In Cooperation with the National Park Service 

Gravity Investigations of the Chickasaw 

National Recreation Area, south-central 

Oklahoma 

By Daniel S. Scheirer and Allegra Hosford Scheirer 

Any use of trade, firm, or product names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government 

Open-File Report 2006-1083

U.S. Department of the Interior 

U.S. Geological Survey
Contents 

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

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

Geologic Setting ............................................................................................................................................................3

Gravity Observations and Analysis ...............................................................................................................................4

Results ...........................................................................................................................................................................6

Discussion......................................................................................................................................................................8

Faults beneath Chickasaw National Recreation Area................................................................................................8

Basement variation and gravity anomalies ................................................................................................................9

Recommendations for future study............................................................................................................................9

Acknowledgments .......................................................................................................................................................10

Literature Cited............................................................................................................................................................12

Appendix: Continuous Gravity Results .......................................................................................................................32

Figures 

Figure 1. .....................................................................................................................................................................14 

Figure 1 (continued). ................................................................................................................................................15 

Figure 1 (continued). ................................................................................................................................................16 

Figure 2. .....................................................................................................................................................................17 

Figure 2 (continued). ................................................................................................................................................18 

Figure 3. .....................................................................................................................................................................19 

Figure 4. .....................................................................................................................................................................20 

Figure 5. .....................................................................................................................................................................21 

Figure 6. .....................................................................................................................................................................22 

Figure 7. .....................................................................................................................................................................23 

Figure 7 (continued). ................................................................................................................................................24 

Figure 7 (continued). ................................................................................................................................................25 

Figure 7 (continued). ................................................................................................................................................26 

Figure 8. .....................................................................................................................................................................27 

Figure 9. .....................................................................................................................................................................28 

Figure A1. ..................................................................................................................................................................29 

Figure A2. ..................................................................................................................................................................30 

Figure A3. ..................................................................................................................................................................31 

Tables 

Table 1. Depth to base of Vanoss Formation (and top of Arbuckle Group) rocks in three wells in CNRA ...............11

Table 2. Physical properties of rock samples grouped by stratigraphic unit...............................................................11

Table 3. Gravity base stations used for data collected in 2004 and 2005....................................................................11

Table A1. Principal facts for gravity stations collected in 2004 and 2005 in south-central Oklahoma ......................35

Table A2. Physical property measurements of rock samples collected in 2004 and 2005..........................................40
Gravity Investigations of the Chickasaw 

National Recreation Area, south-central 

Oklahoma 

By Daniel S. Scheirer and Allegra Hosford Scheirer 

Abstract 

The geological configuration of the Arbuckle Uplift in the vicinity of Chickasaw National Recreation Area 

in south-central Oklahoma plays a governing role in the distribution of fresh and mineral springs within the park and 

in the existence of artesian wells in and around the park. A confining layer of well-cemented conglomerate lies 

immediately below the surface of the recreation area, and groundwater migrates from an area of meteoric recharge 

where rocks of the Arbuckle-Simpson Aquifer crop out as close as two kilometers to the east of the park. 

Prominent, Pennsylvanian-aged faults are exposed in the aquifer outcrop, and two of the fault traces project beneath 

the conglomerate cover toward two groups of springs within the northern section of the park. We conducted gravity 

fieldwork and analysis to investigate the subsurface extensions of these major faults beneath Chickasaw National 

Recreation Area. By defining gravity signatures of the faults where they are exposed, we infer that the Sulphur and 

Mill Creek Faults bend to the south-west where they are buried. The South Sulphur Fault may project westward 

linearly if it juxtaposes rocks that have a density contrast opposite that of that fault’s density configuration in the 

Sulphur Syncline area. The Sulphur Syncline, whose eastern extent is exposed in the outcrop area of the ArbuckleSimpson Aquifer, does not appear to extend beneath Chickasaw National Recreation Area nor the adjacent City of 

Sulphur. The South Sulphur Fault dips steeply northward, and its normal sense of offset suggests that the Sulphur 

Syncline is part of a graben. The Mill Creek Fault dips vertically, and the Reagan Fault dips southward, consistent 

with its being mapped as a thrust fault. The Sulphur and Mill Creek Synclines may have formed as pull-apart basins 

in a left-lateral, left-stepping strike-slip environment. The character of the gravity field of Chickasaw National 

Recreation Area is different from the lineated gravity field in the area of Arbuckle-Simpson Aquifer outcrop. This 

change in character is not due to the presence of the overlying conglomerate layer, which is quite thin (<100 m) in 

the area of the park with the springs. The presence of relatively high-density Precambrian basement rocks in a 

broader region suggests that significant gravity anomalies may arise from variations in basement topography. 

Understanding of the geological configuration of Chickasaw National Recreation Area can be improved by 

expanding the study area and by investigating complementary geophysical and borehole constraints of the 

subsurface. 

Introduction 

Chickasaw National Recreation Area (CNRA) was established over a century ago to protect the natural, 

cultural, and recreational resources of an area of south-central Oklahoma that is rich in springs, streams, and artesian 

wells (fig. 1a) (Gould and Schoff, 1939; Hanson and Cates, 1994). The juxtaposition of eastern forest and western 

prairie ecosystems yields a unique array of flora and fauna in the Midcontinent, and mineral and fresh water springs 

have attracted people to this area for centuries. Studies of the hydrology of CNRA and its vicinity have been 

important since its inception (for example, Gould, 1906; Gould and Schoff, 1939; Hart, 1974; Goemaat and Willard, 

1983; Fairchild and others, 1990; Hanson and Cates, 1994), with goals of understanding the natural variability of 

groundwater and surface water flows and compositions, and of estimating the impact of water usage outside of 

CNRA to water features within the park. The geologic configuration (figs. 1b, 1c) of CNRA, the adjacent City of 

Sulphur, and the surrounding regions has long been recognized as a primary control on groundwater flow in this 
region, connecting areas of outcrop of the Arbuckle-Simpson Aquifer to the east of CNRA where there is meteoric 

recharge with areas beneath the park where the aquifer is confined. The aims of the geophysical study described in 

this report are to elucidate the geologic structures in the subsurface that might be important in promoting or 

impeding groundwater flow into and within CNRA. The focus of this study is in the northern section of CNRA – 

the Travertine District – where the springs occur. 

The hydrogeology of CNRA was comprehensively described by Hanson and Cates (1994), and the aspects 

that are important to ground and spring waters are summarized in this paragraph. Precipitation recharges the 

Arbuckle-Simpson Aquifer where it crops out in a broad area of elevated topography to the east of CNRA; this 

outcrop area is named the Hunton Anticline (fig. 2). The Arbuckle-Simpson Aquifer outcrop of predominantly 

carbonate rocks, with lesser sandstone and shale units (fig. 3), extends to within about 2 km of the Travertine 

District, where Arbuckle-Simpson Aquifer units dip westward unconformably beneath a thickening wedge of 

younger, tightly-cemented conglomerate (fig. 1b). Prominent faults segment the Arbuckle-Simpson Aquifer 

lithologic units into distinct blocks. The overlying conglomerate of the Vanoss Formation acts as a confining layer 

to the Arbuckle-Simpson Aquifer, and the intersection of the potentiometric surface of the aquifer with the land 

surface produces the springs within CNRA. Wells that penetrate into the Arbuckle-Simpson Aquifer are commonly 

artesian, especially at lower ground elevations. Two fresh water, bicarbonate-type springs, Buffalo and Antelope, 

are located in the eastern Travertine District (fig. 1c), and they have 1 to 2 orders of magnitude greater discharge 

than the mineral springs of the park. The mineral springs are located in the central and western Travertine District 

(fig. 1c). Those in the west (Bromide and Medicine) have greater salinity and total dissolved solid content than those 

in the center. This configuration has led to the suggestion that fresh and mineralized groundwaters mix in the center 

of the Travertine District (for example, Hanson and Cates, 1994). Water wells tapping the Arbuckle-Simpson 

Aquifer in the Sulphur area have water compositions that mimic the general east-to-west gradient, from fresher to 

more mineralized, of the springs. The best-known well, Vendome Well, was originally drilled in 1922, and it 

supplied a public swimming pool for about 50 years. Vendome Well is now within the Travertine District of 

CNRA, and it continues artesian flow. 

Flow in the springs and wells has varied significantly over the past century. On numerous occasions, flow 

in Buffalo and Antelope Springs ceased; these periods are well-correlated with times of drought (Hanson and Cates, 

1994). Relative to the fresh water springs, discharge at the mineral springs and the artesian wells is less correlated 

with precipitation, but flow of mineralized waters has nonetheless varied significantly. Many wells were artesian 

when drilled in the first half of the 20th century, but now they must be pumped to produce water. An initial census of 

springs within the park included 33 active springs (Gould, 1906). In 1939 only 19 could be located (Gould and 

Schoff, 1939), and in 1987 about 7 were reported to have measurable flow (Hanson and Cates, 1994). There are 

several reasons for the decreases in the spring counts: in the 1930’s, flows of neighboring diffuse springs were 

combined into single outlet constructions; trees planted in the same decade increased transpiration rates; and many 

artesian wells were drilled in the vicinity of the park in the 1920’s and 1930’s and their flows were unrestricted. 

While the number of flowing mineral springs has decreased through the years in part because of man-made 

plumbing changes, their collective discharge has substantially decreased due to other causes. For example, Gould 

and Schoff (1939) estimate that the flow rates of the mineral springs decreased by about 80% over the prior three 

decades. Hanson and Cates (1994) report that impacts of pumping at City of Sulphur water wells and Oklahoma 

Gas and Electric wells to the north of the Travertine District were not discernible at wells and springs within CNRA, 

although they suggest that there may be impact during prolonged dry periods. They suggest, like prior authors, that 

unrestricted flow from artesian wells may be responsible for decreased discharge of springs within CNRA. In the 

broader Arbuckle-Simpson Aquifer area, applications to remove groundwater from the Hunton Anticline to supply 

communities in other parts of Oklahoma led to the passage of Senate Bill 288 by the Oklahoma State Legislature in 

May 2003. Among other items, this Law initiated a comprehensive study of Arbuckle-Simpson Aquifer hydrology 

to allow more informed management and protection of the region’s water resources (Oklahoma Water Resources 

Board, 2003). 

To delineate subsurface structures that might be important for groundwater flow, Steven Cates conducted a 

study of CNRA based on gravity and magnetic observations and on structural geology (Cates, 1989). Using 

geophysical observations, modeling, and borehole information, he endeavored to define the geometry of faults and 

crustal blocks where they are obscured by Vanoss Formation. In 2004, the National Park Service funded the U.S. 

Geological Survey to conduct additional gravity investigations in and around CNRA, and this report documents new 

insights about the subsurface from these recent studies. 

2 
Geologic Setting 

Chickasaw National Recreation Area is situated on a portion of the northern margin of the Southern 

Oklahoma Aulacogen (Hoffman and others, 1974), a Cambrian rift that initiated a long history of marine 

sedimentation, followed by a structural inversion during Pennsylvanian orogenies and finally erosion to its current 

level. The geology of the region, termed the Arbuckle Uplift, has been extensively studied (for example, Taff, 1904, 

1928; Ham, 1950), and most geological maps of the area derive from Ham and others (1954) (fig. 2b). More recent, 

detailed geological studies of Paleozoic stratigraphy, oil basins surrounding the outcrop area of the ArbuckleSimpson Aquifer, and quarries within the outcrop area are available, as are numerous geological summaries and 

field guides and some reinterpretations of regional geology in a plate tectonic context. 

 The Arbuckle Uplift is one of the few structural highs where Precambrian basement rocks are exposed in 

the Midcontinent. The uplift area, also termed the Arbuckle Mountains, is comprised of three major structural 

elements: from west to east, these are the Arbuckle Anticline, the Tishomingo Anticline, and the Hunton Anticline 

(fig. 2). These structures are variably deformed, are separated by major Paleozoic fault zones, and are surrounded 

by deposits of younger, minimally-deformed sedimentary rocks. The Hunton Anticline region (fig. 2) is comprised 

of a number of smaller structural units, including the Sulphur and Mill Creek Synclines and the Belton Anticline on 

its western margin near CNRA (fig. 1). 

Precambrian granitic and granite-gneiss basement is present in the eastern portion of the Tishomingo 

Anticline (fig. 2b) and has been dated at 1,350 Ma (Ham and others, 1964). The next younger unit present in the 

Arbuckle Uplift is the Colbert Rhyolite that erupted within the active rift graben in the Cambrian. This igneous 

activity was followed by a long period of widespread marine deposition of primarily carbonate units in Cambrian 

through early Ordovician times (rocks of the Timbered Hills Group and the Arbuckle Group) followed by deposition 

of carbonate, sandstone, and shale units in middle Ordovician (the Simpson Group) through early Pennsylvanian 

times (fig. 3). Early Paleozoic sediment accumulation in the area of the present-day Arbuckle Anticline was much 

greater than in other areas of the uplift because of enhanced subsidence of the aulacogen trough. Over 11 km of 

sediment accumulated in the axis of the aulacogen in Cambrian through Pennsylvanian times, while the 

accumulation on the Hunton Anticline, considered to be on stable continental shelf was about one quarter as much 

(Ham, 1969). In Pennsylvanian time, a series of orogenies deformed the earlier Paleozoic sedimentary section, first 

uplifting the Hunton Anticline and then culminating in the late Pennsylvanian Arbuckle Orogeny, which produced 

intense thrust faulting and folding that formed the Arbuckle Anticline (fig. 2). The Washita Valley Fault Zone is 

thought to be the northern boundary of the aulacogen rift (fig. 2), and this Cambrian extensional structure was likely 

reactivated as a steep reverse and strike-slip fault in Pennsylvanian time. The syn- and post-tectonic erosional 

products of the Arbuckle Mountains are preserved as proximal coarse-grained clastic rocks. In the vicinity of 

CNRA, these rocks primarily consist of the conglomerate of the Vanoss Formation. The conglomerate forms a thin 

veneer over older Paleozoic units in the Travertine District of CNRA. In water wells in central and eastern 

Travertine District, the base of the Vanoss Formation occurs within 100 m of the surface (table 1). At the base of a 

meander scarp (figs. 1b, 1c), Rock Creek has eroded through Vanoss Formation to expose small outcrops of 

Ordovician rocks within CNRA (Cates, 1989; Johnson, 1990). 

 The rocks of the Arbuckle Group are carbonates, dominantly limestones in the western portion of the 

Arbuckle Uplift and dolomites in the Hunton Anticline region. Karst features are present in some locales. Rocks of 

the Simpson Group overlie the Arbuckle Group and include carbonates, but clastic units such as quartz sands and 

shales become increasingly common. The hydrologic properties of the rocks of the Arbuckle and Simpson Groups, 

along with those of the thinner and older Timbered Hills Group, are similar, and most investigators include these 

units in the definition of the Arbuckle-Simpson Aquifer (fig. 3). 

 Faulting during the Pennsylvanian orogenies was complex. Major faults and fault zones that offset 

stratigraphic units and that are exposed over scales of tens of kilometers are highlighted in the geologic maps of 

Figures 1 and 2. These faults are oriented consistently ~N70W, and they are considered to be the result of 

widespread wrench and thrust deformation. Based on stratigraphic offsets, wrench (strike-slip) offsets are 

interpreted to be in a left-lateral sense and are estimated to range from a few kilometers to perhaps 60 km for 

individual faults (Ham, 1955; Tanner, 1967). Fault dip was not determined in early geologic mapping. A 

predominance of thrust deformation, versus earlier interpretations of strike-slip faulting, has been suggested by 

Denison (1982) and Brown (1984), who infer that some of the apparent left-lateral offset of units across the faults 

may arise from juxtaposition of previously-dipping units by the thrust motion. Numerous smaller faults are present 

3 
throughout the Arbuckle Uplift and are displayed as dashed lines in the geological maps of this report. These 

subsidiary faults are characterized by short lengths, small offsets of stratigraphic units, and a wide range of 

orientations; they terminate against each other and against the major WNW-ESE oriented faults. The hydrologic 

importance of the major faults relative to their minor counterparts is unknown. 

