Excerpts From USGS Sedona Magnetic Anomaly Survey:

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Preliminary Report 

on Geophysical Data 

in Yavapai County, 

Arizona

by V.E. Langenheim1, J.P. Hoffmann2, K.W. Blasch2, Ed 

Dewitt3, and Laurie Wirt3

Open-File Report 02-352

2002

This report is preliminary and has not been reviewed for conformity with U.S. 

Geological Survey editorial standards or with the North American Stratigraphic Code.  

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

imply endorsement by the U.S. Government.  

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

1Menlo Park, California

2Tucson, Arizona

3Denver, Colorado

AEROMAGNETIC DATA, MAPS, AND DERIVATIVE PRODUCTS

About the Aeromagnetic Method

Geologic structures (such as faults or igneous intrusions) often produce small magnetic 

fields that distort the main magnetic field of the Earth (Fig. 3).  These distortions, called 

anomalies, can be detected by measuring the intensity of the Earth's magnetic field on or 

near the surface of the earth.  By analyzing magnetic measurements, geophysicists are able 

to learn about geologic structures, even though these structures may be buried beneath the 

Earth's surface (e.g., Dobrin and Savit, 1988; Blakely, 1995).  Magnetic measurements are 

often made from airplanes (or helicopters) flown along closely spaced, parallel flight lines.  

Additional flight lines are flown in a perpendicular direction to aid in data processing.  

These measurements are then processed into a digital aeromagnetic map.  Assisted by 

computer programs, the geophysicist builds a geologic interpretation from these data, 

incorporating geologic mapping, well information, and other available geophysical 

information (e.g., gravity, radiometric, electrical, seismic-reflection).

Magnetic Lithologies

Volcanic rocks are the most prevalent magnetic lithology of this region, and we expect 

high-amplitude, short-wavelength anomalies over volcanic terranes, especially in the Black 

Hills and the area between Page Springs and Sedona.  Volcanic rocks in the area consist of 

(1) the Sullivan Buttes latite (24-26 Ma; also known as the andesite unit of Krieger, 1965), 

(2) an older sequence of basalts of the Hickey Formation, and (3) a younger section of 

basalts of the Perkinsville Formation.  The basalts in this region typically include lava 

flows which individually may have a uniform direction of magnetization.  Steeply dipping 

faults that offset subhorizontal units often produce magnetic anomalies that appear as linear 

trends on aeromagnetic maps (e.g., Bath and Jahren, 1984).  The latites, on the other hand, 

often are extruded from volcanic plugs and thus tend to produce intense, somewhat circular 

magnetic anomalies (often as magnetic lows, because the latites are generally reversely 

polarized).  The latites are exposed at Sullivan Buttes and are inferred to underlie parts of 

Little Chino Valley (Langenheim and others, 2000).

The magnetic properties of sedimentary rocks, such as the Paleozoic sequence of 

dolomite, limestone, and sandstone, are usually weak, such that the resulting magnetic 

anomalies are very small in amplitude or undetectable by airborne surveys.  The 

Precambrian metasedimentary rocks are generally incapable of producing detectable 

magnetic anomalies, although there are some notable exceptions.  Exceptions include an 

iron-rich metachert that forms a minor lithologic constituent exposed south of the towns of 

Prescott and Prescott Valley (Krieger, 1965) and metagraywacke exposed on the western 

side of Sullivan Buttes.  Metavolcanic rocks, gabbros and some of the intrusive rocks can 

produce prominent magnetic anomalies.  Magnetic susceptibility is a measure of how 

magnetic a rock becomes when placed in an external magnetic field and is mostly a function 

of the amount of magnetite in that rock.  Magnetic susceptibility measurements of 

Precambrian metavolcanic rocks range from 2 (meta-rhyolite) to 8000 (metabasalt) 10-6 SI 

units (Ed DeWitt, written commun., 2000).  Gabbros also show a high range in 

susceptibility for 29 samples, from 30 to 8000 x 10-6 in the SI system.  Granodiorites and 

granites show a wide range in susceptibility, although individual granodiorite and granite 

plutons are characterized by narrower ranges in susceptibility.  For example, the Prescott 

granodiorite has a range of 900 to 1600 x 10-6 for 6 samples (mean of 1100 x 10-6).  The 

Minnehaha granodiorite, on the other hand, is less magnetic, showing a range in 

susceptibility of 10 to 350 x 10-6 for 5 samples (mean of 48 x 10-6).  

Data Acquisition and Processing

Goldak Airborne Survey conducted the aeromagnetic survey under contract to Yavapai. 

