Digital data from the Questa-San Luis and Santa 

Fe East helicopter magnetic surveys in Santa Fe 

and Taos Counties, New Mexico, and Costilla 

County, Colorado. 

By Viki Bankey, V.J.S. Grauch, B.J. Drenth, and Geophex, Ltd. 

Prepared in cooperation with the New Mexico Office of the State Engineer, New Mexico Bureau of 

Geology & Mineral Resources, and Santa Fe County 

Open-File Report 2006–1170 

U.S. Department of the Interior 

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

Dirk A. Kempthorne, Secretary 

U.S. Geological Survey 

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia 2006 

Revised and reprinted: 2006 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

Telephone: 1-888-ASK-USGS 

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

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

endorsement by the U.S. Government. 

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

copyright owners to reproduce any copyrighted material contained within this report.

ii
Digital data from the Questa-San Luis and Santa Fe 

East helicopter magnetic surveys in Santa Fe and 

Taos Counties, New Mexico, and Costilla County, 

Colorado. 

By Viki Bankey1, V.J.S. Grauch1, B.J. Drenth1, and Geophex, Ltd.2

Abstract 

This report contains digital data, image files, and text files describing data formats and 

survey procedures for aeromagnetic data collected during high-resolution aeromagnetic surveys in 

southern Colorado and northern New Mexico in December, 2005. One survey covers the eastern 

edge of the San Luis basin, including the towns of Questa, New Mexico and San Luis, Colorado. A 

second survey covers the mountain front east of Santa Fe, New Mexico, including the town of 

Chimayo and portions of the Pueblos of Tesuque and Nambe. 

Several derivative products from these data are also presented as grids and images, 

including reduced-to-pole data and data continued to a reference surface. Images are presented in 

various formats and are intended to be used as input to geographic information systems, standard 

graphics software, or map plotting packages. 

Introduction 

This report describes data collected from high-resolution aeromagnetic surveys flown over 

the east-central San Luis basin and the southeastern Española basin during December, 2005, by 

Geophex, Ltd., on contract to the U.S. Geological Survey (USGS). One survey, Questa-San Luis, 

covers the eastern edge of the San Luis basin, including the towns of Questa, Taos County, New 

Mexico, and San Luis, Costilla County, Colorado (fig. 1). A second survey, Santa Fe East, covers 

the mountain front east of Santa Fe, Santa Fe County, New Mexico, including the town of Chimayo 

and portions of the Pueblos of Tesuque and Nambe (fig. 2). 

These surveys were designed to complete or extend the coverage of surveys previously 

acquired in the central San Luis basin (Bankey and others, 2005), the southern Española basin 

(USGS and others, 1999), and along the eastern borders of the San Luis Basin over the vicinities of 

Taos, New Mexico (Bankey and others, 2004a), and Blanca, Colorado (Bankey and others, 2004b). 

Our overall objective in conducting these surveys is to improve knowledge of the subsurface 

geologic framework in order to understand ground-water systems in populated alluvial basins along 

the Rio Grande. These USGS efforts are conducted in collaboration with other federal, state, and 

local governmental entities where possible. Additional funding was provided by the New Mexico 

 

1

 U.S. Geological Survey, Denver, CO. 

2

Contractor, Raleigh, NC. 

1
Office of the State Engineer, Santa Fe County, and Taos County through a grant to the New 

Mexico Bureau of Geology and Mineral Resources from the Healy Foundation. 

Organization of Data 

The "readme.txt" file provides summaries of the file contents. The folders are organized as 

follows. Files pertaining to this report are contained in the "report" folder; files pertaining to the 

gridded data are contained in the "grids" folder; files pertaining to the flight-line data are contained 

in the "linedata" folder; and files pertaining to the color shaded relief images are contained in the 

"images" folder. In the data folders, ASCII files with the extension “.txt” describe the format and 

contents of the data files. Please read the ".txt" files before using the data files.

Description of Data 

Aeromagnetic data are collected using airborne geophysical sensors that measure subtle 

variations in the Earth’s magnetic field. Aeromagnetic surveys are designed to map the variations 

caused by the irregular distribution of naturally occurring magnetic minerals associated with 

geologic features. High-resolution surveys are flown closer to the ground and with narrower line 

spacing than conventional aeromagnetic surveys to better detect weakly magnetic rocks and 

sediments. Mapping the three-dimensional distribution of these features underground helps 

scientists understand the geologic controls on regional ground-water systems. 

The aeromagnetic surveys (figs. 1 and 2) employed a helicopter flying along traverse lines 

oriented east-west, spaced 200 m (about 650 ft) apart, and flown a nominal 130-150 m (426 - 492 

ft) above ground. Orthogonal lines were flown north-south at a 1,500 m (4,920 ft) spacing. Total 

flight-line coverage was 3,904 km (2,440 mi) for Questa-San Luis and 3,344 km (2,090 mi) for 

Santa Fe east. The east-west orientation of traverse lines was chosen because it is oblique to the 

predominant northerly geologic strike of the area. The flight-line data from the contractor are 

included in this report. Contractor-provided details of the flight specifications, survey procedures, 

and data processing are included in Appendix A. 