 Two of the major faults exposed in the Hunton Anticline are close to the Travertine District of CNRA, and 

they have long been suspected as having a role in groundwater flow beneath CNRA. The main strand of the Sulphur 

Fault terminates in outcrop about 2 km east of CNRA, and it projects linearly to a position very near to the freshwater Buffalo and Antelope Springs in easternmost Travertine District (fig. 1c). The South Sulphur Fault projects to 

the mineral springs in the western Travertine District. 

The variations in lithology across the study area are associated with changes in physical property values 

(table 2). In this area, the Precambrian basement has the greatest density (seen better in regional gravity variations 

than in table 2), and the Pennsylvanian coarse-grained clastic rocks have the lowest densities. Changes in density, 

especially at interfaces such as faults or basement contacts, produce gravity anomalies at the earth’s surface that can 

be detected and characterized by gravity studies. 

Gravity Observations and Analysis 

In 2004 and 2005, the U.S. Geological Survey collected gravity observations in two field sessions, each 

utilizing two gravity meters: a conventional LaCoste-Romberg land G-meter and a continuous BGM-5 air/sea 

gravimeter. Our deployment of the latter instrument on a land vehicle is the first such deployment to our 

knowledge. This report will focus on results from the discrete gravimeter stations, and the Appendix summarizes 

the continuous gravimetry results to-date. 

We collected gravity at 257 new stations (table A1) to supplement the 183 stations of Cates (1989) (fig. 4). 

In addition, we included 24 gravity stations from the GeoNet - United States Gravity Data Repository System 

compilation (http://paces.geo.utep.edu/gdrp/) for regional coverage. For the recent fieldwork (2004/2005), we 

established local gravity base stations in CNRA at Vendome Well, at the parking lot of Vendome Well, and at the 

Rock Creek Campground (table 3). Values of gravity at these local bases were tied to the IGSN71 gravity datum 

(Jablonski, 1974) via double-loop surveying to the permanent gravity base station at the Coalgate (Okla.) fire 

station. Cates (1989) also used a gravity base at Vendome Well for his surveying, but he assigned it an observed 

gravity value that differed from our calculated IGSN71 value by 28.8 mGal. To merge the new gravity stations with 

the Cates (1989) observations, we used that Vendome base difference as a constant correction for the Cates (1989) 

gravity values, and this correction resulted in acceptable matches of new and old observations across the entire study 

area. The 24 regional gravity stations did not require a gravity correction to provide a suitable match with the 

2004/2005 gravity values. 

Geographic positions of the new gravity stations were recorded with a Trimble GeoXT Global Positioning 

System (GPS) receiver. By using fixed GPS reference stations within ~100 km of the gravity observations, latitude 

and longitude values were calculated via post-processing to have a precision generally better than 1 meter, and 

elevations had precisions of about 1 meter. Most LaCoste-Romberg stations were collected adjacent to the field 

vehicle, but in a number of instances we hiked to places that did not have road access. At gravity stations situated 

on rock outcrop, we collected 21 rock hand-samples, and we measured their density and magnetic susceptibility 

properties in the lab (Johnson and Olhoeft, 1984) (table A2). In addition, we measured the physical properties of 9 

rocks collected for us by personnel of the Oklahoma Water Resources Board (OWRB). 

Values of observed gravity were calculated at the new stations by accounting for tidal acceleration effects 

and for instrument drift constrained by gravity base ties at the beginning and end of each field day. We calculated a 

series of predictable gravity effects at all of the stations to account for: the global gravity field, the reduction in 

gravity with increasing elevation (free-air), the mass between the station and the geoid (simple Bouguer), the 

topographic variation near the station (terrain), and the compensating mass near the base of the crust (isostatic). 

(These effects are often termed corrections in the geophysical literature.) The final gravity anomaly after 

incorporation of these effects is termed the isostatic gravity anomaly, and it is useful for interpretation because it 

primarily reflects the density variations in the upper- and mid-crust (Simpson and others, 1986). For new gravity 

stations, field estimates were made of the terrain effect in a zone from the station out to a radius of 68 m; for the 

4 
Cates (1989) and GeoNet stations, this innermost terrain effect was not available. For all stations, digital terrain 

effects beyond 68 m were calculated from digital elevation models (DEMs) in two stages: from 68 m to 2 km and 

from 2 km to 167 km using the algorithms of Plouff (1977). Other parameters that were used in the calculation of 

gravity effects are typical for continental gravity studies; these include an upper crustal density of 2.67 g/cm3

, a 

mantle-crust density contrast of 0.4 g/cm3

, and a nominal crustal thickness at sea level of 25 km. A typical 

uncertainty of a gravity anomaly from a new gravity stations is estimated to be ~0.2 mGal, and uncertainty from a 

prior gravity station is more variable and thought to be somewhat larger. In all cases, the greatest component of 

gravity error arises from uncertainties in elevation. 

Gravity station data were gridded with a 100 m spacing (fig. 5), which is somewhat finer than the average 

station-spacing. We investigated whether any of the stations had gravity values significantly different from their 

neighbors by upward-continuing (for example, Blakely, 1996) the isostatic gravity field by 100 to 500 m, then 

calculated the difference between the original and upward-continued grids. This difference highlights shortwavelength anomalies in the grid, and in this way we determined that no such noisy stations were present. In 

another data validity test, we compared the tabulated elevations of the 464 gravity stations in the study area with 

elevations interpolated from 30 m DEMs, and in all cases, tabulated elevations were within 10 m of their DEM 

counterparts. For gravity stations collected in 2004 and 2005, virtually all were within 1 to 3 m of the DEM 

elevations, confirming the high quality of the elevations derived from the GeoXT system. 

Gridded isostatic gravity anomaly data were used to detect significant lateral density interfaces in the 

subsurface using a maximum horizontal gradient technique (Blakely and Simpson, 1986). Maximum horizontal 

gradients of gravity fields are situated above vertical or near-vertical density boundaries in the subsurface, especially 

at shallow depths. The positions of local maxima of the gravity gradient can help delineate lithologic contacts at 

depth, especially those related to faults juxtaposing rocks of different densities. The magnitude of the gradient is a 

function of both the depth to the density boundary and the size of the density contrast. In practice, the presence of 

noise in the gravity grid may lead to false identifications of geological boundaries using this technique, so the 

interpretive power of this method is best where maximum gradients are spatially identified in clusters or lineaments. 

To create models of the subsurface that can account for the observed gravity variation, we utilize a twodimensional forward modeling approach by configuring crustal blocks and adjusting their boundaries and densities 

such that their predicted gravitational effect matches the observed gravity anomaly at the surface (Talwani and 

others, 1959). A significant limitation of this gravity modeling is the non-uniqueness intrinsic to any potential field 

interpretation: in general, there are unlimited physical property configurations that can explain a given observation 

set. That said, by constraining the subsurface model geometry based on geologic mapping of significant features 

and based on realistic rock density values, important aspects of the subsurface may be surmised. Seven north-south 

profiles were selected for analysis (fig. 6). The profiles are separated by 1 minute in longitude and they correspond 

approximately to the mile-spaced section line roads where many of the stations were collected. In the east, these 

profiles cross the outcrop area of the Arbuckle-Simpson Aquifer and the major faults of the Pennsylvanian 

orogenies; towards the west, the profiles cover areas that are progressively buried by Vanoss Formation. An 

assumption of the profile analysis is that geologic structures are oriented perpendicular to the profiles; in our 

analysis, we make a geometric correction to account for the major fault directions that are oriented about 20o from 

east-west. The western three profiles cross gravity anomalies that are oriented in a variety of directions, and twodimensional models of these lines are not as quantitatively reliable as those from where the anomalies are lineated. 

Rock property measurements are presented in table A2 and are summarized in table 2 grouped by 

stratigraphy. While physical properties of individual hand-samples may not represent well the bulk properties of in 

situ volumes of rock, especially those at depth, the measurements can aid in characterizing the density variations 

among the units. The most common rock type sampled is carbonate, largely of dolomitic composition, with a 

saturated bulk density very similar to the 2.67 g/cm3

 assumed for the Bouguer and isostatic gravity calculations. The 

clastic rocks generally have densities 0.1 to 0.2 g/cm3

 lower than the 2.67 g/cm3 Bouguer reduction density. The 

youngest rocks sampled, arkosic and conglomeratic units of the Vanoss Formation, generally have even lower 

densities. The densities of Precambrian granite samples average to about the Bouguer reduction density, and one of 

the Precambrian basement samples has the highest density measured in the sample suite. As discussed below, the 

pattern of gravity anomalies indicates that the in situ density of the basement rocks is on-average significantly 

greater than the density of the overlying carbonate and clastic units. In terms of magnetic properties, the basement 

rock samples are the only ones that have significantly non-zero magnetic susceptibilities (table 2).

5 
Results 

The gridded isostatic gravity anomaly of CNRA and adjacent Hunton Anticline is illustrated in fig. 5. Two 

fundamental observations may be made: beneath the Hunton Anticline, gravity anomalies trend sub-parallel to the 

major WNW-ESE oriented faults, and west of the Hunton Anticline, gravity anomalies are less lineated and they do 

not correspond to simple, linear extensions of the anomalies beneath the anticline (a possible exception is noted 

below). Beneath the outcrop area of the Arbuckle-Simpson Aquifer, isostatic gravity lows correspond to structural 

lows; the Sulphur and Mill Creek Synclines are relative gravity lows, and the Belton and Tishomingo Anticlines are 

relative gravity highs. The magnitude of these gravity variations are 4 to 10 mGal, significantly greater than the 

uncertainty of the anomaly measurements. The positive gravity anomaly of the Tishomingo Anticline reflects the 

presence of dense Precambrian basement at the surface (fig. 2b) and in the shallow subsurface. The western portion 

of the Mill Creek Syncline is marked by an oval-shaped gravity low that extends to the southern portion of CNRA 

(fig. 5). Beneath CNRA, the isostatic gravity anomalies are broader and less lineated than those to the east. A 

relative gravity high is present beneath the City of Sulphur and the Travertine District of CNRA, a relative gravity 

low lies to the west of CNRA, and a SW-NE-trending high extends through the Park parallel to Rock Creek (fig. 5). 

The gravity high of the Belton Anticline merges with the perpendicularly-oriented Rock Creek gravity high; the 

gravity low of the Sulphur Syncline terminates before reaching the Travertine District of CNRA. The exception to 

the observation that gravity anomalies of the Arbuckle-Simpson Aquifer outcrop area do not extend linearly towards 

the WNW occurs where the gravity anomaly associated with the South Sulphur Fault is collinear with a gravity 

gradient of the opposite sense (positive isostatic gravity anomaly to the north, negative to the south) west of the 

Travertine District (fig. 5). This case and its significance will be discussed below. 

A number of possible conclusions, some compatible with each other and others incompatible, can be drawn 

from the differing character of the gravity beneath the Hunton Anticline and that beneath CNRA. One alternative is 

that the tectonic structures of the Hunton Anticline region terminate or change trend near the edge of the ArbuckleSimpson Aquifer outcrop area. Another possibility is that a westward-thickening wedge of Vanoss Formation (table 

1), having either homogeneous or heterogeneous density, obscures gravity signatures that may exist if the major 

faults extend linearly beneath CNRA. A final possibility is that the gravity anomalies beneath CNRA arise from 

deeper variations in basement topography. These possibilities will be explored below. 

The positions and magnitudes of maximum horizontal isostatic gravity gradients (Blakely and Simpson, 

1986) are illustrated as crosses in fig. 5. In agreement with the visual assessment of the gravity field, the maximum 

gradients are greatest in magnitude and best-aligned beneath the Hunton Anticline region (east of CNRA). The 

South Sulphur and Mill Creek Faults correspond to lineated maximum gradient locations. The Reagan Fault is not 

as well surveyed by gravity, but a line of maximum gradient positions extends across the portion of the fault that is 

obscured by the Lake of the Arbuckles. The Sulphur Fault is associated with small gravity gradients and does not 

have maximum gradient locations associated with it. West of the Arbuckle-Simpson Aquifer outcrop area, the 

maximum gradients are lower in magnitude than to the east. 

For the faults marked by maximum gradient alignments, a comparison of the outcrop location of the fault 

and the maximum gradient locations can provide a first-order indication of fault-dip. If the fault is vertical, then the 

maximum gravity gradients are centered on the fault trace. If the fault dips and if the density contrast across the 

fault increases with depth, then the maximum gradient locations are displaced in the direction of the dip. Thus, the 

Mill Creek Fault Zone appears to be largely vertical, given the close spatial correspondence of maximum gradients 

and fault trace (fig. 5). The South Sulphur Fault appears to be north-dipping and to have a greater density contrast 

with depth, given the 500 to 800 m offset of the maximum gradients to the northeast of the fault trace. The Reagan 

Fault, although poorly surveyed by gravity and obscured by the Lake of the Arbuckles, appears to be south-dipping 

in a number of places. A northern dip to the South Sulphur Fault is consistent with normal fault displacement at the 

southern edge of the structural Sulphur Syncline, and a southern dip to the Reagan Fault is consistent with a reverse 

component of motion, in agreement with interpretation from geological mapping (Denison, 1982). 

In a complementary approach, we constructed two-dimensional models of subsurface structures to explain 

the observed gravity anomalies (fig. 7). Because subsurface well information is sparse beneath the Hunton Anticline 

and CNRA, these models have been assigned only very simple block geometry. The primarily utility of the models 

will be to constrain the geometry of the major faults at depth, including fault dip where the faults crop out and the 

fault positions where they are buried. Densities of the blocks are prescribed based on the average characteristics of 

the measured rock units: the Arbuckle Group carbonates are assumed to range in density from the Bouguer reduction 

6 
value of 2.67 g/cm3

 to about 2.60 g/cm3; Simpson Group rocks are assumed to be 0.10 to 0.15 g/cm3 less than the 

Bouguer density; and Pennsylvanian rocks are assumed to be 0.20 g/cm3

 less than the Bouguer density. The blocks 

correspond to the structural synclines and anticlines of the westernmost Hunton Anticline region; their depth extent 

was arbitrarily set to 1 km below the land surface. Changes in the depth extent of individual blocks trade-off with 

the density perturbations required to explain the observed gravity anomalies. One geometric constraint was 

available: the depth to the base of the Vanoss Formation at three locations within the Travertine District (table 1 and 

fig. 1c), and this was utilized when constructing the block models. The depth to the base of Vanoss Formation 

ranges from 37 m at the East Well in the easternmost part of the district to nearly 100 m at Vendome Well near the 

center of the district (table 1). 

