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Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the Los Alamos National Security, LLCfor the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. By acceptanceof this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce thepublished form of this contribution, or to allow others to do so, for U.S. Government purposes. Los Alamos National Laboratory requeststhat the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. Los Alamos NationalLaboratory strongly supports academic freedom and a researcher’s right to publish; as an institution, however, the Laboratory does notendorse the viewpoint of a publication or guarantee its technical correctness.
Title:
Author(s):
Intended for:
05-6863
Technological Advancements: Seismic Refraction on thePajarito Plateau, Northern New Mexico
Jennifer E. Nisengard, John F. Ferguson, Emily Hinz,John S. Isaacson, Rory Gauthier
71st Annual Society for American Archaeology Meetings inSan Juan, Puerto Rico
Nisengard et al. 2006 LA-UR-05-6863
Technological Advancements: Seismic Refraction on the Pajarito Plateau, Northern New Mexico
LA-UR-05-6863
by
Jennifer E. Nisengard (Los Alamos National Laboratory), John F. Ferguson (University of Texas at Dallas), Emily Hinz (University of Texas at Dallas), John S. Isaacson (Los Alamos National Laboratory), and Rory Gauthier (Bandelier National Monument)
Paper presented at the 71st Annual Society for American Archaeology Meetings in San Juan, Puerto Rico. Invited paper in the symposium The Role of Archaeogeophysical, Archaeogeochemical, and Airbourne Remote Sensing in Southwestern US Archaeology.Sponsored by Carothers Environmental, LLC.
ABSTRACT
Geophysical techniques can be used for non-invasive surveys at archaeological sites. Seismic refraction is one such technology that has many potential applications, although it has been under-utilized. It is an inexpensive, efficient way to characterize subsurface deposits, especially at sites in shallow contexts over bedrock. Archaeologists and geophysicists participating in the Summer of Applied Geophysics Experience (SAGE), from Los Alamos National Laboratory (LANL), and Bandelier National Monument are working together to characterize Ancestral Pueblo (A.D. 1200 to 1600) sites. We present the results from three seismic refraction surveys and provide an overview of how seismic refraction works.
Archaeology and Federal Law
Federal laws require archaeologists to identify, evaluate, and protect
archaeological and historic properties eligible for the National Register of Historic Places
(NHRP). The National Historic Preservation Act (NHPA) of 1966 requires that federal
agencies take into account the effect of their actions on sites that are eligible for inclusion
on the National Register of Historic Places (Register). Until a determination is made, all
sites are considered eligible for the Register and require active monitoring and protection.
Ironically, only a fraction of the potentially eligible sites will actually meet Register
criteria once they are evaluated and tested. Therefore, these determinations are a key
process in identifying the number of archaeological sites that warrant NHPA protection.
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Nisengard et al. 2006 LA-UR-05-6863
Register eligibility is traditionally determined using a combination of test
excavations and systematic shovel probes to determine site boundaries, the depth of
cultural deposits, and site integrity (Isaacson et al. 1999). Test excavation and artifact
analysis are the principle methods used to determine a site's inclusion on the Register
under Criterion D. In the case of sites with buried features, the ability to accurately
identify these features is limited by the small sample size of most testing methods. In
addition, subsurface testing is time consuming, labor intensive and expensive. Finally,
testing methods miss many buried features that can lead to inadvertent discoveries during
construction projects causing long delays and additional unanticipated costs. The Pajarito
Plateau, upon which LANL and Bandelier National Monument are situated, contains
unusually high archaeological site densities, with a ratio of approximately 1-site/10 acres
(Figure 1), making the problem of adequate site characterization more acute. Thus,
completing eligibility determinations reliably and cost effectively is of crucial importance
in determining land use options in the area.
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Nisengard et al. 2006 LA-UR-05-6863
Figure 1. Distribution of Cultural Properties at Los Alamos National Laboratory.
The Native American Graves Protection and Repatriation Act (NAGPRA) also
affects the treatment of archaeological sites. Passed in 1990 and revised in 1992,
NAGPRA has resulted in a partial excavation moratorium, in response to concerns raised
by Native American tribal organizations. Consultation with culturally affiliated Native
American tribes is required under the 1992 and 2000 amendments to the NHPA. This
combined with the guidelines set forth in the NHPA, has brought about the need for
alternative methods of site characterization that do not include extensive excavations. In
an effort to limit costs, reduce impacts to archaeological sites and to decrease compliance
time, archaeologists are applying of a variety of non-invasive, geophysical techniques,
like those discussed above and in subsequent sections, which can aid in site eligibility
testing and guide those excavations that do take place.