Topographic and radar-altimeter gridded data are also provided. The topographic grid was 

created from 1-arc-second (30 m) digital elevation data that were resampled to 50 m. The radar 

altimeter data were smoothed using a running average before gridding to minimize irregularities 

caused by variations in vegetative cover or sensor orientation. 

Data Processing 

Initial data processing and data reduction were accomplished by the contractor (Appendix 

A). The additional data and grid processing described in this section was done to display 

maximum detail while removing unwanted noise and simulating a flying surface draped at a 

consistent level over terrain. 

A linear interpolation was made to the contractor's radar altimeter channel to generate a 

radar value at each magnetometer measurement. Radar values at the ends of flight lines, 

commonly erroneous, were eliminated (producing channel 'raltl_trim') and a running-average filter 

was applied to eliminate high-frequency noise in the radar values (producing channel 'radar_sm'), 

The Questa-San Luis magnetic data required extra processing prior to gridding. Some flight 

lines displayed undetermined instrument noise of 1 to 3 nT amplitude, with wavelengths spanning 

about 80 sample points. These effects were minimized by applying a one-dimensional Fast Fourier 

transform (FFT) low-pass filter to 62 affected lines. 

2
The magnetic and smoothed radar data were interpolated onto a grid at 50 m intervals. Map 

projection is Universal Transverse Mercator (UTM), zone 13 (central meridian of 105o W 

longitude, a false easting of 500,000 m, a false northing of 0 m), and North American Datum of 

1927 (NAD27). 

Low-amplitude data mismatches between flight lines commonly remain after gridding. This 

type of noise appears as east-west linear "corrugations" in the grid. To minimize this noise without 

removing significant geologic effects, the data were microlevelled following the procedure 

implemented in OASIS MontajTM. A high-pass Butterworth filter was first applied to the gridded 

magnetic data perpendicular to the flight line direction using a wavelength cut-off of four times the 

flight-line spacing. Inspection of the resulting high-pass grid suggested that values above 2 

nanoTeslas (nT) were due to geologic signal rather than noise. After limiting the values removed 

by the high-pass filter to no greater than 2 nT, the values were subtracted from the original flightline data to derive a microlevelled channel in the flight-line database. The microlevelled data were 

then gridded as the final total-field magnetic anomaly grid. 

To enhance details and provide better consistency for interpretation, the final magnetic grid 

was continued to a simulated flight surface of 100 m above the ground, using the draping method of 

Cordell (1985). A digital elevation grid is provided to represent the ground surface. In this 

continuation method, the continued data are extrapolated from a series of parallel continuation 

surfaces that have each been computed using standard FFT techniques. The grid of smoothed 

radar-altimeter values determined the distances to upward or downward continue the data to a 

common terrain clearance of 100 m. 

Supplementary filtering was required to minimize noise that was enhanced during 

continuation. First, additional decorrugation filtering was applied, following the method of 

Urquhart (1988), modified for use with a Blackman window (Oppenheim and Schafer, 1975; 

described in Sweeney and others, 2002). A Blackman window with a filter of 11 grid points was 

convolved with data along the grid rows (the direction of the traverse lines), resulting in both a 

high-pass grid and a low-pass grid. The same Blackman window was then convolved with the data 

along the columns of the low-pass grid (the direction orthogonal to the traverse line) to remove 

flight-line “corrugations”. The high-pass grid was added to this filtered grid to retain the shortwavelength information in the direction of the flight lines. 

Residual high-frequency noise that didn’t necessarily correlate with the flight-line direction 

was removed by applying a low-pass, eighth-order Butterworth filter with a cut-off wavelength of 

120 m to the decorrugated magnetic anomaly grid. This wavelength is slightly greater than the 

minimum sampling (or Nyquist) wavelength of two times the grid interval. 

Finally, a standard reduction-to-pole operator was applied to the grid. This operation, 

resulting in reduced-to-pole (RTP) data, corrects for shifts of the main anomaly from the center of 

the magnetic source that occur at most latitudes owing to the oblique orientation of the measured 

magnetic field with respect to the Earth’s surface (the field is vertical only at the magnetic poles). 

Declination and inclination of the Earth's field were assumed to be 10o

 and 64 o for Questa-San Luis 

and 11o and 63o for Santa Fe east surveys. To apply the reduced-to-pole transformation, one must 

assume that the total magnetizations of most rocks in the study area align parallel or anti-parallel to 

the Earth's main field Based on considerations of rock type and age in the area, this assumption is 

generally valid (Grauch and others, 2004). 

