MAGNETOMETER SURVEYS IN THE GAP HILL SITE (LA 125819)      

IN LUNA COUNTY, NEW MEXICO©

by

Meade F. Kemrer

Report to

Bureau of Land Management

Las Cruces Field Office

Las Cruces, New Mexico

September, 1999

ACKNOWLEDGEMENTS

            I thank the Bureau of Land Management Las Cruces Field Office, particularly Archeologist Tom Holcomb, for the opportunity to perform these magnetometer surveys as a portion of my volunteer agreement. I am also pleased to have the opportunity to help Ann Owen as a component of her Master’s thesis. Finally, I extend a big thank-you to BLM volunteers Robert Dragon, Bud Lunders, Ann Owen, and Martha Sharp who helped me perform the fieldwork.
 

Meade F. Kemrer

Las Cruces, NM

September 20, 1999      

TABLE OF CONTENTS

ACKNOWLEDGEMENTS

TABLE OF CONTENTS

Introduction

Instrumentation and Methods

Blocks 1 and 2 Surveys

Block 3 Survey

Block 4 Survey

Block 5 Survey

Block Survey Findings

Interpretation and Methodological Findings

References Cited

Introduction

 

            During May 25, 27 and 28, 1999, Bureau of Land Management - Las Cruces Field Office volunteers Robert Dragon, Meade Kemrer, Bud Lunders, Ann Owen, and Martha Sharp performed a series of magnetometer surveys within the Gap Hill Site (LA 125819) a Mimbres Classic (A.D. 1000-1150) pueblo in southwestern New Mexico. Pothunting by mechanical equipment reduced much of the site to a series of backdirt mounds and pothole depressions. The magnetic surveys were designed to characterize subsurface site content and condition, particularly architectural elements and intramural features.
 

            Prior to the survey, Ann Owen and Meade Kemrer visited the site on May 19, 1999. The purpose was twofold: to identify areas to be surveyed, particularly places that could contain intact architectural elements, and to examine the geological setting.


 

            Several promising areas were found. One locality contained two visible intact walls, ideal for identifying a magnetic signature useful for locating and mapping buried walls.


 

            The geological environment of the Gap Hill site was conducive for magnetometer surveying. Iron-bearing minerals are evident. The soil is clayey loam containing mostly andesite cobble and boulder lenses derived from the nearby mountains and ridges. These stones were used exclusively for pueblo masonry based on the sitewide examination.


 

            A model of the geomagnetic characteristics of the Gap Hill site used the GEOMAG30 programs (Defense Mapping Agency 1995). Magnetometric surveys locate lateral changes (anomalies) in the magnetic field of a site. Prospection at an archaeological site consists of measuring the magnitude of the earth’s magnetic (geomagnetic) field on a closely spaced grid mapped onto the study area. Archaeological feature detection is dependent on the present of a lateral change in the magnetic field. The vector properties of the earth’s magnetic field are represented in Figure 1.

Figure 1. Earth’s Magnetic Field Properties (Defense Mapping Agency 1995).


 

            The Earth's magnetic field is described by seven parameters. These are declination (D), inclination (I), horizontal intensity (H), vertical intensity (Z), total intensity (F) and the north (X) and east (Y) components of the horizontal intensity. The parameter most frequently requested and most often misunderstood is magnetic declination or variation, D. This is the angle made between the trace of the total magnetic field in the horizontal plane, H, and true north. D is considered positive when the angle measured is east of true north and negative when west. The inclination or dip, I, is the angle between the horizontal plane and the total magnetic field. Inclination, also called magnetic dip, is considered positive when downward pointing. These elements, D, I and H give a full vector representation of the magnetic field, F. Vertical intensity is the trace of the total intensity in the vertical plane and is considered positive when I is positive, that is downward pointing. The east component, Y, is considered positive when pointing east and the north component, X, is positive when pointing towards geographic north (Defense Mapping Agency 1995). The geomagnetic properties of the Gap Hill site are listed in Table 1.

Table 1. Geomagnetic Properties of the Gap Hill Site (LA 125819).