In the outcrop area of the Arbuckle-Simpson Aquifer, it is straightforward to generate simple block models 

whose gravity predictions match the observations (figs. 7a, b). The gradients associated with the Sulphur Syncline 

gravity low require a north-dipping South Sulphur Fault. The modeled dip is steep, 60o, in the profiles at 96o54’W 

and 96o55’W. Sensitivity tests conducted with forward modeling allow dips within about 15o of that value. The 

main strand of the Sulphur Fault has a smaller gravity signature that does not allow constraint of its geometry very 

well: it may be vertical or relatively steeply dipping (>45o) in either direction. Gravity observations are not 

available to characterize the Mill Creek Fault in the Arbuckle-Simpson Aquifer outcrop area (figs. 7a, b, c) via twodimensional modeling. Moving west of the Arbuckle-Simpson Aquifer outcrop area (fig. 6), the low-density 

Sulphur Syncline becomes narrower, and the low densities associated with the Mill Creek Syncline are detected on 

the profiles at 96o56’W and 96o57’W (figs. 7c, d). Again, the lack of gravity data south of the Mill Creek Fault does 

not permit characterization of the fault itself. The southernmost observations of high gravity in these profiles reflect 

the influence of shallow, high-density basement of the neighboring Tishomingo Anticline, but these effects are not 

modeled with the sparse data. 

The gravity anomaly of the profile at 96o58’W (fig. 7e) was difficult to model as a simple extension of its 

neighbor 1 mile to the east (fig. 7d). To illustrate the change in density structure required between the easternmost 

modeled profiles and the profile at 96o58’W, we extrapolated the positions of the major tectonic blocks from the 

Arbuckle-Simpson Aquifer outcrop profiles along the orientation of the faults. The predicted and observed gravity 

variations are anti-correlated (fig. 7e, top), emphasizing the change in structure that was inferred from the map-view 

analysis of the isostatic gravity anomaly. 

To test the possibility that the Vanoss Formation obscures gravity signatures from underlying units, we 

conducted a number of forward modeling tests from which we conclude that the cover of Vanoss Formation (<100 

m) is not thick enough in the eastern Travertine District to mask gravity anomalies arising from underlying units. 

By artificially raising the elevation of the gravity observations by 100 m, the gravity anomalies from prePennsylvanian rock units observed in the outcrop area are attenuated only minimally (<5%). In addition, the Vanoss 

layer is not thick enough to produce significant gravity anomalies, even if it were heterogeneous with densities 

spanning a realistic range of values (+/-0.2 g/cm3

). Thus, we conclude that the structural changes in the prePennsylvanian units inferred from gravity in our analysis above are significant and are primarily responsible for the 

observed gravity anomalies. 

For the westernmost profiles at 96o59’W (fig. 7f) and 97o00’W (fig. 7g), we cannot sensibly generate 

crustal blocks that are simple extensions of the structures modeled to the east. Using a different approach, we 

consider that the gravity anomalies arise from a different source, namely the interface between relatively lowdensity Paleozoic units and higher-density Precambrian basement rocks. The densities assigned to these units were 

chosen to be 2.52 g/cm3

 and 2.77 g/cm3 (-0.15 g/cm3 and +0.10 g/cm3 relative to the Bouguer density), respectively. 

Using these densities, basement topography was modeled with both steep and gradual gradients and with a 

maximum relief of about 2.5 km; the fit to the observed gravity anomalies is excellent, as expected by constraining 

structure only by matching an observed gravity profile. As noted above, the gravity field for these westernmost 

profiles is not two-dimensional, so the modeled interfaces are only approximate. The steepest and greatest relief 

occurs at the boundary between the Mill Creek Syncline and the Tishomingo Anticline, consistent with the nearsurface position of Precambrian granites in this portion of the Tishomingo Anticline and with a thickness of Mill 

Creek Syncline deposits exceeding 8,000 feet (2.4 km) (Ham, 1955). 

7 
Discussion 

Faults beneath Chickasaw National Recreation Area 

The primary goal of this study was to use gravity observations to learn how the major faults of the 

Arbuckle-Simpson Aquifer outcrop area extend westward beneath CNRA. Like Cates (1989), we do not see a clear, 

linear projection of these structures beneath CNRA. That said, we argue that gravity observations support the notion 

of curvilinear projection of the Sulphur Fault and the Mill Creek Fault Zones towards CNRA and, with a change of 

density configuration, of the linear projection of the South Sulphur Fault across CNRA near the mineral springs of 

the Travertine District. 

Just east of the Travertine District, the NE-SW-oriented gravity contours (fig. 5) suggested to Cates (1989) 

that the main strand of the Sulphur Fault deviated to the WSW beyond where it is exposed in the Arbuckle-Simpson 

Aquifer outcrop area. Cates (1989) noted that the observation of Arbuckle Group, rather than Simpson Group, rocks 

beneath a cover of Vanoss Formation at East Well and West Well (Figures 1c and 6) in the eastern Travertine 

District supports the conclusion that the Sulphur Syncline, which would juxtapose Simpson Group units beneath 

Vanoss Formation cover, starts to close off immediately to the west of the Arbuckle-Simpson Aquifer outcrop area. 

The results of this study confirm these conclusions of Cates (1989). By expanding beyond the study area of Cates 

(1989) and by utilizing forward modeling methods, this study makes a number of additional conclusions. 

First, in a similar manner to the Sulphur Fault, the Mill Creek Fault Zone appears to deviate to the WSW 

beyond its exposure in the Arbuckle-Simpson Aquifer outcrop area (fig. 5). The Belton Anticline gravity high 

merges with a gravity high beneath Rock Creek that is perpendicular to the main fault traces. The Mill Creek 

Syncline is marked by a prominent, oval gravity low that terminates against the Rock Creek gravity high. The 

Reagan Fault extends linearly across the southern Lake of the Arbuckles, and the inferred extension of the Mill 

Creek Fault Zone appears to terminate against the Reagan Fault. 

Second, a gravity gradient west of Travertine District has the same orientation and is collinear with the 

gravity gradient that marks the South Sulphur Fault where it is exposed southeast of the Travertine District (fig. 5), 

suggesting that the two are related and that the South Sulphur Fault extends linearly through the southern Travertine 

District. A change in polarity of the density contrast across the South Sulphur Fault is required for this 

interpretation to be viable: west of the Travertine District, denser rocks would be expected to occur north of the 

South Sulphur Fault extension, opposite of the configuration of less-dense rocks of the Sulphur Syncline juxtaposed 

against more-dense rocks of the Belton Anticline in the Arbuckle-Simpson Aquifer outcrop area. Such changes in 

density along a fault are relatively common in strike-slip settings where different rock units are juxtaposed along the 

fault, but this inference from gravity for the South Sulphur Fault should be substantiated by other geophysical and 

sampling methods to verify this interpretation. 

Third, there is strong evidence supporting a northward dip to the South Sulphur Fault, both from analysis of 

gravity anomaly amplitude and gradient and from forward models. Given that younger rocks of the Simpson Group 

to the north of the fault are juxtaposed with older Arbuckle Group rocks to the south, this northward dip supports the 

interpretation that the Sulphur Syncline is at least in part formed by a structural graben. Whether this graben is the 

product of simple tectonic extension or whether it formed in the environment of a pull-apart basin in a strike-slip 

dominated zone is unknown given available geological mapping and gravity interpretations. If the westward 

extension of the South Sulphur Fault is indeed linear, as inferred above, and if the Sulphur Fault curves and 

terminates against the South Sulphur Fault, then the Sulphur Syncline occupies a pull-apart setting in a left-stepping, 

left-lateral fault configuration. The similarity of the inferred western terminations of the Sulphur Fault and the Mill 

Creek Fault Zones indicates that the Mill Creek Syncline may have a similar tectonic setting to the Sulphur 

Syncline. 

Figure 8 illustrates the inferred extensions of the major faults beneath CNRA within the study area. The 

precise location of the faults cannot be determined from gravity alone, but any linear extension of the South Sulphur 

Fault places the fault very close to the locations of Bromide and Mineral Springs in the western Travertine District. 

The Sulphur Fault appears to deviate to the south of the fresh-water Buffalo and Antelope Springs, but approaches to 

within about 500 m of them. The springs in the central Travertine District are 1.5 km or more from the nearest 

inferred fault. In the outcrop area, fault traces are mapped with single strands in some instances and multiple 

8 
strands, separated by hundreds of meters, in others (fig. 8), suggesting that the width of deformation varies among 

the major faults. This, in turn, suggests that the zones about which some faults influence groundwater flow may 

vary with similar dimensions. 

Thus, the mineral springs of the western Travertine District are arguably situated directly above the South 

Sulphur Fault, while the freshwater springs of the eastern Travertine District may be related to the Sulphur Fault if 

its deformation occurs across a zone hundreds of meters wide. The springs of the central Travertine District are not 

particularly close to any of the buried faults inferred from gravity. The location of springs depends not only on the 

conditions of the underlying aquifer but also on factors such as land elevation, piezometric surface, and properties of 

the overlying confining layer. A more definitive characterization of the buried faults and of the hydrologic 

properties of the aquifer and confining layers requires observations to complement the gravity analysis, such as from 

borehole stratigraphy or other geophysical methods. 

Basement variation and gravity anomalies 

The ability to model observed gravity anomalies in the western profiles (fig. 7f, g) by varying Precambrian 

basement depth suggests that basement may be an important interface for gravity variations throughout the Arbuckle 

Uplift. The eastern gravity profiles could have contributions from basement topography that were not modeled in 

fig. 7, suggesting that the block densities and thicknesses would require modification. Structural highs and lows 

developed by the faults of the Hunton Anticline are expected to be reflected in the basement topography; thus, 

density variations in the Paleozoic section would be correlated with basement topography, and both sources would 

contribute to surface gravity anomalies. On the other hand, pre-existing topography of the basement surface 

(described as “local granite islands that stood in the Late Cambrian sea” by Ham, 1969) might contribute gravity 

anomalies that are uncorrelated with Paleozoic structures. 

The significance of basement topography as a contributor to gravity anomaly in southern Oklahoma is 

illustrated by the Bouguer gravity anomaly map of Oklahoma (fig. 9). The Arbuckle Uplift and the Wichita Uplift 

to its west are prominent gravity highs, and the Ouachita Mountains are a significant gravity low. The uplifts are 

associated with very shallow basement, and the Ouachita fold-belt is associated with a thickened Phanerozoic 

section, displacing relatively dense basement and mantle materials downwards and leading to the negative gravity 

anomaly. North of the Wichita Uplift are gravity lows over the Anadarko and Ardmore Basins that have deep 

basement. 

The gravity anomaly patterns for southern Oklahoma suggest that gravity may be a promising proxy for 

Precambrian basement depth, but a number of factors must be known before one can convert the observed gravity 

variation into basement depth variation. Primarily, the gravity effects of overlying igneous (restricted to the 

aulacogen) and sedimentary rocks must be known. Away from the Arbuckle-Simpson Aquifer outcrop area, 

petroleum exploration has been active, and seismic and borehole observations are available. By constraining 

gravity-inferred basement depths at discrete sites or profiles, the gravity method can be calibrated and its utility as a 

quantitative predictor of basement geometry can be evaluated. 

Recommendations for future study 

The gravity results presented in this study have constrained the nature of the Sulphur and South Sulphur 

Faults as they approach CNRA from the east. These faults appear to extend towards the fresh water and mineral 

springs, respectively, of the Travertine District (fig. 1c), so these faults likely play a role in the location of these 

springs. Gravity indicates that the Sulphur Syncline is lozenge-shaped, with a northern margin just south of the 

eastern Travertine District. To provide more definitive images of the subsurface extension of these faults, other 

geophysical methods are required. 

Cates (1989) collected ground magnetic observations at his gravity stations, and he detected significant 

magnetic anomalies that were associated with some, but not all, of the gravity anomalies near CNRA. Cates (1989) 

suggested that in the vicinity of the Sulphur Syncline, magnetic highs might arise from accumulation of sediment 

enriched in magnetic minerals at the base of some of the active fault scarps. His discrete magnetic data do not 

permit a high-resolution assessment of magnetics as a detector of fault position and character, but it is possible that 

9 
continuous ground magnetics or low-altitude aeromagnetic observations might provide complementary evidence of 

the faults. 

Seismic and electromagnetic methods are sensitive to different physical properties than gravity and 

magnetics methods, and thus would also provide useful constraints on subsurface structure. Geophysical anomalies 

would be expected to arise both from lithologic interfaces that are offset across the fault and from damage zones at 

the fault interface. Under the Vanoss Formation cover beneath CNRA and eastwards to the Arbuckle-Simpson 

Aquifer outcrop area, these other geophysical techniques would need to penetrate at least tens of meters to detect 

rocks of the aquifer formations; ideally, they would penetrate several hundreds of meters with sufficient resolution 

to characterize fault zones and offset strata in the uppermost Arbuckle-Simpson Aquifer units. 

Borehole lithologic and geophysical information will also contribute to the understanding of the faulted 

structures beneath CNRA. An effort is underway to compile water well and oil and gas well information and to 

generate structural contour and fault maps of the Arbuckle-Simpson Aquifer in the western Hunton Anticline, under 

the sponsorship of the Arbuckle-Simpson Hydrology Study (J. Puckette, personal comm.). These results will allow 

the testing and refinement of models based on geophysical analysis. In a broader area, the analysis of deeper oil and 

gas well information, especially to the west of CNRA, will allow characterization of major crustal structures well 

beyond the borders of the Arbuckle-Simpson Aquifer outcrop area. Cates (1989) performed this analysis with a 

subset of the available well data, and a modern synthesis will allow construction of a three-dimensional geological 

model of the major structures in this area. A USGS/Geology Discipline effort is underway to pursue this goal (C. 

Blome, personal comm.). 

From the perspective of understanding the gravity field in the vicinity of CNRA, additional gravity station 

data would be very helpful to refine the measured anomalies (fig. 4) associated with the Arbuckle-Simpson Aquifer 

outcrop area and with the western margin of progressive burial. While most of the Sulphur Syncline is well 

characterized, additional data to the east of existing coverage would confirm the relative significance of the physical 

anomalies associated with the Sulphur and South Sulphur Faults, and such coverage would help constrain the nature 

of their merger just to the east of the study area. Additional gravity observations on either side of the Mill Creek 

Fault Zone in the Arbuckle-Simpson Aquifer outcrop area would improve our interpretation of this major fault and 

would help to characterize the Mill Creek Syncline where existing coverage is absent. Also, additional data on 

either side of the Reagan Fault would refine our interpretation of this as a dipping, thrust structure and would help to 

constrain the importance of high-density Precambrian basement on the gravity signature of the Tishomingo and 

southern Hunton Anticlines. More observations north of CNRA would characterize better the gravity field in the 

area where the Arbuckle-Simpson Aquifer is covered by Pennsylvanian units and would allow an assessment of how 

basement topography variations might explain observed gravity anomalies. 

Acknowledgments 

We thank the National Park Service, Water Resources Division for project funding and Jennifer Back 

(NPS/WRD Water Rights Branch) for facilitating this research. Support was also received from USGS National 

Cooperative Geologic Mapping Program project on the geohydrology of the Edwards and southern Trinity aquifers 

(Charles Blome, Project Chief). We are indebted to Randall Herr, Scott Langford, and Michael Ducre of the Naval 

Oceanographic Office (NAVOCEANO) for extensive help in installing and trouble-shooting the marine gravimeter 

during our field sessions, and we thank Daniel Fornari (WHOI) for facilitating our use of the NAVOCEANO 

gravimeter. Scott Christenson (USGS, Oklahoma City) provided the differential GPS system, and he assisted our 

field operations in many ways. Maurice Tivey (WHOI) provided the electronic compass. Kendall Truett, Darryl 

Carter, Steven Burrough, and Sue Braumiller (CNRA) provided assistance throughout the fieldwork. Noel Osborn 

(OWRB) and other investigators of the Arbuckle-Simpson Hydrology Study provided data and advice at all stages 

of this work. Jeremy Lui (USGS) conducted lab measurements, and Don Plouff (USGS) provided programming 

support. This report was improved by reviews of Neil Suneson (OGS) and Robert Jachens (USGS). 

