virginia. The Coffeewood Correctional Facility property contains 5 wells that were 

logged in this study. Coffeewood wells OBS–1 and OBS–5 contain optical televiewer data. The location map contains the positions of 

the wells projected on a line. Data for the surveys may be downloaded by clicking on: data.

Appendix V 
U.S. Geological Survey (USGS) National Center, Reston, Virginia.

Eastern structural margin of the Culpeper basin. (Fairfax County, Virginia)

Figure AIV-4. The U.S. Geological Survey National Center observation wells (52V–1D) and (52V–2D), in Reston, Virginia. These 

boreholes were completed on the 105-acre National Center campus and are located at the extreme eastern edge of the Culpeper basin. 

Data for these surveys may be downloaded by clicking on: data.
Hydraulic conductivity: The hydraulic conductivity of 

an aquifer is the ability of the rock mass to transport aqueous fluids under the drive provided by hydraulic gradients 

and resisted by the properties of the fluid. It is defined under 

‘permeability’ below, and is symbolized as K.

Permeability: The permeability, k, may be defined in 

terms of the hydraulic conductivity, K, as:

K = kg/h shear = kg/hkinematic

Where g is the fluid specific weight, and , g = rG where G is 

the accelleration of gravity and r is the fluid density. In the 

permeability expression, hshear and hkinematic are the dynamic 

shear viscosity and the kinematic viscosity, respectively. Note 

in particular that the expression for hydraulic conductivity 

contains entities that relate only to the properties of the porous 

medium (k), and entities that relate only to the properties of 

the fluid passing through it (g, h , and h shear kinematic).The dimensions of permeability are L2

, and are usually expressed as m2. 

Sometimes k is called the intrinsic permeability.

Piezometric head: The total of the elevation head and 

the pressure head is referred to as the piezometric head. The 

piezometric head is measured with a piezometer, or an open 

tube whose diameter is sufficiently large to avoid capillary 

effects, and screened in its lower sections to freely admit the 

fluids sampled. Collectively, piezometric heads measured 

independently over an extended region are referred to as a 

piezometric surface. Each measurement point within such a 

data group potentially contains an elevation component and a 

pressure component, thus the total piezometric surface must 

also be viewed as the conceptual sum of a elevation surface 

and a pressure surface.

Storativity: The storativity (S) of a body is defined as 

the volume of fluid released from (or taken into) the body 

(of unit horizontal cross-section) per unit decline (or rise) in 

the piezometric head. Alternately, it may be viewed as the 

volume of fluid added to the body under conditions promoting 

a decrease (or increase) of the piezometric head. Storativity is 

also referred to as the coefficient of storage.

Transmissivity: The transmissivity (T) of a body indicates an aquifers ability to transmit fluid through its entire 

thickness, h. The formation transmissivity is the product of 

the hydraulic conductivity (K) and the aquifer thickness. The 

parameter is dimensionless.

T=Kh

For example, basal flow beneath confining diabase sheets 

and lopoliths may occur within the underlying hornfels and 

within the deeper sandstone/siltstone sequence, and these units 

may act collectively as an aquifer transmitting groundwater 

horizontally. Transmissivity is an indirect measure of fluid 

properties through its component hydraulic conductivity.

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Levy, 2000, On the geological and geophysical framework 

of Eastern North American Mesozoic basin hydrology: 

Flow deflection and compartmentalization induced by 

diabase intrusions. EOS Transactions of the American Geophysical Union. Spring Annual Meeting, Washington, D.C.; 

Spring Meeting Program. 81, No. 19, p.S-262.

Ryan, M.P, D.M. Sutphin, D.L. Daniels, H.A. Pierce, and J.P. 

Smoot, 2002, Three-dimensional compartmentalization of 

subsurface ground water flow in eastern North American 

Mesozoic basins. EOS. Transactions of the American Geophysical Union. Spring Annual Meeting, Washington, D.C., 

Spring Meeting Program. 83, No.19, p.S-151.

Ryan, M.P., D.L. Daniels, D.M. Sutphin, H.A. Pierce, P.M. 

Carr, J.P. Smoot, J.A. Costain, C. Coruh, and L.E. Kearns, 

2007a, (in prep.), Compartmentalization and fracture flow 

of crustal fluids in the Mesozoic Culpeper and Barboursville basins: Conceptual, analytic and numerical models for 

paleo- and contemporary flow in volcanic and non-volcanic 

rift zone complexes. U.S. Geological Survey. Scientific 

Investigations Report. 224 ms pages, 185 figures, 9 tables.

Ryan, M.P., P.M. Carr, A. Ingerov, D.L. Daniels, D.M. 

Sutphin, H.A. Pierce, and J.P. Smoot, 2007b, (in prep.), 

Solitary wave-mediated magma ascent and emplacement, 

Rayleigh-Taylor and Saffman-Taylor instabilities, and the 

dynamics of compartmentalized hydrothermal fluid flow: 

Application to the crustal magma conduits and intrusions 

of the Mesozoic basins of eastern North America. U. S. 

Geological Survey Professional Paper. 57 ms pages, 142 

figures, 1 table.

Ryan, M.P., H.A. Pierce, A. Ingerov, G. Elliott, G. Thompson, G. Torrico, C. Finateu, Y. Avram, D.L. Daniels, D.M. 

Sutphin, J.P. Smoot, T. Musek, D. Fox and L. Fox, 2007c, 

(in prep.), Magnetotelluric (MT) surveys of the Mesozoic 

Culpeper basin of Virginia: Data, inversions, and structural 

systheses of basin cross-sectional structure and diabase 

intrusions. U.S. Geological Survey Open File Report. (OnLine and DVD).

Ryan, M.P., H.A. Pierce, D.L. Daniels, P.M. Carr, and D.M. 

Sutphin, 2007d, (in prep.), Reconnaissance AudioMagnetotelluric (AMT) surveys of the island of Hawai’i: Data and 

comments related to the hydrogeologic structure of saddles, 

rift zones, dike-induced compartmentalization and the subsurface saline-freshwater interface. 53 ms pages; 23 figures 

and maps. U.S. Geological Survey Open File Report.
Smoot, J.P., 1989, Fluvial and lacustrine facies of the Early 

Mesozoic Culpeper basin, Virginia and Maryland. Field 

Trip Guidebook T213. American Geophysical Union. Washington, D.C. 15 p.

Smoot, J.P., 1991, Sedimentary facies and depositional environments of early Mesozoic Newark Supergroup basins, 

eastern North America. Paleogeography, Paleoclimatology 

and paleoecology. 84, 369-423.

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Weems, R.E. and Olsen, P.E., 1997, Synthesis and revision 

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