            Given the known geological environment and the geomagnetic characteristics of the site, magnetic anomaly magnitudes expected from the site were modeled. Magnetic anomalies are produced by the interaction between the magnetic susceptibility variation of the local constituent materials and the geomagnetic field. Susceptibility is the ability of a material to affect the local magnetic field. The magnitude of an anomaly produced by a particular material in a local field is computed by:


 

            For the Gap Hill site, the target material, andesite, was used for masonry. It has an estimated magnetic susceptibility value (ko) of 0.0001 cgs (Breiner 1973: 8). The background magnetic susceptibility for the soil (ks) contains some iron minerals derived from decomposed andesite and is estimated at 0.00005 cgs. Magnetic contrast from the soil and the andesite object (ko-ks) is 0.00005 cgs. The local field intensity (F) is 50,000 nT (from Figure 1 and Table 1). The volume of a cobble of andesite (Vo) is arbitrarily set at 1600 cubic cm, a 20 cm diameter sphere. The depth of the andesite cobble below the surface is arbitrarily set as 10 cm ( r 3= 1000 cubic cm). Using the equation, the andesite cobble will create an anomaly (T) of 4 nT. The result is acceptable, for many of the cobbles employed in the pueblo walls are at least four times larger than the cobble used in this example. Thus buried masonry should be magnetic visible for mapping.


 

            Several tasks to assure project success were completed prior to the survey. Solar magnetic storms can curtail a survey. Predicted solar weather reports from NOAA were daily consulted during the survey period to ensure that the magnetometer would operate properly. Finally, the survey area was inspected and removed all visible metal items that would produce strong anomalies that could mask archaeological features.


 

Instrumentation and Methods
 

            The instrument used for these tests is a Geometrics G-856AX proton precession magnetometer with a rated accuracy and sensitivity of 0.1 nT, sufficiently capable to detect buried prehistoric features. Two configurations were used for the various block surveys. The first two block surveys used the instrument as a gradiometer with two sensors separated between 0.8 m by a vertical aluminum staff. The bottom sensor was 40 cm above the ground surface. Gradiometers are designed to eliminate extraneous solar (diurnal) and regional magnetic sources. Another characteristic is their pinpoint accuracy locating buried archaeological features and materials.
 

            The remaining surveys used the instrument as a single-sensor magnetometer to reduce the time required to collect data from two sensors. Sensor height was 40 cm above the ground surface. A base station designed to remove solar fluctuation was established for each survey block. A base station magnetic reading was made before starting each line of data points within the survey block.


 

            Survey methods were consistent and designed to minimize operator/instrument magnetic effects (cf. Bevan 1996: 36; Kemrer 1999: 9). All survey blocks were oriented to magnetic north. Data collection intervals were 0.25 m with transects oriented north-south. All transects were performed unidirectional from south to north within the blocks.


 

            A total of 5 blocks were surveyed. A map (Figure 2) describes the spatial relationships among the survey blocks within the site. Magnetic variation within block surveys is displayed in isopleth maps. The contour nT value intervals are shown in the vertical scale at the right of each map. Distance values along the block X- and Y-axes are in meters.

Figure 2. Map of Magnetic Survey Blocks, Gap Hill Site (LA 125819).


 

Blocks 1 and 2 Surveys
 

            Blocks 1 and 2, combined forming a contiguous rectangle, measures 10 m north-south by 8.75 m east-west (Figure 2). Block 2 represents an addendum to Block 1. Both portions were surveyed with the instrument configured as a gradiometer.


 

            The Blocks 1 and 2 contour map shows the presence of a series of large intense magnetic anomalies (Figure 3). Their sources were identified during survey. One intense anomaly on the southern edge located 5 m east of the X,Y origin was produced by a steel item carried inadvertently by one of the team members. The remaining anomalies were caused by gopher colonies in the survey unit. Note that the steel item produced a strong positive anomaly and the gopher colonies produced intense negative anomalies. It is likely that the negative values are caused by voids produced by gopher colony tunnels and rooms.

             

Figure 3. Gap Hill Site (LA 125819) Gradiometer Survey Blocks 1 and 2.
            Line and Station Intervals = 0.25m.



            The nT range shown in the bar graph (Figure 3) greatly exceeded that expected for the andesite stone masonry. Therefore, the steel and gopher colony anomalies masking the archaeological targets were removed. Figure 4 shows the re-mapped results.


 

            Subsequent to removing the intense anomalies, the magnetic variation dropped into the narrower expected range (Figure 4). Numerous of small anomalies occur throughout the unit. The sizes of these anomalies are consistent with buried individual andesite wall masonry rocks. Most important, many of the anomalies form linear arrays that are likely wall alignments. Virtually all of the larger anomalies are those that are aligned, probably produced by sets of stones vertically stacked or horizontally coursed within a wall.