County.  Goldak is headquartered in Saskatoon, Sasketchewan, Canada, and has years of 

experience in acquiring and processing aeromagnetic data.  Data acquisition and processing 

were accomplished under guidelines established by the U.S. Geological Survey over the 

last several decades.  

The aeromagnetic data were acquired with a Piper PA-31 Navajo, a fixed-wing aircraft.  

The airborne magnetic sensor was a Geometrics G-822A cesium-vapor magnetometer 

located at the tip of a fiberglass stinger (boom).  A theoretical flight surface, based on a 

digital topograhic model, was computed in advance of the survey, and a real-time, 

differentially corrected Global Positioning System (GPS) was used during flight to 

maintain this theoretical surface.  Flight lines were oriented east-west and flown at a 

nominal altitude of 150 m (500 ft) above terrain, or as low as permitted by the Federal 

Aviation Adminstration and safety considerations.  Figure 4 shows the height of the 

airplane above the ground measured by radar altimetry.  The Prescott, Williamson Valley, 

and most of the Black Hills and Cottonwood blocks were flown at a flightline spacing of 

200 m.  The Upper Big Chino block and the northern parts of the Black Hills and 

Cottonwood blocks were flown at a spacing of 300 m.  North-south control lines were 

spaced 3.0 km (1.83 mi) apart.  Total flight distance was 21,691 km (13,478 mi).

Two base station magnetometers were deployed for this survey.  The primary base 

station, a cesium-vapor magnetometer identical to the airborne sensor, was just east of 

Prescott.  The secondary base station, a proton-precession magnetometer, was located at 

the Ernest Love municipal airport north of the city of Prescott.  A base station 

magnetometer measures the time-varying magnetic field and has two important functions:  

(1) it records the normal daily changes of the external field (diurnal variation), which are 

subtracted from the aeromagnetic data and (2) it records the onset and dissipation of 

magnetic storms.  Airborne operations were interrupted if magnetic storm activity exceeded 

the limits established by the U.S. Geological Survey.  The limits were as follows:  (a) 5 

nanoteslas (nT) for monotonic changes during any 5 minute period, (b) 2 nT for pulsations 

with periods of 5 minutes or less, (c) 4 nT for pulsations with periods between 5 and 10 

minutes, and (d) 8 nT for pulsations with periods between 10 and 20 minutes.  Time 

between aircraft and base stations was synchronized with GPS time.

Post-survey data processing was performed by Patterson, Grant and Watson (PGW) of 

Toronto, Canada.  This included removal of diurnal fields, subtraction of the International 

Geomagnetic Reference Field (e.g., Barton and others, 1996), navigational corrections, 

and adjustment of total-field values between crossings of flight lines and tie lines.  A 

preliminary version of the completed survey was provided to the USGS for evaluation in 

September, 2001.  Final data were delivered in November, 2001.  Accuracy of the data is 

estimated to be on the order of 0.5 to 1 nT.

Aeromagnetic Map and Derivative Products

Figures 5a and 5b show the improved resolution of the new, high-resolution data 

compared to the preexisting, regional aeromagnetic data.  Color scale in these maps 

indicates the intensity of the Earth's magnetic field relative to a global standard (the 

International Geomagnetic Reference Field updated to the date of the survey).  The regional 

digital coverage (Sweeney and Hill, 2001) consists of east-west flightlines flown at a 

spacing of 1 or 3 miles and data from 3 higher-resolution surveys (Dempsey and Hill, 

1963; USGS, 1981; USGS, 1982).  We also incorporated the high-resolution 

aeromagnetic survey that covers the headwaters region of the Verde River (Langenheim and 

others, 2000).  Figure 5b shows the new survey merged into the regional digital database.  

As expected, volcanic regions produce distinctive magnetic anomalies, high in amplitude 

and short in wavelength.  These anomalies are particularly evident over much of Lonesome 

Valley, the Black Hills and the area between Cornville and Sedona.  For example, the 

preexisting regional coverage indicates only a broad magnetic high in the Page Springs area 

(Fig. 5a).  Virtually all of the individual magnetic anomalies seen in the new high-

resolution data (Fig. 5b) are absent in the pre-existing regional coverage.

Large magnetic highs are present over the weakly magnetic Paleozoic sedimentary 

rocks exposed on Big Black Mesa and in the vicinity of Sedona.  Thus, the sources of 

these anomalies are likely concealed by Paleozoic units.  Krieger (1967a) shows several 

small exposures of Precambrian granitic rocks along the Big Chino fault; these rocks are 

the most likely source of the Big Black Mesa magnetic high.  The high is truncated by the 

Big Chino fault on its southwestern margin (Langenheim and others, 2000).  Oil-test wells 

in the Sedona area encountered Precambrian granite beneath 300-500 m of Paleozoic 

sedimentary rocks (Peirce and Scurlock, 1972); 

"Precambrian crystalline basement is the most likely source of the broad 

magnetic high in the Sedona area. The long-wavelength nature of the magnetic 

high indicates that the source of the anomaly is buried".