10 
Table 1. Depth to base of Vanoss Formation (and top of Arbuckle Group) rocks in three wells in CNRA 

[Based on Irwin (1972), Hanson and Cates (1994), and Nord (1996); Datums: latitude and longitude, NAD27; elevations, NGVD29] 

Well name Year drilled Latitude Longitude Surface 

elevation (m) 

Depth to base of 

 Vanoss Formation (m) 

East Well 1972 34° 30.28’N 96° 56.25’W 348 37 

West Well 1972 34° 30.37’N 96° 56.95’W 329 46 

Vendome 1997134° 30.33’N 96° 58.38’W 288 99 

1

most recently drilled hole 

Table 2. Physical properties of rock samples grouped by stratigraphic unit 

[Properties are averages except for the column containing the range of saturated bulk density] 

Stratigraphic grouping Number of 

samples 

Saturated bulk 

density (g/cm3

) 

Saturated bulk 

density range (g/cm3

) 

Porosity 

(%) 

Susceptibility 

(10-3 SI) 

Vanoss Formation 5 2.44 2.24-2.59 7 0.02 

Sycamore Ls./Weldon 

Ls./Woodford Shale 2 2.52 2.45-2.59 3 0.01 

Hunton Group 1 2.57 2.57-2.57 4 0.01 

Sylvan Shale/Viola Group 2 2.66 2.66-2.66 0.7 0.02 

Simpson Group 6 2.48 2.26-2.66 5 0.02 

Arbuckle Group 10 2.67 2.55-2.80 4 0.03 

Precambrian granite 4 2.66 2.58-2.82 1 8.5 

 

 

Table 3. Gravity base stations used for data collected in 2004 and 2005 

[Datums: latitude and longitude, NAD27; elevations, NGVD29; gravity values, IGSN71] 

Base name Latitude Longitude Elevation (m) Observed gravity 

(mGal) 

Description 

COALGATE134° 32.23’N 96° 13.11’W 185.9 979,626.22 

Coalgate, Okla. Fire 

Station (codes ACIC 

3841-2; IGB 11846B) 

VENDOME 34° 30.348’N 96° 58.378’W 288.4 979,648.02 Vendome Well, 

CNRA, Okla. 

PVENDOME 34° 30.346’N 96° 58.353’W 287.8 979,648.11 

Parking lot of 

Vendome Well, 

 CNRA, Okla. 

RCC12 34° 29.865’N 96° 59.211’W 284.3 979,647.26 

Rock Creek 

Campground, Site 12, 

CNRA, Okla. 

1

Part of the World Reference Gravity Network (Jablonski, 1974).

11 
Literature Cited 

Barthel, C. J. J., 1985, Hydrogeologic investigation of artesian springs flow, Sulphur, Oklahoma area: Norman, Okla., University of 

Oklahoma, M.S. Thesis, 234 p. 

Bell, R.E., and A.B. Watts, 1986, Evaluation of the BGM-3 sea gravity meter system onboard R/V Conrad: Geophysics, v. 51, p. 1480-

1493. 

Blakely, R.J., 1996, Potential theory in gravity and magnetic applications: Cambridge University Press, 441 p. 

Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source bodies from magnetic or gravity anomalies: Geophysics, v.

51, p. 1494-1498. 

Brown, W. G., 1984, Washita Valley fault system—a new look at an old fault: Technical Proceedings of the 1981 Midcontinent 

Section Meeting of the American Association of Petroleum Geologists, p. 68-80. 

Brown, R. L., and G. D. Morgan, 1991, Aulacogen collapse— a model for the compressive stage of aulacogen evolution: Norman, 

Okla., Oklahoma Geological Survey Circular 92, p. 185-188. 

Cates, S. W., 1989, Fault distribution in the Sulphur, Oklahoma area based on gravity, magnetic and structural data: Norman, Okla., 

University of Oklahoma, M.S. Thesis, 106 p. 

Cederstrand, J.R., 1996, Digital geologic map of Ardmore-Sherman quadrangles, south-central Oklahoma: U.S. Geological Survey 

Open-File Report 96-370. 

Denison, R. E., 1973, Basement rocks in the Arbuckle Mountains, Regional Geology of the Arbuckle Mountains: Norman, Okla., 

Oklahoma Geological Survey Special Publication 73-3, p. 43-49. 

Denison, R. E., 1982, Geologic cross section from the Arbuckle Mountains to the Muenster arch, southern Oklahoma and Texas: 

Geological Society of America Map and Chart Series MC-28 R, 8 p. 

Fairchild, R. W., R. L. Hanson, and R. E. Davis, 1990, Hydrology of the Arbuckle Mountains area, south-central Oklahoma: Norman, 

Okla., Oklahoma Geological Survey Circular 91, 112 p. 

Goemaat, R., and C. Willard, 1983, Ground-water records for the area surrounding the Chickasaw National Recreational Area, 

Murray County, Oklahoma: U.S. Geological Survey Open-File Report, 8 p. 

Gould, C. N., 1906, Platt National Park: Letter of August, 1906, to honorable B.A. Hitchcock, Secretary of Interior, Washington, D.C., 

11 p. 

Gould, C. N., and S. L. Schoff, 1939, Geological report on water conditions at Platt National Park, Oklahoma: Oklahoma Geological 

Survey and National Park Service Report no. 249, Open-File Report 39-14, 38 p. 

Ham, W. E., 1950, Geology and petrology of the Arbuckle limestone in the southern Arbuckle Mountains, Oklahoma: New Haven, 

Conn., Yale University, Ph.D dissertation, 162 p. 

Ham, W. E., 1955, Field conference on geology of the Arbuckle Mountain region, Part 1— Geology of the Arbuckle and Timbered 

Hills Groups, Part 2— Regional stratigraphy and structure of the Arbuckle Mountain region: Norman, Okla., Oklahoma 

Geological Survey Guide Book 3, 61 p. 

Ham, W. E., 1969, Regional geology of the Arbuckle Mountains, Oklahoma: Norman, Okla., Oklahoma Geological Survey Guidebook 

17, 52 p. 

Ham, W.E., McKinley, M.E., and others, 1954, Geologic map and sections of the Arbuckle Mountains, Oklahoma: Oklahoma 

Geological Survey Miscellaneous Map A-2, scale 1:72,000. 

Ham, W. E., R. E. Denison, and C. A. Merritt, 1964, Basement rocks and structural evolution of southern Oklahoma: Norman, Okla., 

Oklahoma Geological Survey Bulletin 95, 302 p. 

Hart, D. L. J., 1974, Reconnaissance of the Water Resources of the Ardmore and Sherman Quadrangles, Southern Oklahoma: 

Oklahoma Geological Survey Hydrologic Atlas HA-3, scale 1:250,000. 

Hanson, R. L., and S. W. Cates, 1994, Hydrogeology of the Chickasaw National Recreation Area, Murray County, Oklahoma: U.S. 

Geological Survey Water-Resources Investigations Report 94-4102, 86 p. 

Hoffman, P.F., Dewey, J.F., and Burke, K., 1974, Aulacogens and their genetic relations to geosynclines, with a Proterozoic 

example from Great Slave Lake, Canada, in Dott, R.H. and Shaver R.H. eds, Modern and ancient geosynclinal 

sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 19, p. 38-55. 

Irwin, J.H., 1972, Letter to G. Moore, National Park Service, re. Platt National Park, dated November 6, 1972. 

Jablonski, H. M., 1974, World relative gravity reference network North America, Parts 1 and 2: U.S. Defense Mapping Agency 

Aerospace Center Reference Publication no. 25, originally published 1970, revised 1974, with supplement of IGSN 71 

gravity datum values, 1261 p. 

Johnson, G.R., and Olhoeft, G.R., 1984, Density of rocks and minerals, in Carmichael, R.S., ed., CRC Handbook of Physical 

Properties of Rocks, Vol. 3: Boca Raton, Fla., CRC Press, Inc., p. 1-38. 

Johnson, K.S., 1990, Geologic map and sections of the Arbuckle Mountains, Oklahoma, revision of Ham, W.E., McKinley, M.E., and 

others, 1954: Norman, Okla., Oklahoma Geological Survey Circular 91, scale 1:100,000. 

12 
Nord, J. D., 1996, Geologic and hydrologic controls on flow at Vendome Well, Sulphur, OK: Stillwater, Okla., Oklahoma State 

University, M.S. Thesis, 147 p. 

Oklahoma Water Resources Board, 2003, The Arbuckle-Simpson hydrology study— Management and protection of an Oklahoma 

water resource: Oklahoma Water Resources Board Fact Sheet, 4 p. 

Plouff, D., 1977, Preliminary documentation for a FORTRAN program to compute gravity terrain corrections based on topography 

digitized on a geographic grid: U.S. Geological Survey Open-File Report 77-535, 45 p. 

Schornick, H. M., J. F. Harp, and J.G. Laguros, 1976, Subsurface and surface water flows at Platt National Park, Sulphur, 

Oklahoma: Norman, Okla., School of Civil Engineering and Environmental Science, University of Oklahoma, 49 p. 

Simpson, R.W., Jachens, R.C., Blakely, R.J., and Saltus, R.W., 1986, A new isostatic residual gravity map of the conterminous 

United States with a discussion on the significance of isostatic residual anomalies: Journal of Geophysical Research, v. 

91, p. 8348-8372. 

Taff, J. A., 1904, Preliminary report on the geology of the Arbuckle Mountains in Indian Territory and Oklahoma: U.S. Geological 

Survey Professional Paper 31, 97 p. 

Taff, J. A., 1928, Preliminary report on the geology of the Arbuckle and Wichita Mountains in Indian Territory and Oklahoma: 

Norman, Okla., Oklahoma Geological Survey Bulletin 12, 95 p. 

Talwani, M., Worzek, J.L, and Landisman, M., 1959, Rapid gravity computations for two-dimensional bodies with application to the 

Mendocino submarine fracture zone: Journal of Geophysical Research, v. 64, p. 49-59. 

Tanner, J.H., III, 1967, Wrench fault movements along Washita Valley Fault, Arbuckle Mountains area, Oklahoma: American 

Association of Petroleum Geologists Bulletin, v. 51, p. 126-141. 

Tapp, B., 1995, Inversion model for the structural styles in the Southern Midcontinent, 1992 Symposium: Norman, Okla., Oklahoma

Geological Survey Circular 97, p. 113-118. 

13 
-97 ̊ 04' -97 ̊ 02' -97 ̊ 00' -96 ̊ 58' -96 ̊ 56' -96 ̊ 54' -96 ̊ 52'

34 ̊ 23'

34 ̊ 24'

34 ̊ 25'

34 ̊ 26'

34 ̊ 27'

34 ̊ 28'

34 ̊ 29'

34 ̊ 30'

34 ̊ 31'

34 ̊ 32'

34 ̊ 33'

34 ̊ 34'

34 ̊ 35'

S

D

Lake of the Arbuckles

Rock Creek

SF

SSF

MCF

RF

Sulphur Syncline

Belton Anticline

Mill Creek Syncline

Tishomingo Anticline

0 2 4

km

Major faults

Minor faults

CNRA boundary

Roadways

Highways

Edge of ArbuckleSimpson Aquifer

Well

Spring

Cates (1989)

study area

Travertine

District

Figure 1. 1a) Index map of the study area. S=City of Sulphur; D=Town of Dougherty; SF=Sulphur Fault; SSF=South Sulphur Fault;

MCF=Mill Creek Fault; RF=Reagan Fault. Inset shows our study area (blue box) within the State of Oklahoma; outcrop area of

Arbuckle-Simpson Aquifer is shown in yellow.

14
-97 ̊ 04' -97 ̊ 02' -97 ̊ 00' -96 ̊ 58' -96 ̊ 56' -96 ̊ 54' -96 ̊ 52'

34 ̊ 23'

34 ̊ 24'

34 ̊ 25'

34 ̊ 26'

34 ̊ 27'

34 ̊ 28'

34 ̊ 29'

34 ̊ 30'

34 ̊ 31'

34 ̊ 32'

34 ̊ 33'

34 ̊ 34'

34 ̊ 35'

SF

SSF

MCF

RF

Sulphur Syncline

Belton Anticline

Mill Creek Syncline

Tishomingo Anticline

0 2 4

km

Penn. Vanoss and

Ada Formations

Precambrian

granite

Camb. Timbered

Hills Group

Camb.-Ordovician

Arbuckle Group

Ordovician 

Simpson Group

Late Ordovician --

Mississippian rocks

Penn. pre-Arbuckle

Orogeny

Penn. post-Arbuckle Water body

Orogeny

Major faults

Minor faults

CNRA boundary 

Spring

Isolated outcrop

15

Figure 1 (continued). 1b) Bedrock geology and faults of the study area, based on by mapping of Ham and others (1954),

transcribed by Hart (1974), and digitized by Cederstrand (1996) with edits by the Oklahoma Water Resources Board (2004, personal

commun.). Two filled stars mark the locations from Cates (1989) of isolated outcrops of uppermost Ordovician Viola Limestone

along Rock Creek and of Arbuckle Group units just east of the Travertine District.
-97 ̊ 00' -96 ̊ 59' -96 ̊ 58' -96 ̊ 57' -96 ̊ 56' -96 ̊ 55' -96 ̊ 54'

34 ̊ 28'

34 ̊ 29'

34 ̊ 30'

34 ̊ 31'

West

East Vend

A&B

B&M

SF

SSF

Sulphur Syncline

Belton Anticline

0 1 2

km

Penn. Vanoss and

Ada Formations

Camb.-Ordovician

Arbuckle Group

Ordovician 

Simpson Group

Late Ordovician --

Mississippian rocks Water body

Major faults

CNRA boundary

Spring

Well

Roadways

Isolated outcrop

16

Figure 1 (continued). 1c) Geology of the Travertine District and vicinity, highlighting the locations of three wells and springs and

the termination of the SF and SSF traces at the western edge of Arbuckle-Simpson outcrop. East=East Well; West=West Well;

Vend=Vendome Well; A&B = Antelope and Buffalo Springs; B&M=Bromide and Mineral Springs.
150 175 200 225 250 275 300 325 350 375 400 

Elevation (m)

-97 ̊ 20' -97 ̊ 10' -97 ̊ 00' -96 ̊ 50' -96 ̊ 40' -96 ̊ 30' 

34 ̊ 10'34 ̊ 20'

34 ̊ 30'

34 ̊ 40'

HUNT

TISH

ARB

SF

SSF MCF

RF

WVF

0 10 20

km

Major faultsMinor faults CNRA boundary

Edge of ArbuckleSimpson Aquifer

17

Figure 2. 2a) Topographic map of the Arbuckle Uplift. The Arbuckle, Tishomingo, and Hunton Anticlines are designated by ARB, TISH, and HUNT, respeSSF=South Sulphur Fault; MCF=Mill Creek Fault; RF=Reagan Fault; WVF=Washita Valley Fault.
-97 ̊ 20' -97 ̊ 10' -97 ̊ 00' -96 ̊ 50' -96 ̊ 40' -96 ̊ 30' 

34 ̊ 10'

34 ̊ 20'

34 ̊ 30'

34 ̊ 40'

SF

SSF MCF

RF

WVF

0 10 20

km

Penn. Vanoss andAda Formations

PrecambriangraniteCamb. ColbertRhyoliteCamb. TimberedHills GroupCamb.-OrdovicianArbuckle GroupOrdovician Simpson Group

Late Ordovician --Mississippian rocksPenn. pre-ArbuckleOrogeny

Water body

CretaceousrocksPenn. post-ArbuckleOrogeny Major faults

Minor faultsCNRA boundary 

Edge of ArbuckSimpson Aquife

18

Figure 2 (continued). 2b) Bedrock geology and faults of the Arbuckle Uplift are based on mapping of Ham and others (1954), transcribed by Hart (1974), and digitwith edits by the Oklahoma Water Resources Board (2004, personal commun.).
8000 7000 6000 5000 4000 3000 2000 1000

metersfeet

00

500 1000 1500 2000 2500

Arbuckle-Simpson Aquifer Units

Timbered

Hills Group

Arbuckle

Group

Simpson

Group

Hunton

Group

Penn. Miss. Dev.

