Excerpts From USGS Sedona Magnetic Anomaly Survey:
Preliminary Report 
on Geophysical Data 
in Yavapai County, 
by V.E. Langenheim1, J.P. Hoffmann2, K.W. Blasch2, Ed 
Dewitt3, and Laurie Wirt3
Open-File Report 02-352
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.  
1Menlo Park, California
2Tucson, Arizona
3Denver, Colorado
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 anomaliesMagnetic 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 
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.
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, 
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 
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)
in hertz
Coil spacing,
in meters
Maximum depth of investigation, in meters
          Vertical Dipole                      Horizontal Dipole