Subdued magnetic anomalies are present in the Verde Valley.  The subdued nature of 

these anomalies is probably caused by two factors:  (1) the increased height of the magnetic 

sensor above the ground surface in this region (Fig. 4) and (2) the increased thickness of 

relatively non-magnetic Verde Formation in the downdropped block of the Verde fault 

zone.  To emphasize and sharpen the anomalies in this region, filtering of the data and 

comparison to the gravity data will be needed.

The new aeromagnetic data are of sufficient quality to permit the application of well-

established processing and filtering techniques that emphasize subtle features.  Figures 6 

and 7 show the aeromagnetic data processed to enhance and define near-surface sources.  

Figure 6 shows residual magnetic anomalies, a technique that emphasizes shallow 

magnetic sources.  This residual magnetic map was computed by analytically continuing the 

aeromagnetic data to a slightly higher surface (100 m; Blakely, 1995), in other words, 

mathematically transforming the data as if they were collected at a higher altitude, and then 

subtracting that result from the original data.  The anomalies that remain are commonly 

called residual magnetic anomalies.  This method, essentially a discrete vertical derivative, 

emphasizes anomalies caused by shallow magnetic sources (approximately < 1 km) while 

subduing anomalies caused by deep sources.  It is particularly useful in identifying shallow 

crustal faults that separate rocks of contrasting magnetic properties.  Shallow sources 

produce short-wavelength anomalies, such as the anomalies present over exposed volcanic 

rocks.

Subtle magnetic anomalies that are not apparent in Figure 5b are accentuated in the 

filtered aeromagnetic data (Fig. 6).  The magnetic field over the alluvial deposits of 

Lonesome Valley shows several northwest- and north-trending anomalies.  Because 

alluvium is often weakly magnetic, some of these anomalies may originate from volcanic 

rocks concealed beneath the surface.  Other possible sources are shallowly buried 

Precambrian basement, or alteration along buried fault zones.  A linear, north-striking 

magnetic high extends from the town of Prescott Valley north towards Perkinsville.  Its 

source probably resides within the Precambrian basement, because the magnetic anomaly 

can be traced onto Precambrian rocks.

Figure 7 shows magnetization boundaries, automatically computed from the 

aeromagnetic data (Blakely and Simpson, 1986).  This calculation assumes that magnetic 

contacts are vertical; calculated positions will be shifted slightly over contacts that are not 

vertical.  Figure 7 shows the magnetization boundaries plotted on the regional geology.  

Langenheim and others (2000) used the magnetization boundaries to map the extent of the 

Big Chino Fault, as indicated by a lineament on the basin margin of Big Black Mesa.  The 

new data extend the Big Chino fault to the north of the 1999 high-resolution aeromagnetic 

survey and are consistent with Kriegers (1967b) geologic mapping of the fault.  The 

magnetic data can also be used to extend the Bear Wallow Canyon fault west of its mapped 

extent into the northern part of Verde Valley.

The magnetization boundaries also define structures related to buried volcanic rocks.  

For example, the magnetization boundaries in the northeast corner of the Upper Big Chino 

block delineate the extent of buried basalt (possibly a northeast-striking paleochannel filled 

with basalt?) on the upthrown side of the Big Chino fault.  Future analysis should focus on 

establishing the depth, thickness, and geometry of the volcanic rocks beneath upper Big 

Chino, Lonesome, Verde, and Williamson Valleys.

SURFACE GEOPHYSICAL DATA

Gravity data and map

As part of this project, we compiled and reprocessed the existing gravity coverage of 

the region (National Geophysical Data Center, 1999; Frank, 1984; Smith, 1984; 

Langenheim and others, 2000) and added 628 new gravity stations using a global 

positioning system (GPS) to determine location and elevation.  These data have been 

processed to provide information on subsurface density variations.  The gravitational 

attraction at any point depends on many factors, including the latitude and elevation of the 

measurement, earth tides, terrain, deep masses that isostatically support the terrain, and 

variations in density within the Earth's crust and upper mantle.  The last of these quantities 

is of primary interest in geologic investigations and can be obtained by calculating and 

removing all other quantities.  The resulting field is called the isostatic residual gravity 

anomaly and reflects, to first order, density variations within the middle and upper crust 

(Simpson and others, 1986).

The gravity field is dominated by gravity highs along the northeastern part of the study 

area, with lower gravity values in the southwestern part of the study area (Fig. 8).  