Sil.Ordovician Cambrian Precambrian

Tishomingo

and Troy

granites

(1350 Ma)

19

Figure3.StratigraphicsectionofPrecambrianandPaleozoicunitsintheHuntonAnticlinearea,adaptedfromHam(1969).
-97 ̊ 04' -97 ̊ 02' -97 ̊ 00' -96 ̊ 58' -96 ̊ 56' -96 ̊ 54' -96 ̊ 52'

34 ̊ 23'

34 ̊ 24'

34 ̊ 25'

34 ̊ 26'

34 ̊ 27'

34 ̊ 28'

34 ̊ 29'

34 ̊ 30'

34 ̊ 31'

34 ̊ 32'

34 ̊ 33'

34 ̊ 34'

34 ̊ 35'

SF

SSF

MCF

RF

0 2 4

km

Major faults

Minor faults

CNRA boundary

Roadways

Highways

pre-Arbuckle

Orogeny units

Edge of ArbuckleSimpson Aquifer

Gravity station (2005)

Gravity station (2004)

Gravity station (Cates)

Gravity station (PACES)

20

Figure 4. Gravity stations of the study area, with geological features outlined. Fault abbreviations as in fig. 1.
32 34 36 38 40 42 44 46 48 50 52

Isostatic Gravity Anomaly (mGal)

-97 ̊ 04' -97 ̊ 02' -97 ̊ 00' -96 ̊ 58' -96 ̊ 56' -96 ̊ 54' -96 ̊ 52'

34 ̊ 23'

34 ̊ 24'

34 ̊ 25'

34 ̊ 26'

34 ̊ 27'

34 ̊ 28'

34 ̊ 29'

34 ̊ 30'

34 ̊ 31'

34 ̊ 32'

34 ̊ 33'

34 ̊ 34'

34 ̊ 35'

SF

SSF

MCF

RF

Rock Creek

0 2 4

km

Major faults

Minor faults

CNRA boundary

pre-Arbuckle

Orogeny units

Edge of ArbuckleSimpson Aquifer Gravity station

Max. horizontal

gravity gradient

21

Figure 5. Isostatic gravity anomaly map of the study area, contoured every 1 mGal, based on a 100 m-spaced grid. Grid locations

more than 1.2 km from the nearest gravity station are assigned a void value (gray shade). Orange crosses denote locations of

maximum horizontal gravity gradients, with sizes varying as to whether the magnitude of the gradient falls within the lower,

middle, or upper thirds of the distribution. Fault abbreviations as in fig. 1.
32 34 36 38 40 42 44 46 48 50 52

Isostatic Gravity Anomaly (mGal)

-97 ̊ 04' -97 ̊ 02' -97 ̊ 00' -96 ̊ 58' -96 ̊ 56' -96 ̊ 54' -96 ̊ 52'

34 ̊ 23'

34 ̊ 24'

34 ̊ 25'

34 ̊ 26'

34 ̊ 27'

34 ̊ 28'

34 ̊ 29'

34 ̊ 30'

34 ̊ 31'

34 ̊ 32'

34 ̊ 33'

34 ̊ 34'

34 ̊ 35'

West Vend East

97_00

96_59

96_5896_5796_56

96_55

96_54

SF

SSF

MCF

RF

Sulphur Syncline

Belton Anticline

Mill Creek Syncline

Tishomingo Anticline

Rock Creek

0 2 4

km

Major faults

Minor faults

CNRA boundary

pre-Arbuckle

Orogeny units

Edge of ArbuckleSimpson Aquifer

Gravity station

2D analysis profile

22

Figure 6. Isostatic gravity anomaly map of the study area with the locations of 7 profiles for two-dimensional analysis

superimposed as hachured lines. Fault and water well abbreviations as in fig. 1.
b)

96_55

Gravity (mGals)

42.00

45.00

48.00

-10.00 -5.00 0.00 5.00

V.E.=2.8 Distance (km) 

Depth (km) 

1.40

0.70

0.00

-0.70

Belton Hunton Anticline

Anticline

Mill Creek Syncline Sulphur Syncline

Pennsylvanian

D=2.6

D=2.67

D=2.57 D=2.52

D=2.47

South North

MCF SSF SF

23

Figure 7. Two-dimensional gravity models along north-south profiles at: 7a) 96o54’W, 7b) 96o55’W. For each model, the top panel

displays gravity observations (filled circles) and modeled gravity (line); the bottom panel shows a cross-section with annotated

crustal density blocks. Where faults crop out, symbols are superimposed that illustrate dips of vertical and 60o to the south and

north; multiple strands of the fault systems are shown with separate symbols. Fault abbreviations are as in fig. 1, and densities are

listed in g/cm3.

a)

96_54

Gravity (mGals)

42.00

45.00

48.00

-10.00 -5.00 0.00 5.00

V.E.=2.8 Distance (km) 

Depth (km) 

1.40

0.70

0.00

-0.70

Belton Anticline Sulphur Syncline Hunton Anticline Mill Creek Syncline

D=2.67 D=2.52 D=2.57 D=2.57

South North

MCF SSF SF
d)

96_57

Gravity (mGals)

42.00

45.00

48.00

-10.00 -5.00 0.00 5.00

V.E.=2.8 Distance (km) 

Depth (km) 

1.40

0.70

0.00

-0.70

Belton Anticline Hunton Anticline Mill Creek Syncline

Pennsylvanian

Sulphur

D=2.60 Syncline D=2.60 D=2.52

D=2.47

D=2.52

South North

MCF SSF SF

24

Figure 7 (continued). Two-dimensional gravity models along north-south profiles at: c) 96o56’W, d) 96o57’W.

c)

96_56

Gravity (mGals)

42.00

45.00

48.00

-10.00 -5.00 0.00 5.00

V.E.=2.8 Distance (km) 

Depth (km) 

1.40

0.70

0.00

-0.70

Pennsylvanian

Belton Anticline

Mill Creek Syncline

Hunton Anticline

Sulphur

Syncline

D=2.47

D=2.64

D=2.52

D=2.60

D=2.50

South North

MCF SSF SF
96_59

Gravity (mGals)

42.00

45.00

48.00

-6.00 0.00 6.00

V.E.=1.2 Distance (km) 

Depth (km) 

4.00

2.00

0.00

Dense Basement

Uppermost Crust

D=2.77

D=2.52

f)

South North

25

Figure 7 (continued). Two-dimensional gravity models along north-south profiles at: e) 96o58’W, f) 96o59’W.

96_58

Gravity (mGals)

42.00

45.00

48.00

-6.00 0.00 6.00

V.E.=3.0 Distance (km) 

Depth (km) 

1.40

0.70

0.00

-0.70

Pennsylvanian

Tishomingo Mill Creek Syncline

Anticline

Sulphur Syncline Hunton AnticlineBelton Anticline

D=2.47

D=2.52

D=2.72

D=2.52

D=2.60

D=2.60

e)

South North

RF MCF SSF? SF?
g)

97_00

Gravity (mGals)

42.00

45.00

48.00

-6.00 0.00 6.00

V.E.=1.2 Distance (km) 

Depth (km) 

4.00

2.00

0.00

Dense Basement

Upper Crust

D=2.77

D=2.52

South North

Figure 7 (continued). Two-dimensional gravity models along north-south profiles at: g) 97o00’W.

26
32 34 36 38 40 42 44 46 48 50 52

Isostatic Gravity Anomaly (mGal)

-97 ̊ 04' -97 ̊ 02' -97 ̊ 00' -96 ̊ 58' -96 ̊ 56' -96 ̊ 54' -96 ̊ 52'

34 ̊ 23'

34 ̊ 24'

34 ̊ 25'

34 ̊ 26'

34 ̊ 27'

34 ̊ 28'

34 ̊ 29'

34 ̊ 30'

34 ̊ 31'

34 ̊ 32'

34 ̊ 33'

34 ̊ 34'

34 ̊ 35'

SF

SSF

MCF

RF

Rock Creek

0 2 4

km

?

? ? ?

?

Inferred, buried

 faults

Major faults

Minor faults

CNRA boundary

pre-Arbuckle

Orogeny units

Edge of ArbuckleSimpson Aquifer

Spring

27

Figure 8. Isostatic gravity anomaly map of the study area, similar to fig. 5, but with inferred, buried faults indicated by thick

dashed lines. Question marks indicate where the inferences are least certain.
-160 -140 -120 -100 -80 -60 -40 -20 0 

Bouguer Gravity Anomaly (mGal)

-103 ̊ -102 ̊ -101 ̊ -100 ̊ -99 ̊ -98 ̊ -97 ̊ -96 ̊ 

33 ̊ 30'34 ̊ 00'

34 ̊ 30'

35 ̊ 00'

35 ̊ 30'

36 ̊ 00'

36 ̊ 30'

37 ̊ 00'

37 ̊ 30'

0 

W A

28

Figure 9. Bouguer gravity anomaly map of the state of Oklahoma and the panhandle of Texas. Contour interval is 10 mGal. Yellow line denotes the outcroAquifer in south-central Oklahoma. W=Washita Uplift; A=Arbuckle Uplift; O=Ouachita Mountains.
-0.2

-0.1

-0.0

0.1

0.2

Gravity (mGal)

0 1 2 3 4 5 6 7 8 9

Elapsed time (hr)

2004jun11

a)

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

-0.0

0.2

0.4

Gravity (mGal)

0 1 2 3 4 5 6 7 8 9

Elapsed time (hr)

2005may03_overnight b)

29

Figure A1. Time-series plot of BGM-5 gravity data (thin line) for 9 hour periods when the vehicle was stationary on A1a) 2004jun11

and A1b) 2005may03. Thick gray line indicates predicted tidal accelerations from the positions of the moon and sun.
979625 979630 979635 979640 979645 979650 979655

Observed Gravity (mGal)

-97 ̊ 04' -97 ̊ 02' -97 ̊ 00' -96 ̊ 58' -96 ̊ 56' -96 ̊ 54' -96 ̊ 52'

34 ̊ 23'34 ̊ 24'34 ̊ 25'34 ̊ 26'34 ̊ 27'34 ̊ 28'34 ̊ 29'34 ̊ 30'34 ̊ 31'34 ̊ 32'34 ̊ 33'34 ̊ 34'34 ̊ 35'

0 2 4

km

979625 979630 979635 979640 

Observed Gravit

-97 ̊ 04' -97 ̊ 02' -97 ̊ 00' -96 ̊ 58' 

34 ̊ 23'34 ̊ 24'34 ̊ 25'34 ̊ 26'34 ̊ 27'34 ̊ 28'34 ̊ 29'34 ̊ 30'34 ̊ 31'34 ̊ 32'34 ̊ 33'34 ̊ 34'34 ̊ 35'

0 2 4

km

CNRA boundary

HighwaysEdge of ArbuckleSimpson Aquifer

Gravity station (2004/2005 traditional)

HighwaysEdge of ArbuckleSimpson Aquifer

a) b)

30

Figure A2. A2a) Map of observed gravity across the study area using the traditional, discrete LaCoste-Romberg gravity stations collected in 2004 and 20Because this gravity has not had any elevation effects removed, the dominant gravity signal is anti-correlated with land elevation. A2b) Map of observed using the continuous BGM-5 values when the vehicle was stationary at LaCoste-Romberg stations, contoured every 1 mGal.
979625

979630

979635

979640

979645

979650

BGM-5 Observed Gravity (mGal)

979625 979630 979635 979640 979645 979650

LaCoste-Romberg Observed Gravity (mGal)

31

Figure A3. Scatter plot of observed gravity values measured by the LaCoste-Romberg meters versus those of the BGM-5 meters

at identical sites. Red symbols are for stations collected in 2004; blue symbols are for stations collected in 2005. Error bars

indicate plus and minus one standard-deviation values about the mean gravity measured by the BGM-5 meter when the vehicle

was stationary. Gray field about the 1:1 reference line spans 1 mGal vertically and horizontally.
Appendix: Continuous Gravity Results 

A major component of the geophysical fieldwork conducted in 2004 and 2005 was the use of the BGM-5 

gravimeter to collect continuous gravity across the study area. The full analysis of this gravity has yet to be 

completed, but in this appendix we describe our acquisition strategy and some preliminary results from the 

continuous gravity observations. 

The Bell Aerospace BGM-5 Marine Gravity Meter System is an axially symmetric, forced feedback 

accelerometer that is mounted on a gyro-stabilized platform. The BGM-5 gravimeter is a successor to the BGM-3 

system (Bell and Watts, 1986), and both systems have been deployed on ships and aircraft world-wide. To our 

knowledge, our experiment is the first deployment of this type of continuous gravity system on a land vehicle. In 

theory, collecting gravity continuously will allow significantly greater numbers of observations to be made in a 

field-day relative to traditional stationary gravity collection. Furthermore, by driving a vehicle slowly, observations 

can be made at close spacing, allowing resolution of shallow density variations. The challenges of collecting gravity 

in this manner are several-fold: providing sufficient AC power of adequate quality to the meter electronics, driving 

the vehicle in ways that allow the platform to remain stable, and acquiring synchronous ancillary data to determine 

geographic position and to allow correction for motion of the gravity sensor from the raw accelerometer record. The 

vehicle motions that must be removed are of two types: horizontal velocities via the Eotvos correction (Blakely, 

1996) and vertical accelerations of the sensor. These motions are calculated as first and second time-derivatives of 

GPS locations, described below. 

During our two field sessions, the Naval Oceanographic Office (NAVOCEANO) provided a BGM-5 meter 

under its agreement with the Academic Gravimeter Pool operated by the Woods Hole Oceanographic Institution 

(WHOI). In 2004, the gravimeter system was mounted in a 21’-long recreational vehicle (RV), and the equipment 

was powered by a 30 Amp generator. In 2005, the gravimeter system was mounted in a passenger van with its seats 

removed, and the equipment was powered by a 2.5 kW (~22 A at 110 V AC) modified-sinewave inverter connected 

to the van’s battery. Overnight, the system was plugged into land power sources. Tests of the power draw of the 

BGM-5 system indicated that the maximum draw was ~14 A upon power-up, and the sustained usage was ~11 A 

(M. Ducre, personal comm.). Power must be supplied to the BGM-5 sensor at all times or else it requires servicing, 

and a battery backup provided power to the sensor subsystem when an AC source was disconnected for short times. 

The AC power provided by the generator and by the inverter, while adequate for powering the gravimeter system 

electronics, was not of high enough quality to support uninterruptible power supply (UPS) systems – both high-end 

and low-end UPS units. We never discovered the source of this incompatibility but we suspected problems with the 

wiring in the RV circuitry and with modified (non-true) sinewave AC from the inverter. Because we did not have a 

working UPS, transitions from land- to vehicle-power required a complete system shutdown and reboot, a timeconsuming (30-45 min) operation at the beginning and end of each field-day. 