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Nisengard et al. 2006 LA-UR-05-6863
Archaeology and Geophysics
Geophysical techniques have an inherent advantage over sampling approaches to
eligibility testing in that they can provide synoptic views of sites and their contents
(Isaacson et al. 1999). Several geophysical techniques have been used to assess
archaeological sites (Isaacson et al. 1999; Witten et al. 1993), including resistivity and
magnetometer surveys (Bevan 1983; Carr 1982), ground penetrating radar (GPR) surveys
(Conyers and Goodman 1997; Scollar et al.1990), and thermal infrared remote sensors
(Behrens and Sever 1991). The potential for non-invasive geophysical techniques in site
assessment is promising, as they allow researchers to survey at depth large areas at low
cost. Yet this potential is under-utilized in the United States, as many North American
prehistoric sites are characterized by ephemeral campsites and features that are hard to
detect using geophysical techniques (Isaacson et al. 1999:228).
Geophysical techniques vary in their effectiveness based on the nature of the
archaeological deposits and the local geological conditions (Isaacson et al. 1999).
Therefore the decision to use one method over another should be based on the particular
strengths of a given technique for the particular conditions under investigation. Also the
results of geophysical surveys can vary based on the way the preferred technique is
applied in the field. For example, thermal remote sensing must be tuned to the seasonal,
regional, and diurnal variations in thermal optimal conditions. GPR reflections, which
are sensitive to the dielectric properties of soils, are also susceptible to interference
caused by the presence of metal objects, telephone poles, and even trees (Conyers 1995;
Conyers and Goodman 1997; Nisengard et al. 2004). When these and other
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Nisengard et al. 2006 LA-UR-05-6863
environmental factors are taken into consideration, it is possible to produce a high-
resolution map of the subsurface.
Several geophysical techniques have been used to assess archaeological sites
(e.g., Camerynck 1994; Conyers 1995; Desvignes and Tabbagh 1995; Fredrick and
Abbott 1992; Isaacson et al. 1999; Witten et al. 1993). However, the most often used
methods are ground penetrating radar (GPR) (Scollar et al.1990; Conyers 1995; Conyers
and Goodman 1997), electromagnetic (Bevan 1983; Camerlynck et al. 1994), resistivity
(Carr 1982), and magnetic surveys (Fredrick and Abbot 1992; Desvignes and Tabbagh
1995).
Geophysical methods are designed to detect variations in fundamental material
properties of the subsurface. For example, seismic methods take advantage of the fact
that different types of soils and rocks have different inherent velocities and densities that
can be distinguished from each other. Yet even in the best of conditions, geophysical
techniques have limitations and the methodologies associated with each must be
understood (Isaacson 1999). Once this understanding of the methodology is acquired, the
appropriate method(s) for the suspected target can be determined.
On the Pajarito Plateau, the majority of sites contain architectural features such as
masonry and adobe walls, prepared plaster floors, collard hearths, kivas cut into bedrock,
and grid gardens that make geophysical surveys promising (Nisengard et al. 2004).
Buried pueblo walls are often constructed from tuff blocks derived from the nearby
canyon walls (Hoagland 2000; Nisengard et al. 2004; Steen 1977, 1982; Vierra and
Hoagland 2000; Vierra et al. 2002). These canyon walls also provide the weathered
material that make up the surface soils. Since these tuff blocks and surrounding soils
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Nisengard et al. 2006 LA-UR-05-6863
have the same fundamental mineralogy, geophysical methods could have difficulty
distinguishing them since these methods observe changes in fundamental physical
properties (e.g. density, percent of water content, mineralogy, percent of iron content,
porosity, etc.). For example, wall blocks do not contain any significant amount of ferrous
(iron) minerals, so detecting them with a magnetic survey is unlikely to be successful.