Figure 4.Gap Hill Site (LA 125819) Blocks 1 and 2 Gradiometer Survey, Steel and
Gopher Colony Anomalies Removed. Station and Line Intervals = 0.25m.



            Figure 5 is an interpreted map of the Blocks 1 and 2 unit. Areas # 5 and #6, bounded by dashed lines in Figure 5, magnetically represent the two known surficial visible walls. These walls produced magnetic responses by the gradiometer noted during the survey. Both walls exhibited double-coursed masonry manifested by pairs of andesite cobbles. Wall #5 is less distinct because the removal of gopher colony anomalies included masonry magnetic values for the three gopher colonies adjacent to the wall (see Figure 3).


 

            Another wall segment, designated as #4 in Figure 5, was buried, but gradiometer responses were clearly noted during the survey. This wall may extend further to the east, but a gopher colony precludes identifying the easternmost end (see Figure 3).


 

            North-south walls #3 and #7 are buried. Both are not straight indicating wall collapse, either by natural or by mechanical pothunting agencies. The numerous of small anomalies between the walls are probably individual rocks produced by cobble wall-fall.  Areas #1 and #2 illustrate a magnetic signature of intensive vandal excavation.
 

            Area #1 corresponds to a clearly visible pothunter depression. This area is relatively magnetically “quiet” with few small probable cobble anomalies. Area #2 lacks a depression, but does exhibit little magnetic variation, a fact that was also noted during the survey. This excavation zone was probably refilled.

Figure 5.Gap Hill Site (LA 125819) Blocks 1and 2 Interpreted Gradiometer Survey.
Station and Line Intervals = 0.25m.

 

Block 3 Survey
 

            Block 3 measures 6 m north-south by 6.5 m east-west (Figure 6). It was selected on the basis of a scatter of andesite cobbles that suggested the remains of pothunted pueblo rooms. This block was surveyed with the instrument configured as a single-sensor magnetometer. The mapped data within this unit is the residual magnetic variation after removal of solar flux from the base station data.


 

            Three large anomalies occur in the northern portion of the unit (Figure 6). The central and the northeastern anomalies are gopher colonies. The northwestern anomaly is a small intense object and likely is a buried steel or iron item.
 

            An analytic procedure identical to that applied to the Blocks 1 and 2 units. The three anomalies were removed from the data and the unit was re-mapped. In this case, no walls or pothunter excavation areas could be identified. If this area was vandalized, no trace of puebloan architecture likely survived.

Figure 6.Gap Hill Site (LA 125819) Magnetometer Survey Block 3.
Line & Station Intervals = 0.25m.

Block 4 Survey


 

            Block 4 measures 4m north-south by 4.5 m east-west (Figure 7). It was believed to be a pothunted pueblo room basis of the configuration of surface andesite cobbles. This block was surveyed by a single-sensor magnetometer. Solar magnetic variation has been removed from the magnetic data prior to mapping.


 

            Two large anomalies occur in the eastern portion of the unit (Figure 7). Both are attributable to a large gopher colony. 


 

            Again, the Blocks 1 - 2 and 3 units analytic procedure was applied to the Block 4 data. The two anomalies were removed from the data and the unit was re-mapped. No walls or pothunter excavation areas could be identified. If this area was vandalized, no trace of puebloan architecture survived.

             

Figure 7.Gap Hill Site (LA 125819) Magnetometer Survey Block 4.
Station and Line Intervals = 0.25m

Block 5 Survey


 

            Block 5 measures 7 m north-south by 5 m east-west (Figures 8). It is located approximately 35 m south-southeast of Block 4 (Figure 2). It is an unmistakable pothunted locality and was selected for study. It contains an excavation depression and adjacent backdirt and cobble piles. This block was surveyed by a single-sensor magnetometer. Solar magnetic variation collected by the base station placed at the southwest 0,0 unit corner was removed from the magnetic data prior to mapping. 


 

            The depressed excavation area located in the west-central portion of the unit (Figure 8) exhibits little magnetic variation, consistent with the findings in the Blocks 1 and 2 study (Figure 5). Anomalies in the northeastern and southeastern portions correspond to backdirt cobble piles. Anomalies along the center of the western block edge are attributable to an adjacent gopher colony. No evidence of prehistoric architecture could be identified within Block 5.