The BGM-5 gravimeter system consists of the gyro-stabilized platform that contains the accelerometer 

sensor, a GPS system that provides time and navigation information, a computer that logs the acceleration and 

navigation streams and processes the data to provide a smoothed gravity anomaly, and the electronics that supports 

the other subsystems. For both deployments in 2004 and 2005, we mounted the gyro-stabilized platform on the 

floor as near as possible to the center-point of the wheels, to minimize motions of the sensor related to the vehicle 

suspension. The GPS antenna was mounted on the roof of the vehicle, as close to vertically above the sensor as 

possible. The accelerometer and GPS data streams were recorded at 1 Hz, and frame count output of the sensor was 

converted to acceleration using bias and scaling calibration factors specific to the individual BGM-5 system. Minor 

adjustments to the bias were made daily in the study area when we recorded data over local gravity base stations. In 

addition to the raw, 1 Hz gravimeter data, the logging computer smoothes the raw readings with a 3-minute halfwidth Gaussian filter, and it calculates and applies Eotvos corrections based on smoothed GPS navigation to 

generate gravity anomalies. This smoothing filter works well in marine deployments of BGM systems. On ships, 

the vertical accelerations from heave average to zero over the 6-minute width of the Gaussian filter, so the vertical 

acceleration correction may be ignored in marine settings. The filtered output is a measure of free-air gravity 

anomaly. In land vehicle deployments, horizontal velocities are both comparable to and exceed the steaming 

velocities of marine craft, so land-based Eotvos corrections are of similar and greater magnitude than marine 

corrections. In land-deployments, the magnitude of vertical accelerations is typically much less than that of ship32 
heave. However, rather than averaging to zero, vertical accelerations are non-zero when traversing land topography, 

so careful collection of vertical positions is necessary. 

The GPS unit integrated with the BGM-5 system is autonomous, and it utilizes the Civilian Access Code 

signal. To investigate how navigation quality varies among different types of GPS systems, we collected GPS data 

using two other autonomous, code-based units and using one dual-frequency, carrier-phase-based differential GPS 

system that consisted of a fixed GPS base station unit and a rover unit. For the differential GPS system (Trimble 

4700), the observations were made in real-time-kinematic mode with a radio link in 2004 and in post-processed 

kinematic mode by recording the GPS waveforms in 2005. All GPS antennas were mounted on the vehicle roof, and 

their 3-D offsets from the gravimeter sensor were measured. The GPS base stations were established at locations 

with unobstructed views of the sky and at local highs in elevation. The quality of the GPS navigation varies due to a 

number of factors: the GPS constellation geometry changes through the day; there are environmental effects at 

observation sites; and the navigation algorithms within each GPS unit differ. During our field sessions, the GPS 

constellation geometry was generally very good, with only single ~1 hour periods of slightly degraded coverage 

during field-days. In CNRA, tree foliage absorbed some of the GPS signals, and measures of navigational precision 

degraded noticeably when we were beneath leaf cover. The carrier-phase-based GPS system requires high signal-tonoise reception of the GPS satellite broadcast to retain the high precision of the differential solutions, and we rarely 

obtained differential-quality navigation within CNRA. (The differential GPS system provides code-based, lowerquality locations when conditions are not sufficient for differential solutions.) In the ranchland surrounding CNRA, 

trees are much less common and differential GPS solutions were prevalent, although at creek-crossings and near 

some homes, large trees overhanging the road were often sufficient to cause loss of differential GPS acquisition. 

In addition to the multiple GPS observations, we collected vehicle attitude (heading, pitch, and roll) data 

with an electronic compass. These observations will allow precise calculation of the sensor position by correcting 

the GPS antenna navigation for the offsets between the gravity sensor and the GPS antenna. 

The typical field-day started and ended with a LaCoste-Romberg observation at one or more of the local 

gravity base-stations, during which time the BGM-5 collected continuous, stationary data. We then drove slowly 

along the roadways of CNRA, the City of Sulphur, and surrounding areas. We collected data at very slow speeds (5-

10 mph) where it was safe to do so and where high-resolution gravity was deemed most desirable. For the most 

part, we drove at 10 to 15 mph, but occasionally we increased that value to 20 mph and greater. During transits 

from the NAVOCEANO laboratory at Stennis Space Center, Miss. to the study area and back, we collected data at 

speeds as great at 70 mph. In the study area, we drove with an intent to minimize all accelerations of the vehicle: we 

accelerated slowly, avoided sharp stops, tried to maintain constant survey speeds, made turns as smoothly as 

possible, and avoided rough patches of roadway. We continuously monitored all data streams, and we designed 

surveys to cover all of the park roads in CNRA and the publicly accessible roads in the surrounding area. In a few 

instances that were facilitated by CNRA Resources personnel, we obtained permission to collect observations on 

private ranches. 

The partial analysis of the BGM-5 data presented in this report consists only of times when the vehicle was 

stationary. This subset of the data is by far the easiest to analyze because corrections due to sensor motion are, by 

definition, zero. Most of the times that the vehicle was stationary were times either when the vehicle was parked 

overnight or when the vehicle was stopped for 3 to 10 minutes while acquiring a traditional LaCoste-Romberg 

station. Thus, these continuous gravity observations do not contribute new information to the gravity field as 

described in the main part of this report. That said, this analysis of stationary BGM-5 data is critical in establishing 

the signal and noise characteristics of the BGM-5 system to help guide filter design and navigation corrections for 

the moving BGM-5 data. 

Figure A1 displays the BGM-5 gravity output of the NAVOCEANO smoothing filter (3-minute half-width 

Gaussian) for two 9-hour periods when the vehicle was parked. On 2004jun11 (fig. A1a), the measured gravity 

fluctuations varied by <0.2 mGal, and much of the variation can be explained by the 0.1 mGal signal predicted for 

lunar and solar tidal accelerations and the Earth’s response to them. The residual of filtered gravity vs. predicted 

tidal accelerations has a maximum of 0.1 mGal, with agreement generally better than 0.05 mGal. This agreement 

between stationary BGM gravity and tidal predictions is typical of that in quiet, laboratory-bench deployments (R. 

Herr, personal commun.). The filtered gravity is not randomly distributed about the predicted gravity line; there are 

oscillations with periods of tens of minutes to two hours. The example from 2004jun11 is typical of the best 

stationary field-deployment periods that we measured. In other cases, such the overnight period starting on 

2005may03 (fig. A1b), the filtered gravity has periods of relative quiet and others of higher noise. Prior to 6 hours 

33 
of elapsed time, the BGM-5 gravity varies slowly and broadly in-synch with the predicted tidal acceleration, 

although it is somewhat noisier than the 2004jun11 case. After 6 hours of elapsed time, there are abrupt changes in 

filtered gravity, with steps of 0.5-2 mGal, and the measured gravity does not reflect the lower-amplitude predicted 

tidal accelerations. The cause of these rapid excursions is known in some cases but not in others. For instance, in 

some of the overnight deployments, the temperature inside of the vehicle decreased below the minimum operating 

temperature, 15oC (59oF), of the BGM-5 system and the measurements became erratic. When temperature changes 

were not a factor, we inspected the electronic compass records to see if there were vehicle motions that might 

indicate physical disturbance. In all of these cases, we did not detect such a physical disturbance, so the source of 

the noise in the gravimeter is unknown. Based on noise observed on some of the early overnight tests in 2005, we 

conducted a number of tilt and damp tests of the BGM-5 gyro-stabilized platform system when the vehicle was 

parked in an effort to improve the platform’s stability and calibration. After these tests, noise levels were decreased, 

as expected, but in some cases they were still greater than the quiet results observed in times such as 2004jun11 (fig. 

A1a). 

During extended, stationary time periods when the meter is quiet (for example, fig. A1a), filtered BGM-5 

gravity data are as precise as 0.05 mGal. This precision is not as good as that obtainable by LaCoste-Romberg Gmeter measurements (estimated to be 0.02 mGal through a typical survey day), but it is better than the gravity 

uncertainty related to elevation precision, where a 1 m uncertainty in elevation (common to many mid-level GPS 

systems) corresponds to a ~0.2 mGal uncertainty in elevation effect (free-air plus Bouguer terms). During noisy 

times such as the latter part of fig. A1b, the filtered BGM-5 data can be considered to be good only to 1 to 2 mGal, 

seriously limiting its utility for land-surveys at those times. In the 2004 and 2005 survey days, the vehicle 

temperature was always within the BGM-5 operating range, so that source of error observed in some overnight 

deployments is not present during the surveys. The collection of >250 traditional LaCoste-Romberg gravity stations 

in this study allows comparisons with the BGM-5 gravity values, permitting the identification of times of BGM-5 

noise and their subsequent omission from further analysis. 

As a first step in this analysis, we compare observed gravity values measured by traditional meters with 

those measured by the BGM-5 when the vehicle was stationary. To obtain a BGM-5 gravity value when stopped, 

we extracted times when the unfiltered GPS velocity was below an empirically-derived threshold of 0.02 m/sec 

(0.05 miles/hour). We then considered only those time intervals longer than 2 minutes. Because the system does 

not respond instantaneously to changes in acceleration, we investigated how long it took after stopping for the raw 

accelerometer signal to reach a near-constant value. Most of the decay of the raw signal from its transient to its 

near-constant state occurs within the first 30 seconds after the speed threshold is met. This indicates that the timeconstant of the BGM-5 system is considerably less than the 3-minute half-width Gaussian filter utilized in marine 

applications, and it suggests that less smoothing will be necessary in the moving, land-vehicle application. To be 

conservative, we omitted the first 60 seconds of stationary gravimeter data to avoid the transient, and then we passed 

the remaining data through a 3-point median filter to exclude any single-point spikes before averaging the data to 

provide a single gravity value. The standard deviations of the raw gravity values contributing to each stationary 

gravity observation ranged between 0.02 mGal and about 0.3 mGal. Finally, bias corrections were made for the 

BGM-5 stationary gravity values to force them to match the observed gravity values at the gravity base stations at 

the beginning and end of each field day. These bias corrections varied by 1 to 2 mGal from day to day and may be 

related to the operational requirement of cycling the power to the BGM-5 system twice per day. 

The observed gravity values measured by the LaCoste-Romberg meter (fig. A2a) with those of the BGM-5 

when it was stationary (fig. A2b) are strikingly similar. The major features of the observed gravity field are 

consistently recorded at the 1 mGal contour level using both data sets. Because these gravity values have not had 

any elevation effects incorporated, the dominant gravity signal of these maps is anti-correlated with land elevation. 

Individual BGM-5 “station” gravity values match their LaCoste-Romberg counterparts by less than the 1 mGal 

contour interval in all instances. 

Another display of the agreement of LaCoste-Romberg and stationary BGM-5 gravity observations is 

shown in fig. A3, where uncertainties of the BGM-5 values are plotted as standard-deviation error bars of the raw 

gravity counts. The gray band about the 1:1 line has a vertical dimension of 1 mGal. The agreement in observed 

gravity between the traditional and continuous meters points to the promise of this continuous technique in other 

land studies.

34 
Table A1. Principal facts for gravity stations collected in 2004 and 2005 in south-central Oklahoma 

[Datums: latitude and longitude, NAD27; elevations, NGVD29. FAA, free-air gravity anomaly; ITC, inner terrain correction calculated out to 2 

km; TTC, total terrain correction; CBA, complete Bouguer gravity anomaly, ISO, isostatic gravity anomaly] 

Station Latitude 

north 

(deg min) 

Longitude 

west 

(deg min) 

Elevation 

(m) 

Observed 

gravity 

(mGal) 

FAA 

(mGal) 

ITC 

(mGal) 

TTC 

(mGal) 

CBA 

(mGal) 

ISO 

(mGal) 