3
Acknowledgments 

We are thankful to the people of southern Colorado and northern New Mexico for their 

tolerance during the low-level flying over their homes and businesses. We also thank the Pueblos 

of Nambe and Tesuque for their cooperation during flying. 

Additional funding was provided by the New Mexico Office of the State Engineer, Santa Fe 

County, and Taos County through a grant to the New Mexico Bureau of Geology and Mineral 

Resources from the Healy Foundation. 

References Cited 

Bankey, Viki, Grauch, V.J.S., and Fugro Airborne Surveys Corp., 2004a, Digital aeromagnetic data 

and derivative products from a helicopter survey over the town of Taos and surrounding areas, 

Taos County, New Mexico: U.S. Geological Survey Open-File Report 2004-1229A, CD-ROM. 

Bankey, Viki, Grauch, V.J.S., and Fugro Airborne Surveys Corp., 2004b, Digital aeromagnetic data 

and derivative products from the Blanca airborne survey, covering areas in Costilla and Alamosa 

County, Colorado: U. S. Geological Survey Open-file Report 2004-1229B, CD-ROM. 

Bankey, Viki, Grauch, V.J.S., Webbers, Ank, and PRJ, Inc., 2005, Digital data and derivative 

products from a high-resolution aeromagnetic survey of the central San Luis basin, covering parts 

of Alamosa, Conejos, Costilla, and Rio Grande Counties, Colorado, and Taos County, New 

Mexico: U. S. Geological Survey Open-file Report 2005-1200, CD-ROM. 

Cordell, L., 1985, Techniques, applications, and problems of analytical continuation of New 

Mexico aeromagnetic data between arbitrary surfaces of very high relief, Proceedings of the 

International Meeting on Potential Fields in Rugged Topography, Institute of Geophysics, 

University of Lausanne, Switzerland, Bulletin no. 7, p. 96-101. 

Grauch, V.J.S., Bauer, P.W., and Kelson, K.I., 2004, Preliminary interpretation of high-resolution 

aeromagnetic data collected near Taos, New Mexico: New Mexico Geological Society 

Guidebook 55, p. 244-256. 

Oppenheim, A.V., and Schafer, R.W., 1975, Digital signal processing, 1st edition: Prentice-Hall, 

Inc., 585 p. 

Sweeney, R. E., Grauch, V. J. S., and Phillips, J. D., 2002, Merged digital aeromagnetic data for the 

Albuquerque and southern Española Basins, New Mexico: U. S. Geological Survey Open-file 

Report 02-205, 15 p., http://pubs.usgs.gov/of/2002/ofr-02-0205/. 

U.S. Geological Survey, Sander Geophysics, Ltd., and Geoterrex-Dighem, 1999, Digital 

aeromagnetic data from the Sandoval-Santa Fe, Belen, and Cochiti airborne surveys: U. S. 

Geological Survey Open-file Report 99-0404, CD-ROM. 

Urquhart, T., 1988, Decorrugation of enhanced magnetic field maps: Expanded Abstracts and 

Biographies, Society of Exploration Geophysicists 58th Annual Meeting, Anaheim, CA, 1988, p. 

371-372. 

4
2005 

QuestaSan Luis 

helicopter 

survey 

0 10 km

37o 30' 

37o 0' 

38o 0' 

36o 30' 

36o 45' 

36o 15' 

37o 15' 

37o 45' 

38o 15' 

106o 0' 105o 30' 

106o 0' 105o 30' 

2004 fixed-wing 

survey area 

(Bankey and 

others, 2005) 

2003 

 Blanca survey 

(Bankey and 

others, 2004b) 

2003 

Taos survey 

(Bankey and 

Figure 1. Location of aeromagnetic surveys others, 2004a) 

 ( Questa-San Luis = red, this report; 

 Central San Luis, Blanca, and Taos = gray) 

Taos

Alamosa

Great Sand 

Dunes 

National

Park

Blanca

San

Luis

SAN LUIS BASIN

NEW MEXICO

COLORADO

TAOS

PLATEAU

VOLCANIC

FIELD

Del Norte

Questa

Rio Grande R. 
Santo Domingo

Cochiti

Tesuque

Nambe

San Ildefonso

Santa Clara

San Juan

Pojoaque

0 2 4 6 8 10 

Kilometers 

0 2.5 5 

Miles 

I-25 

I-25 

US 84 

NM 

US 84 

NM 68 

502 

US 285 

E l d o r a d o 

Pojoaque

Espanola

Santa Fe River 

Santa 

Fe

Rio 

Grande 

La 

Cienega

Figure 2. Location of Santa Fe East 

aeromagnetic survey 

HELICOPTER 

SURVEY - 

SANTA FE EAST 

2005 

S. Española Basin 

Survey (USGS and 

others, 1999)