Figure 8.Gap Hill Site (LA 125819) Magnetometer Survey Block 5.
Station and Line Intervals = 0.25m.


 

Block Survey Findings
 

            The Gap Hill study produced important findings. High-resolution magnetometer surveys using 0.25 m stations and line width intervals plus virtually eliminating operator/instrument noise successfully detected buried pueblo mural features. The surveys also identified the magnetic signature of intense pothunter excavations. Both findings have significance for the assessment of pueblo structure condition and data potential content. This study emphasizes the importance of utilizing geophysical methods applied to archaeology as efficient, cost effective alternatives to extensive testing and excavations for site characterization.


 

Interpretation and Methodological Findings
 

            This study builds upon the methodological research performed on a test plot with the cooperation of the Bureau of Land Management Las Cruces Field Office (Kemrer 1999). The BLM-sanctioned Gap Hill Site study produced important interpretation and methodological findings applied to the magnetometry of Classic Mimbres pueblos.


 

            In terms of enhancing interpretation accuracy, the Block 5 survey provided an opportunity to illustrate the relation between surface features and magnetic variation. A sketch map of the Block 5 area (Figure 9) and the magnetic map (Figure 10) share the same scale. The excavation depression area exhibits little magnetic variation, consistent with the findings in the Blocks 1 and 2 study (Figure 5). The three cobble piles produce magnetic anomalies. Feature and magnetic anomaly locations correspond exactly to each other.


 

            The magnetic map also contains subsurface information (Figure 10). The low magnetic variation area extends beyond the southwestern portion of the block, indicating that the pothunter excavation area is significantly larger than specified by the depression. A number of small anomalies, probably produced by buried andesite cobbles, extend between the northern and southern cobble piles. A relatively intense anomaly on the western boundary 3 m north of the 0,0 origin suggest that gopher subterranean diggings extends further to the southeast than the surface colony indicators.

Figure 9. Sketch Map, Block 5.

Figure 10. Magnetic Map, Block 5.

            The Blocks 1 and 2 studies produced important data concerning survey resolution. The question is: What level of resolution is needed to meaningfully characterize and map pueblo subsurface content? Data from the 0.25 m magnetic readings were subtracted to produce precisely accurate simulated surveys performed at 0.5 m and 1.0 m station and line width intervals. Magnetic maps with survey intervals at 0.25 m (Figure 11), 0.5 m (Figure 12) and 1.0 m (Figure 13) are presented below.
 

            The simulated 0.50 m map (Figure 12) shows that wall alignments are present, but some details visible in the 0.25 map (Figure 11) disappear. Wall segments include the short east-west wall in the northwest part of the map and the surface north-south wall on the east side of the map. There are anomalies suggesting that other walls are present, but an analyst could easily discount them as “noise”. The pothunter excavation area in the present northeastern part of the block is visible, but the pothole excavation signature is not discernible.


 

            At the 1.0 m survey level (Figure 13), only one alignment is present. This north-south series of anomalies corresponds to a buried wall in the center of the block (Figure 11). The other four wall segments, including the two surface walls, are not present. The pothunter excavation signatures also disappear.


 

            This analysis indicates that a resolution of 0.25 m interval or higher is necessary to characterize and map subsurface features with sufficient detail to assess pueblo content and condition. Basic magnetometer principles explain the need for this level of fine-grained resolution. The size of the cobble anomalies are small and the instrument must be located nearly directly above the buried rocks to detect then and to distinguish then from them neighbors.

Figure 11.Blocks 1 and 2, Survey Intervals = 0.25 m.

Figure 12.Blocks 1 and 2, Survey Intervals = 0.50 m.

Figure 13.Blocks 1 and 2, Survey Intervals = 1.0 m.

References Cited

Bevan, Bruce

1996 Geophysical Exploration for Archaeology Volume B: Introduction to geophysical exploration. Geosight Technical Report Number 4. Pitman.


 

Breiner, Sheldon

1973 Application for Portable Magnetometers. Sunnyvale.


 

Defense Mapping Agency 

1995 1995 World Magnetic Model. Fairfax.


 

Kemrer, Meade F.

1999 Controlled Near-Surface Magnetometer Studies in Dona Ana County, New Mexico. On File, Bureau of Land Management Las Cruces Field Office. Las Cruces.