GEM001 34 30.03 -96 58.34 308.2 979643.64 48.16 0.06 0.13 13.38 46.07 

GEM002 34 30.26 -96 58.29 286.0 979648.38 45.70 0.05 0.11 13.43 46.15 

GEM003 34 30.22 -96 56.98 316.9 979641.12 48.04 0.10 0.19 12.34 44.96 

GEM004 34 29.81 -96 57.91 321.4 979640.61 49.50 0.05 0.14 13.24 45.87 

GEM005 34 30.34 -96 58.35 289.7 979648.08 46.43 0.04 0.10 13.73 46.47 

GEM006 34 26.55 -97 01.38 269.3 979644.89 42.27 0.02 0.04 11.82 44.24 

GEM007 34 26.88 -97 00.96 266.3 979646.32 42.34 0.03 0.05 12.23 44.67 

GEM008 34 27.07 -97 01.12 268.4 979645.88 42.26 0.04 0.06 11.93 44.41 

GEM009 34 27.11 -97 01.13 267.4 979645.95 41.98 0.04 0.06 11.76 44.25 

GEM010 34 27.06 -97 01.15 270.6 979645.44 42.53 0.04 0.06 11.94 44.42 

GEM011 34 27.10 -97 01.15 270.1 979645.70 42.56 0.04 0.06 12.03 44.51 

GEM012 34 27.09 -97 01.14 269.2 979645.89 42.50 0.04 0.06 12.07 44.55 

GEM013 34 27.46 -97 01.01 268.7 979645.14 41.08 0.04 0.06 10.71 43.24 

GEM014 34 27.44 -97 01.34 289.6 979640.90 43.32 0.09 0.12 10.64 43.18 

GEM015 34 27.12 -97 01.40 295.0 979640.03 44.56 0.12 0.16 11.32 43.82 

GEM016 34 27.18 -97 01.58 293.7 979639.83 43.86 0.07 0.11 10.72 43.24 

GEM017 34 27.10 -97 01.66 291.1 979640.13 43.49 0.10 0.14 10.66 43.17 

GEM018 34 27.08 -97 01.68 290.8 979640.59 43.89 0.13 0.16 11.12 43.63 

GEM019 34 27.06 -97 01.68 285.0 979641.26 42.80 0.07 0.10 10.62 43.13 

GEM020 34 27.04 -97 01.70 282.1 979642.22 42.89 0.07 0.10 11.03 43.54 

GEM021 34 27.04 -97 01.72 277.3 979643.01 42.18 0.06 0.08 10.86 43.37 

GEM022 34 27.04 -97 01.75 272.9 979643.87 41.68 0.06 0.08 10.85 43.37 

GEM023 34 27.08 -97 01.80 276.2 979643.36 42.14 0.06 0.08 10.94 43.47 

GEM024 34 27.81 -97 01.75 313.0 979635.47 44.59 0.14 0.22 9.36 41.98 

GEM025 34 27.57 -97 02.11 295.6 979639.46 43.55 0.15 0.19 10.26 42.87 

GEM026 34 27.83 -97 01.38 293.2 979640.49 43.45 0.06 0.10 10.35 42.95 

GEM027 34 28.21 -97 01.61 303.3 979638.01 43.57 0.10 0.15 9.38 42.05 

GEM028 34 28.03 -97 01.04 296.4 979639.80 43.49 0.08 0.12 10.05 42.65 

GEM029 34 28.84 -97 00.79 307.2 979639.10 44.98 0.04 0.10 10.29 42.98 

GEM030 34 29.48 -97 00.55 315.7 979638.81 46.40 0.08 0.15 10.81 43.57 

GEM031 34 28.79 -97 01.86 271.9 979645.45 40.49 0.05 0.07 9.77 42.54 

GEM032 34 29.20 -97 01.71 268.3 979646.32 39.68 0.06 0.08 9.37 42.18 

GEM033 34 28.75 -97 01.36 307.6 979638.12 44.24 0.07 0.13 9.54 42.26 

GEM034 34 29.49 -96 59.91 294.9 979643.31 44.48 0.02 0.06 11.15 43.87 

GEM035 34 29.92 -96 58.61 292.8 979646.85 46.77 0.05 0.11 13.71 46.41 

GEM036 34 29.91 -96 58.25 309.1 979643.10 48.06 0.04 0.11 13.17 45.84 

GEM037 34 25.70 -97 02.26 246.0 979650.04 41.44 0.04 0.06 13.64 46.00 

GEM038 34 25.30 -97 00.07 289.9 979643.01 48.53 0.11 0.15 15.85 47.96 

GEM039 34 25.10 -96 58.69 268.5 979648.29 47.47 0.05 0.08 17.15 49.12 

GEM040 34 23.03 -96 58.09 294.7 979652.74 62.91 0.01 0.06 29.61 61.16 

GEM041 34 17.33 -96 58.46 237.3 979621.12 21.58 0.02 0.01 -5.29 25.24 

GEM042 34 26.70 -97 00.59 274.1 979644.60 43.27 0.05 0.07 12.3 44.68 

GEM043 34 27.07 -97 00.43 299.4 979639.80 45.77 0.28 0.33 12.19 44.60 

GEM044 34 27.60 -97 00.19 306.0 979639.72 46.96 0.12 0.18 12.49 44.96 

GEM045 34 26.86 -96 59.92 297.3 979637.32 42.93 0.07 0.12 9.38 41.72 

GEM046 34 26.46 -96 59.72 276.1 979640.96 40.60 0.04 0.07 9.40 41.67 

GEM047 34 25.98 -97 00.26 267.8 979641.52 39.24 0.02 0.04 8.96 41.20 

GEM048 34 25.58 -96 59.45 267.3 979642.93 41.08 0.07 0.10 10.91 43.02 

GEM102 34 29.84 -96 58.95 283.1 979647.88 44.91 0.09 0.14 12.99 45.70 

GEM103 34 29.84 -96 59.05 282.4 979647.62 44.42 0.13 0.18 12.63 45.35 

35 
GEM104 34 29.80 -96 58.93 284.6 979647.40 44.94 0.13 0.18 12.89 45.59 

GEM105 34 29.83 -96 58.83 282.3 979648.06 44.87 0.11 0.16 13.06 45.76 

GEM106 34 30.27 -96 58.38 286.0 979648.54 45.85 0.05 0.11 13.57 46.30 

GEM107 34 30.18 -96 58.52 286.7 979648.67 46.33 0.05 0.11 13.97 46.69 

GEM108 34 30.15 -96 58.67 284.6 979648.87 45.92 0.08 0.13 13.82 46.55 

GEM109 34 29.96 -96 58.76 282.0 979648.83 45.35 0.09 0.14 13.56 46.28 

GEM110 34 29.91 -96 58.92 285.6 979647.97 45.68 0.05 0.10 13.44 46.16 

GEM111 34 29.71 -96 59.22 284.3 979646.51 44.08 0.08 0.13 12.02 44.73 

GEM112 34 29.57 -96 59.26 294.8 979644.67 45.69 0.07 0.12 12.43 45.12 

GEM113 34 29.65 -96 59.19 288.4 979645.16 44.08 0.09 0.14 11.57 44.27 

GEM114 34 29.81 -96 59.28 279.3 979647.49 43.41 0.06 0.11 11.88 44.60 

GEM115 34 29.76 -96 59.14 293.4 979643.95 44.26 0.11 0.16 11.20 43.91 

GEM116 34 29.47 -96 59.51 277.5 979646.94 42.76 0.05 0.09 11.43 44.13 

GEM117 34 29.43 -96 59.32 279.3 979646.75 43.18 0.11 0.16 11.71 44.39 

GEM118 34 29.54 -96 59.38 287.7 979645.47 44.35 0.07 0.11 11.88 44.58 

GEM119 34 29.45 -96 58.95 296.5 979643.33 45.05 0.07 0.12 11.59 44.24 

GEM120 34 29.50 -96 58.70 298.5 979643.65 45.91 0.10 0.16 12.26 44.90 

GEM121 34 29.69 -96 59.01 312.2 979640.88 47.11 0.19 0.26 12.01 44.70 

GEM122 34 29.69 -96 58.93 322.8 979638.47 47.95 0.42 0.51 11.91 44.59 

GEM123 34 29.68 -96 58.74 319.7 979639.65 48.2 0.29 0.37 12.37 45.04 

GEM124 34 29.56 -96 58.56 313.9 979640.75 47.68 0.13 0.20 12.34 44.98 

GEM125 34 29.58 -96 58.63 308.2 979642.12 47.27 0.09 0.16 12.52 45.17 

GEM126 34 29.66 -96 58.52 297.6 979644.27 46.02 0.09 0.15 12.47 45.13 

GEM127 34 29.71 -96 58.33 310.1 979643.01 48.57 0.06 0.13 13.58 46.23 

GEM128 34 29.59 -96 58.36 316.7 979640.57 48.33 0.09 0.17 12.63 45.26 

GEM129 34 29.62 -96 58.19 317.3 979641.03 48.92 0.06 0.14 13.13 45.75 

GEM130 34 29.61 -96 58.04 311.7 979642.45 48.65 0.06 0.13 13.48 46.09 

GEM131 34 30.11 -96 58.34 303.2 979644.98 47.82 0.11 0.17 13.66 46.36 

GEM132 34 30.22 -96 57.99 295.1 979647.06 47.26 0.06 0.12 13.97 46.67 

GEM133 34 30.29 -96 58.11 292.8 979647.65 47.03 0.05 0.11 13.99 46.70 

GEM134 34 30.31 -96 58.03 291.3 979647.01 45.9 0.09 0.15 13.07 45.78 

GEM135 34 30.26 -96 57.93 291.7 979646.76 45.84 0.09 0.16 12.97 45.67 

GEM136 34 30.13 -96 57.89 300.4 979646.26 48.22 0.06 0.13 14.33 47.01 

GEM137 34 29.48 -96 59.44 278.0 979646.88 42.86 0.06 0.10 11.47 44.16 

GEM138 34 29.52 -96 59.43 279.8 979646.70 43.16 0.05 0.09 11.57 44.27 

GEM139 34 30.26 -96 56.48 328.3 979639.69 50.08 0.08 0.18 13.09 45.67 

GEM140 34 30.15 -96 56.33 330.6 979638.92 50.18 0.09 0.19 12.94 45.50 

GEM141 34 30.18 -96 56.45 328.2 979639.62 50.08 0.09 0.19 13.11 45.68 

GEM142 34 30.26 -96 56.61 324.0 979640.47 49.52 0.07 0.17 13.00 45.59 

GEM143 34 30.24 -96 56.74 321.1 979640.90 49.11 0.07 0.16 12.90 45.50 

GEM144 34 30.24 -96 56.86 319.1 979641.31 48.89 0.07 0.16 12.91 45.52 

GEM145 34 29.50 -96 57.97 308.1 979642.06 47.27 0.08 0.15 12.54 45.13 

GEM146 34 27.19 -96 57.01 319.5 979631.38 43.37 0.02 0.11 7.30 39.47 

GEM147 34 26.88 -96 57.00 320.3 979631.63 44.31 0.04 0.13 8.17 40.29 

GEM148 34 26.83 -96 55.97 321.7 979632.18 45.34 0.04 0.13 9.05 41.08 

GEM149 34 27.07 -96 53.67 328.3 979638.93 53.81 0.02 0.12 16.76 48.62 

GEM150 34 27.20 -96 54.21 334.5 979637.54 54.16 0.02 0.13 16.41 48.34 

GEM151 34 27.02 -96 54.66 348.0 979634.41 55.44 0.03 0.17 16.20 48.14 

GEM152 34 26.95 -96 54.90 336.4 979635.88 53.43 0.03 0.14 15.48 47.43 

GEM153 34 26.87 -96 55.26 319.4 979635.34 47.75 0.07 0.16 11.74 43.72 

GEM154 34 26.92 -96 55.63 313.7 979634.36 44.95 0.06 0.14 9.56 41.58 

GEM155 34 24.45 -96 55.98 296.4 979647.32 56.03 0.03 0.09 22.56 54.18 

GEM156 34 26.35 -96 55.98 306.2 979636.49 45.56 0.02 0.09 10.98 42.93 

GEM157 34 28.63 -96 55.83 354.4 979630.80 51.52 0.04 0.19 11.59 43.88 

GEM158 34 29.02 -96 54.93 355.8 979629.77 50.38 0.03 0.18 10.27 42.55 

GEM159 34 34.21 -96 58.02 338.7 979637.74 45.77 0.05 0.16 7.58 40.73 

GEM160 34 36.29 -96 59.09 333.7 979638.13 41.69 0.06 0.15 4.05 37.47 

GEM161 34 34.61 -96 59.08 325.2 979640.62 43.93 0.05 0.13 7.23 40.49 

36 
GEM162 34 32.11 -96 59.08 312.5 979643.63 46.53 0.03 0.09 11.24 44.23 

GEM163 34 30.40 -96 59.08 288.7 979648.13 46.10 0.03 0.08 13.49 46.28 

GEM164 34 30.38 -97 03.32 305.5 979637.91 41.09 0.03 0.08 6.58 39.63 

GEM165 34 31.25 -97 03.27 303.2 979641.68 42.92 0.03 0.08 8.67 41.81 

GEM166 34 30.66 -96 57.51 322.9 979641.18 49.33 0.04 0.13 12.90 45.61 

GEM167 34 31.05 -96 51.54 381.2 979626.62 52.21 0.02 0.22 9.27 41.51 

GEM168 34 33.85 -96 51.72 368.3 979627.99 45.66 0.02 0.18 4.15 36.75 

GEM169 34 32.98 -96 53.30 366.3 979630.78 49.08 0.02 0.18 7.78 40.44 

GEM170 34 32.10 -96 53.42 355.8 979632.80 49.08 0.02 0.16 8.96 41.53 

GEM171 34 30.52 -96 56.99 314.1 979642.58 48.21 0.07 0.15 12.80 45.46 

GEM172 34 30.48 -96 56.81 332.0 979638.45 49.67 0.06 0.17 12.24 44.88 

GEM173 34 30.40 -96 56.47 345.8 979635.91 51.51 0.08 0.21 12.56 45.16 

GEM174 34 30.45 -96 56.60 340.6 979637.34 51.25 0.07 0.19 12.88 45.49 

GEM175 34 30.42 -96 56.72 335.6 979637.83 50.26 0.05 0.16 12.42 45.04 

GEM176 34 29.43 -96 59.45 278.8 979647.01 43.28 0.05 0.09 11.81 44.50 

GEM177 34 29.25 -96 59.34 275.4 979647.25 42.72 0.15 0.20 11.73 44.39 

GEM178 34 29.35 -96 59.37 273.8 979647.58 42.44 0.10 0.15 11.58 44.25 

GEM179 34 30.19 -96 58.28 285.3 979648.40 45.61 0.08 0.14 13.44 46.15 

GEM180 34 30.33 -96 57.25 308.9 979643.55 47.86 0.07 0.15 13.03 45.68 

GEM181 34 30.80 -96 58.88 298.4 979646.60 47.00 0.02 0.07 13.28 46.11 

GEM182 34 31.04 -96 58.13 296.7 979646.94 46.49 0.03 0.09 12.97 45.78 

GEM183 34 30.76 -96 57.28 328.1 979639.83 49.44 0.04 0.14 12.43 45.14 

GEM184 34 30.59 -96 57.22 332.6 979638.62 49.87 0.08 0.19 12.40 45.08 

GEM185 34 30.71 -96 59.42 298.1 979646.61 47.04 0.02 0.07 13.35 46.20 

GEM186 34 30.65 -96 58.59 291.1 979647.87 46.23 0.03 0.08 13.35 46.14 

GEM187 34 31.25 -96 58.83 306.3 979644.94 47.15 0.02 0.08 12.54 45.42 

GEM188 34 30.35 -96 59.49 294.3 979646.79 46.57 0.02 0.06 13.31 46.12 

GEM189 34 30.05 -96 57.49 304.4 979644.35 47.67 0.06 0.13 13.33 45.96 

GEM190 34 30.15 -96 57.38 307.0 979644.00 47.98 0.06 0.14 13.35 45.98 

GEM191 34 30.05 -96 57.62 297.9 979644.86 46.18 0.10 0.17 12.61 45.25 

GEM192 34 30.27 -96 57.09 317.0 979641.49 48.37 0.06 0.15 12.62 45.25 

GEM193 34 30.34 -96 57.02 327.9 979640.18 50.34 0.14 0.24 13.45 46.08 

GEM194 34 30.45 -96 57.03 314.5 979642.71 48.56 0.08 0.17 13.12 45.77 

GEM195 34 30.41 -96 57.18 313.0 979642.53 47.99 0.07 0.15 12.70 45.36 

GEM196 34 30.22 -96 57.35 306.2 979643.72 47.36 0.08 0.16 12.84 45.48 

GEM197 34 30.31 -96 57.13 317.7 979641.86 48.91 0.06 0.15 13.08 45.72 

GEM198 34 30.39 -96 57.39 310.0 979641.99 46.56 0.08 0.16 11.61 44.28 

GEM199 34 30.32 -96 57.36 309.6 979642.95 47.50 0.06 0.14 12.57 45.22 

GEM200 34 30.10 -96 57.83 294.8 979645.89 46.18 0.08 0.15 12.94 45.61 

GEM201 34 30.16 -96 58.19 288.9 979647.74 46.11 0.07 0.13 13.52 46.22 

GEM202 34 30.06 -96 58.14 304.8 979645.06 48.48 0.08 0.15 14.11 46.79 