Superposed on this regional field are local gravity lows in the valley areas.  Big Chino 

Valley is characterized by a gravity low, bounded on the east by the Big Chino Fault.  The 

deepest part of the basin, as suggested by the lowest gravity value within the valley (-24 

mGal), is about 5 km from the southern edge of the Upper Big Chino aeromagnetic survey 

block.  Gravity values increase to the southeast towards Sullivan Lake.  The northern part 

of Lonesome Valley is characterized by higher gravity values than those over Big Chino 

Valley, suggesting that the basin fill beneath Little Chino Valley is less than 1 km 

(Langenheim and others, 2000).  South of the town of Chino Valley is an east-west 

striking gravity gradient, with a large gravity low to the south.  This low could be caused 

by a deep basin centered near the intersection of Highway 89 and alternate route 89, but a 

more likely explanation is a thick stock of Prescott granodiorite (Cunion, 1985).  The 

gravity low extends over Precambrian rocks and Prescott granodiorite (and Granite Dells 

granite) is less dense than the some of the more mafic metavolcanic and gabbros within 

Precambrian basement.  

A gravity feature of potential hydrologic interest is the gravity low over Williamson 

Valley. The gravity low may reflect either a relatively deep (1-2 km) sedimentary basin or a 

low-density pluton (Langenheim and others, 2000).  The margins of the low are linear and 

strike northwest.  The gravity low also coincides with a magnetic low (Fig. 5b, 6).  The 

basin interpretation is preferred because the gravity low does not extend across 

Precambrian outcrops exposed to the east of the gravity low.  A drillhole recently (2002) 

completed for the city of Prescott supports the basin interpretation; the drillhole penetrated 

460 m (1500 ft) without encountering basement rocks (T. Merrifield, oral commun., 2002; 

Fig. 8).

The Verde Valley is also characterized by a northwest-trending gravity low. Based on 

the amplitude of the gravity low (15-18 mGal) and assuming a reasonable density contrast 

between the basin fill and the basement rocks (-0.4 g/cm3), the basin fill may be as thick as 

1 km.  The low, approximately 35 km long and 8-10 km wide, lies within the western half 

of the valley.  The western margin of the Verde Valley gravity low is nearly coincident with 

the southern part of the Verde fault zone, but lies 1-2 km east of the mapped trace of the 

northern part of the fault zone.  The eastern margin of the basin is more linear, suggestive 

of a fault origin, and steps to the southwest near the intersection of I-17 and Hwy 260, 

dividing the gravity low into two parts.  The lowest gravity values are northwest of the 

step.  A smaller gravity low, about 8 km long, lies to the south of the step.  The southern 

extent of this gravity low is poorly constrained by existing gravity data.  

Another gravity feature of potential hydrologic interest is outside the study area, in the 

southwest corner of Figure 8, where the lowest gravity values lie northeast of the 

intersection of highways 89 and 96.  Although the gravity feature is poorly constrained, the 

low appears to be caused by thick sedimentary fill.  Two drillholes, 518 and 669 m deep 

(1700 and 2195 ft, respectively), did not penetrate basement (Oppenheimer and Sumner, 

1980).

Surface Electromagnetic (EM) Surveys

Surface EM methods measure the apparent electrical conductivity of subsurface 

deposits, which help evaluate the approximate depth and extent of recent alluvium.  The 

depth extent of these data is shallow (<75 m), which complements the generally more 

regional nature and deeper depth of investigation obtained from the gravity data. The 

apparent electrical conductivity of the deposits is a function of grain size, composition, and 

moisture content. Conductivity values for dry alluvium in the arid Southwest commonly are 

less than 10 millimhos per meter (mmhos/m) but can range from 20 to 50 mmhos/m when 

saturated; those of saturated clay and silt commonly are about 100 mmhos/m or greater. 

Paleozoic sandstones and limestones have conductivity values less than 20 and commonly 

less than 10 mmhos/m. Basalt units can be one of the least conductive rock units depending 

on degree of weathering and water content; generally, conductivity values are less than five 

mmhos/m.

Depth of investigation for the EM34-3 instrument used during these surveys (Fig. 9) 

ranges from about 7.5 to 60 m and is a function of transmission frequency, coil spacing, 

and dipole type (Table 1). Although depth of investigation for the electromagnetic-induction 

instruments extends to about 60 m, depth of the material contributing to the signal differs 

Table 1. Depths of investigation using EM-34-3 instrument at various frequencies, coil spacings and 

dipole types (data from McNeill, 1980)

Frequency, 

in hertz

Coil spacing,

in meters

Maximum depth of investigation, in meters

          Vertical Dipole                      Horizontal Dipole