However, there is a density difference between the wall blocks and the soils. Density
differences result in variations in seismic velocity that may be detected. Further, due to
density differences, the wall blocks will have a specific inherent ability to conduct
electric current than the surrounding loose soils (usually due to porosity variation) so
GPR and EM could be used to distinguish the wall blocks from the weathered soils
(Nisengard et al. 2004). Finally, if a structure is dug into bedrock with a high soil-rock
velocity contrast, a seismic refraction survey can map the shape of the interface. Targets
can be outside the sensitivity range of the instrument and even the method used due to
their size, lack of contrast in the geophyscially detectable material properties with
surrounding materials, or a combination of both. Further, poor site understanding and the
possible targets therein relative to detectable geophysical properties often result in the
application of less effective geophysical methods. A greater understanding of a site’s
soils, target composition and size, and expected site architecture can lead to the
application of more effective geophysical methods. In the case of the Pajarito Plateau,
the majority of site locations are located within dry volcanic/sandy soils native to the
upper Rio Grande Valley; such conditions are in fact favorable in theory for several
geophysical survey methods.
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Nisengard et al. 2006 LA-UR-05-6863
Seismic Refraction
The seismic refraction technique (Reynolds 1997; Telford et al. 1990) is a
geophysical method widely used to explore subsurface structures delineated by
differences in seismic wave velocity, which are dependent upon the elastic properties of
the medium through which they propagate (i.e. density, mineralogy, porosity, etc.). A
common use of the shallow refraction method is determining depth to bedrock.
A seismic refraction survey consists of a controlled energy source (as opposed to
an earthquake, which is a natural source) positioned on the surface that is used to
generate and impart seismic energy into the ground. This seismic energy is detected by a
number of geophones, coupled to the ground, that are spaced equally along a profile line.
A geophone or seismometer is a transducer which converts ground motion (usually up
and down) into an electrical signal. Each geophone is connected to a cable that transmits
the signal to a seismograph that digitally records the vibration. To collect the refraction
data needed for analysis, the source is fired at regular intervals (known as shot points)
along the profile line. A seismic record produced at one shot point location is referred to
as a "shot gather" and consists of a seismogram for each geophone.
A shot gather is displayed with geophone location distance along the x-axis and
time along the y-axis. For seismic refraction surveys, the most important information is
the arrival time of the first P-wave (compressional wave) on each seismogram. The first
arrival must be distinguished from any background seismic noise which might be due to
wind or vehicle traffic. The arrival time is “picked” for each geophone location (i.e. each
seismogram) in the shot gather. After picking the first P-wave arrival for all the shot
gathers, the time picks are plotted as a time-distance diagram or traveltime curve to
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Nisengard et al. 2006 LA-UR-05-6863
determine the subsurface velocity information. The slope of the traveltime curve is
determined by the inherent P-wave velocity of the subsurface material and the
topography of interfaces which separate materials with differing velocities. A two-
dimensional layered model of the entire profile is built by determining the depths to
subsurface interfaces where velocities change. Theoretical traveltimes can be evaluated
and matched to the observed traveltimes. Specifically for the Pajarito Plateau area, the
subsurface velocity models identify soil layers with velocities in the range of 110 m/s to
400 m/s and tuff bedrock at about 800 m/s. Note that this process involves skilled
interpretation of both the seismograms and the traveltime curves in the production of the
models.
Geophysical Characterization and Archaeology on the Pajarito Plateau
The Culture History of the Pajarito Plateau
Los Alamos National Laboratory manages more than 2000 heritage resources;
these sites date from 9000 BC, during a period referred to as the Paleoindian period, to
the 1940’s and 1950’s, and are associated with the Manhattan Project and the Cold War
(Table 1). Sites at LANL include large lithic and/or ceramic scatters, rock shelters,
pueblo roomblocks, cavates, and Manhattan Project buildings (Vierra et al. 2002). Prior
to the arrival of Europeans, the Pajarito Plateau was home to the Ancestral Pueblo people
whose descendants still live in the area today.
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Nisengard et al. 2006 LA-UR-05-6863
Table 1. Temporal Sequence Associated with Sites on the Pajarito Plateau.