GEM203 34 29.80 -96 58.19 315.3 979641.14 48.16 0.05 0.13 12.58 45.23 

GEM204 34 30.17 -96 57.29 308.0 979643.50 47.75 0.08 0.16 13.03 45.66 

GEM205 34 30.08 -96 57.31 310.1 979642.39 47.41 0.17 0.25 12.55 45.17 

GEM206 34 29.97 -96 57.44 310.1 979642.38 47.57 0.07 0.15 12.60 45.22 

GEM207 34 30.00 -96 57.66 313.6 979642.36 48.57 0.14 0.22 13.28 45.92 

GEM208 34 29.92 -96 57.86 311.9 979642.31 48.12 0.05 0.13 12.92 45.56 

GEM209 34 29.96 -96 58.08 300.3 979644.97 47.14 0.07 0.13 13.27 45.94 

GEM210 34 29.90 -96 58.04 308.3 979643.02 47.74 0.05 0.12 12.95 45.60 

GEM211 34 29.77 -96 57.96 322.3 979640.74 49.96 0.05 0.14 13.61 46.24 

GEM212 34 29.68 -96 57.86 323.2 979640.24 49.87 0.05 0.14 13.42 46.02 

GEM213 34 30.36 -96 58.90 297.5 979646.33 47.08 0.02 0.07 13.46 46.23 

GEM214 34 27.45 -97 01.74 305.8 979637.11 44.50 0.14 0.20 10.07 42.64 

GEM215 34 28.07 -97 01.62 305.3 979637.09 43.46 0.09 0.15 9.03 41.67 

GEM216 34 28.56 -97 01.35 307.5 979637.60 43.97 0.11 0.17 9.32 42.01 

GEM217 34 29.17 -97 00.96 313.4 979637.87 45.20 0.08 0.15 9.86 42.61 

GEM218 34 29.19 -97 00.42 306.1 979640.35 45.41 0.07 0.13 10.86 43.58 

GEM219 34 30.24 -97 00.16 298.4 979644.07 45.26 0.02 0.06 11.53 44.36 

37 
GEM220 34 30.37 -96 58.60 288.2 979648.24 46.11 0.03 0.08 13.56 46.32 

GEM221 34 28.40 -97 02.28 290.7 979639.55 40.95 0.07 0.10 8.13 40.87 

GEM222 34 28.57 -97 02.02 267.7 979645.22 39.30 0.07 0.09 9.06 41.81 

GEM223 34 28.63 -97 03.31 305.3 979635.39 40.97 0.05 0.11 6.51 39.35 

GEM224 34 29.50 -97 03.19 297.1 979637.49 39.33 0.05 0.09 5.77 38.71 

GEM225 34 29.28 -97 02.00 274.1 979643.94 38.99 0.04 0.06 8.00 40.84 

GEM226 34 30.02 -97 02.25 289.7 979641.59 40.41 0.04 0.07 7.67 40.62 

GEM227 34 26.01 -97 02.03 244.4 979649.23 39.70 0.14 0.16 12.18 44.57 

GEM228 34 26.33 -97 03.03 278.8 979642.95 43.60 0.09 0.11 12.13 44.64 

GEM229 34 26.87 -97 03.29 306.8 979634.67 43.20 0.11 0.18 8.64 41.25 

GEM230 34 27.24 -97 02.99 269.5 979642.56 39.06 0.12 0.13 8.67 41.31 

GEM231 34 27.76 -97 03.33 290.7 979637.21 39.50 0.04 0.07 6.65 39.38 

GEM232 34 34.00 -96 55.76 331.2 979637.53 43.55 0.03 0.13 6.18 39.16 

GEM233 34 37.33 -96 51.20 377.7 979628.67 44.36 0.02 0.20 1.79 34.62 

GEM234 34 33.43 -96 50.68 382.2 979625.23 47.81 0.02 0.21 4.74 37.17 

GEM235 34 30.52 -96 57.05 312.9 979642.51 47.78 0.09 0.17 12.52 45.18 

GEM236 34 30.99 -96 54.87 366.2 979630.81 51.86 0.02 0.19 10.59 43.14 

GEM237 34 31.39 -96 54.88 360.4 979633.35 52.04 0.02 0.17 11.42 44.02 

GEM238 34 31.81 -96 54.88 371.6 979631.54 53.10 0.05 0.23 11.27 43.92 

GEM239 34 32.09 -96 54.88 359.0 979633.55 50.84 0.05 0.20 10.39 43.08 

GEM240 34 30.38 -96 54.88 350.5 979633.33 50.41 0.02 0.15 10.87 43.34 

GEM241 34 27.77 -96 54.93 345.1 979635.28 54.34 0.02 0.15 15.42 47.51 

GEM242 34 28.17 -96 54.93 336.7 979634.41 50.32 0.08 0.19 12.39 44.54 

GEM243 34 28.68 -96 54.93 348.9 979631.26 50.23 0.04 0.17 10.90 43.13 

GEM244 34 29.43 -96 54.93 344.9 979631.10 47.77 0.05 0.17 8.90 41.24 

GEM245 34 29.91 -96 54.93 350.9 979632.03 49.90 0.03 0.16 10.33 42.74 

GEM246 34 30.63 -96 54.87 354.2 979632.70 50.56 0.02 0.16 10.62 43.12 

GEM247 34 27.70 -96 53.77 327.6 979637.90 51.66 0.02 0.12 14.69 46.66 

GEM248 34 28.05 -96 53.89 329.2 979637.20 50.98 0.02 0.12 13.82 45.86 

GEM249 34 28.53 -96 53.88 338.0 979633.19 49.01 0.03 0.14 10.88 42.99 

GEM250 34 28.90 -96 53.88 344.0 979632.98 50.12 0.02 0.14 11.32 43.49 

GEM251 34 29.42 -96 53.88 347.5 979631.82 49.32 0.02 0.15 10.12 42.36 

GEM252 34 29.84 -96 53.88 355.8 979631.96 51.42 0.02 0.16 11.30 43.61 

GEM253 34 30.20 -96 53.88 357.5 979631.84 51.33 0.02 0.17 11.02 43.38 

GEM254 34 30.45 -96 53.82 354.0 979632.20 50.26 0.03 0.17 10.34 42.73 

GEM255 34 30.36 -96 52.77 375.3 979628.42 53.18 0.04 0.23 10.91 43.18 

GEM256 34 30.35 -96 51.62 371.4 979628.84 52.40 0.02 0.20 10.55 42.71 

GEM257 34 31.99 -96 51.22 379.8 979627.05 50.88 0.02 0.21 8.10 40.43 

GEM258 34 37.31 -96 39.61 305.7 979638.26 31.76 0.07 0.18 -2.68 28.24 

GEM259 34 35.58 -96 46.47 362.8 979630.18 43.74 0.03 0.19 2.85 34.92 

GEM260 34 32.97 -96 48.51 361.8 979629.06 45.96 0.03 0.18 5.19 37.31 

GEM261 34 30.66 -96 58.23 288.7 979647.49 45.11 0.06 0.12 12.53 45.30 

GEM262 34 28.83 -96 57.55 346.8 979632.35 50.47 0.04 0.18 11.38 43.83 

GEM263 34 26.02 -96 58.40 294.8 979636.29 42.29 0.07 0.12 9.03 41.13 

GEM264 34 26.23 -97 00.48 268.5 979641.80 39.41 0.03 0.05 9.05 41.35 

GEM265 34 26.14 -97 00.15 280.7 979639.22 40.71 0.06 0.09 9.01 41.26 

GEM266 34 26.52 -96 59.13 285.4 979638.05 40.47 0.06 0.10 8.25 40.49 

GEM267 34 27.11 -96 59.13 302.1 979636.05 42.78 0.05 0.11 8.68 41.01 

GEM268 34 27.70 -96 58.78 318.9 979638.81 49.90 0.03 0.12 13.91 46.30 

GEM269 34 27.38 -96 58.08 323.0 979632.63 45.43 0.04 0.13 8.99 41.27 

GEM270 34 27.76 -96 57.57 329.2 979634.53 48.72 0.04 0.14 11.58 43.87 

GEM271 34 27.97 -96 57.04 331.5 979635.93 50.53 0.04 0.15 13.14 45.43 

GEM272 34 30.39 -96 58.14 297.8 979646.17 46.95 0.04 0.10 13.33 46.05 

GEM273 34 29.45 -96 57.45 327.9 979637.96 49.37 0.06 0.16 12.40 44.94 

GEM274 34 28.63 -96 56.39 363.3 979629.49 52.98 0.07 0.25 12.09 44.42 

GEM275 34 25.33 -96 55.98 323.0 979638.18 53.87 0.09 0.19 17.48 49.25 

GEM276 34 23.83 -96 48.64 311.4 979632.23 46.44 0.02 0.11 11.29 42.00 

GEM277 34 22.61 -96 47.59 290.2 979637.84 47.21 0.03 0.10 14.45 44.79 

38 
GEM278 34 25.16 -96 47.59 344.9 979630.17 52.84 0.03 0.18 13.98 44.80 

GEM279 34 27.72 -96 47.59 336.9 979633.80 50.42 0.02 0.14 12.41 43.68 

GEM280 34 27.77 -96 52.83 330.2 979636.93 51.40 0.02 0.13 14.14 46.04 

GEM281 34 28.63 -96 55.25 367.0 979628.75 53.38 0.06 0.24 12.07 44.31 

GEM282 34 28.64 -96 54.39 338.0 979633.88 49.54 0.03 0.14 11.41 43.58 

GEM283 34 28.64 -96 53.34 335.5 979635.22 50.10 0.01 0.12 12.23 44.31 

GEM284 34 30.04 -96 53.88 360.4 979630.96 51.57 0.03 0.18 10.95 43.28 

GEM285 34 30.33 -96 53.43 355.6 979631.72 50.43 0.02 0.16 10.33 42.67 

GEM286 34 29.98 -96 52.83 356.6 979631.33 50.83 0.03 0.17 10.64 42.88 

GEM287 34 29.68 -96 52.91 356.2 979631.38 51.19 0.03 0.17 11.04 43.24 

GEM288 34 29.53 -96 53.46 356.5 979632.44 52.56 0.03 0.18 12.37 44.59 

GEM289 34 29.36 -96 53.48 344.8 979632.29 49.03 0.02 0.14 10.14 42.34 

GEM290 34 29.08 -96 53.10 350.0 979631.67 50.40 0.02 0.15 10.93 43.05 

GEM291 34 29.70 -96 53.88 351.9 979632.99 51.44 0.02 0.16 11.75 44.04 

GEM292 34 29.51 -96 54.38 339.4 979632.95 47.83 0.02 0.14 9.53 41.84 

GEM293 34 29.72 -96 54.93 347.8 979631.55 48.71 0.03 0.16 9.49 41.87 

GEM294 34 29.50 -96 55.47 355.1 979629.78 49.50 0.02 0.17 9.47 41.87 

GEM295 34 29.50 -96 56.03 370.6 979627.68 52.20 0.05 0.25 10.49 42.92 

GEM296 34 29.49 -96 56.73 336.0 979634.78 48.64 0.06 0.17 10.76 43.25 

GEM297 34 27.98 -96 59.12 323.6 979637.98 50.13 0.04 0.14 13.62 46.07 

GEM298 34 28.27 -96 58.69 332.4 979636.53 51.00 0.04 0.16 13.51 45.97 

GEM299 34 28.75 -96 59.01 321.4 979638.37 48.76 0.10 0.19 12.55 45.11 

GEM300 34 28.86 -96 59.72 289.7 979644.00 44.45 0.07 0.11 11.75 44.37 

GEM301 34 28.92 -96 59.38 302.3 979642.09 46.33 0.07 0.13 12.23 44.83 

GEM302 34 29.08 -96 58.97 312.3 979639.56 46.66 0.07 0.14 11.44 44.04 

GEM303 34 29.26 -96 59.11 300.8 979642.31 45.62 0.06 0.12 11.67 44.31 

GEM304 34 30.43 -96 59.81 290.8 979647.56 46.13 0.02 0.06 13.26 46.10 

GEM305 34 29.71 -96 57.80 323.5 979640.08 49.75 0.05 0.14 13.26 45.86 

GEM306 34 29.76 -96 57.59 320.0 979640.37 48.89 0.06 0.15 12.81 45.41 

GEM307 34 29.58 -96 57.71 331.3 979638.00 50.29 0.06 0.16 12.93 45.51 

GEM308 34 30.07 -96 58.49 292.2 979646.81 46.31 0.05 0.11 13.33 46.04 

GEM309 34 29.74 -96 58.42 310.6 979642.00 47.66 0.05 0.12 12.61 45.27 

GEM310 34 29.81 -96 58.35 311.9 979642.16 48.12 0.05 0.12 12.92 45.58 

GEM311 34 29.86 -96 58.29 311.4 979643.05 48.80 0.05 0.12 13.65 46.32 

GEM312 34 30.05 -96 57.90 301.0 979644.38 46.65 0.07 0.14 12.70 45.37 

GEM313 34 30.14 -96 58.01 302.7 979644.51 47.17 0.06 0.13 13.02 45.70 

GEM314 34 30.22 -96 58.09 295.2 979645.84 46.07 0.07 0.13 12.77 45.47 

GEM315 34 30.17 -96 58.08 299.0 979645.10 46.58 0.05 0.11 12.83 45.53 

GEM316 34 30.04 -96 58.60 287.8 979648.09 46.29 0.05 0.10 13.80 46.51 

GEM317 34 29.97 -96 58.65 291.2 979647.92 47.27 0.05 0.10 14.40 47.10 

GEM318 34 29.89 -96 58.71 282.0 979648.42 45.05 0.11 0.16 13.28 45.98 

GEM319 34 30.07 -96 58.49 291.7 979647.15 46.51 0.06 0.12 13.59 46.30 

GEM320 34 30.43 -96 59.81 290.5 979647.73 46.21 0.02 0.06 13.37 46.21 

GEM321 34 28.90 -96 59.01 316.0 979639.33 47.85 0.08 0.16 12.22 44.8 

GEM322 34 29.08 -96 58.97 314.4 979639.64 47.41 0.09 0.16 11.97 44.57 

GEM323 34 29.18 -96 59.06 306.7 979641.14 46.38 0.06 0.12 11.77 44.39 

GEM324 34 27.65 -96 57.03 332.7 979634.74 50.14 0.04 0.15 12.63 44.87 

GEM325 34 28.51 -96 57.34 355.6 979631.15 52.42 0.04 0.21 12.37 44.75 

GEM326 34 29.21 -96 57.79 328.8 979636.55 48.56 0.10 0.20 11.53 44.06 

GEM327 34 29.73 -96 58.12 317.7 979641.32 49.18 0.04 0.12 13.33 45.96 

 

39 
Table A2. Physical property measurements of rock samples collected in 2004 and 2005 

Station Rock type Circular 

91 code1

Grain 

density 

(g/cm3

) 

Saturated 

bulk density 

(g/cm3

) 

Dry bulk 

density 

(g/cm3

) 

Porosity 

(%) 

Susceptibility 

(10-3 SI) 

GEM149 Sandstone Owk 2.67 2.62 2.59 3.1 0.02 

GEM150 Carbonate Owk 2.77 2.67 2.62 5.9 0.03 

GEM151 Carbonate Owk 2.76 2.64 2.58 7.0 0.03 

GEM152 Carbonate Ooj 2.50 2.30 2.17 14.9 0.02 

GEM158 Carbonate Obm 2.68 2.66 2.66 0.9 0.02 

GEM173 Conglomerate IPvc 2.64 2.56 2.51 5.0 0.01 

GEM175A Arkose IPvc 2.49 2.36 2.27 10.0 0.01 

GEM175B Conglomerate IPvc 2.65 2.59 2.56 3.3 0.02 

GEM205 Arkose IPvc 2.38 2.24 2.14 11.5 0.03 

GEM241 Carbonate Owk 2.75 2.70 2.67 3.1 0.05 

GEM244 Carbonate Obm 2.67 2.65 2.64 1.2 0.01 

GEM254 Carbonate Ow 2.77 2.69 2.64 4.9 0.03 

GEM267 Carbonate Osv 2.67 2.66 2.65 0.7 0.01 

GEM275 Carbonate MDsw 2.64 2.59 2.55 3.5 0.02 

GEM278 Sandstone Ocm 2.71 2.69 2.68 1.3 0.03 

GEM286 Carbonate Ow 2.81 2.80 2.79 0.6 0.02 

GEM296 Arkose IPvc 2.54 2.45 2.40 6.1 0.02 

GEM326 Carbonate Obm 2.67 2.63 2.62 1.9 0.02 

GEM1000 Carbonate Ow 2.66 2.55 2.49 6.8 0.04 

GEM1001 Carbonate Ow 2.75 2.66 2.61 5.6 0.04 

GEM1002 Granite pCgg 2.62 2.58 2.56 2.3 2.1 

GEM1101 Carbonate Ow 2.72 2.71 2.70 0.7 0.02 

GEM1102 Carbonate DSOh 2.63 2.57 2.53 3.9 0.07 

GEM1103 Sandstone Obm 2.37 2.26 2.18 8.8 0.05 

GEM1104 Granite pCtr 2.64 2.64 2.63 0.3 9.4 

GEM1105 Sandstone Obm 2.47 2.40 2.36 4.7 0.03 

GEM1106 Carbonate Osv 2.67 2.66 2.65 0.8 0.04 

GEM1107 Granite pCgr 2.83 2.82 2.81 0.8 22 

GEM1108 Granite pCti 2.61 2.60 2.60 0.5 0.45 

GEM1109 Shale MDsw 2.48 2.45 2.44 1.8 0.01 

1

Circular 91 denotes Plate 1 of Fairchild and others (1990). 

40