Cultural Affiliation Period DatesClovis 9500-9000 BCFolsom 9000-8000 BCPaleoindianLate Paleoindian 8000-5500 BC Jay 5500-4800 BCBajada 4800-3200 BCSan Jose 3200-1800 BC Armijo 1800-800 BCEn Medio 800 BC- AD 400
Archaic
Trujillo AD 400-600Early Developmental AD 600-900 Late Developmental AD 900-1150 Coalition AD 1150-1325
Ancestral Pueblo
Classic AD 1325-1600Spanish Colonial AD 1600-1821 Mexican AD 1821-1846U.S. Territorial AD 1846-1912 Statehood to World War II AD 1912-1945
Native American, Hispanic, and Euro-American
Recent AD 1945-present
Ancestral Pueblo peoples built a large number of multi-room masonry and adobe
pueblos, 1-3 room structures, and cavates between A.D. 1200 and 1325, a time referred to
as the Coalition period (Cordell 1997; Stuart and Gauthier 1981; Vierra and Hoagland
2002). The Pajarito Plateau witnessed population increase, population aggregation, and
agricultural intensification during this period (Cordell 1979, 1997; Steen 1977, 1982;
Stuart and Gauthier 1981; Vierra and Hoagland 2000; Wendorf and Reed 1955).
Seismic Refraction on the Pajarito Plateau: Case Studies
LA 3841 or LA 102996: The Bandelier Site
LA 3841 is one of many Coalition Period sites located on Frijoles Mesa. Edgar
Lee Hewett originally commented on the high number of sites in this area, “One may ride
for some miles along this mesa and never be out of sight of ruins of this class” (Hewett
1953:41). The site was originally recorded by Charles H. Lange in 1959 during his
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Nisengard et al. 2006 LA-UR-05-6863
horseback survey of the southern Pajarito Plateau. The site was again recorded in 1993
by archeologists from Bandelier National Monument and assigned a new site number: LA
102996. In 2003, students and professors from the Summer of Applied Geophysical
Experience (SAGE) and members of LANL’s Cultural Resources Management Team
applied a variety of geophysical techniques to characterize the site. Finally, in 2005,
archaeologists from Bandelier National Monument prepared a detailed map of the site.
The site consists of an L-shaped roomblock containing an estimated 25 rooms.
The long axis of the building is 40 meters long and 5 meters wide and is oriented north-
south. The arm of the “L” shaped roomblock is on the north side and extends 15 meters
to the east. The layout of the roomblocks roughly defines a plaza area within the arms of
the “L.” Located immediately to the east, within the projected plaza area, is a shallow
depression, approximately 10 meters in diameter, which is suspected to contain one kiva.
Two, small, rubble mounds are located east of the roomblock.
As many other Coalition Period roomblocks located within Bandelier National
Monument, LA 3841 appears to have been “robbed” of building stone. Upon a site’s
abandonment, community members frequently removed center posts and other materials
that could be used to build subsequent residences. After sites of this class are abandoned,
they collapse into rubble mounds. The building stone on the surface of the mound is then
removed and these are used to construct farming terraces on or near the original pueblo.
These stones are also used to construct new habitation sites in other locations. Robbing,
re-use, or recycling of building stone is a tradition in the Bandelier/Pajarito Plateau area
of northern New Mexico.
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Nisengard et al. 2006 LA-UR-05-6863
Despite the post-occupation removal of building stone, several alignments of
stone are still visible on the surface of the LA 3841 roomblock, which indicates the
location of the original walls. We project walls from these alignments to estimate the
total number of rooms. Like many other excavated Coalition Period pueblos in the area,
it appears that the easternmost tier of rooms is larger than the western tier of rooms. The
easternmost tier of rooms is usually identified as habitation rooms while the western tier
of rooms are usually defined as storage rooms (Powers and Orcutt 1999).
The refraction data for the LA 3841, the Bandelier site, was modeled using three
layers. The 275 m/s layer represents a thin layer of loose unconsolidated sediment, the
375 m/s layer represents a more compact but still unconsolidated section of sediment, and
the 800 m/s layer represents weathered Bandelier Tuff.
The traveltimes from shot points 3, 8, 75, 77, and 79 in lines 1 and 7 show a
significant time delay when compared to other traveltimes collected on the same line.
This slowness suggests an area of excavation at the 25 m mark on the lines. The change
in the slopes of these curves is not as abrupt as they might be if there was a sudden lateral
change in velocity, such as that produced by a vertical wall. The change between the
excavation and the surrounding 800 m/s geology was softened by sloping the contact
with the introduction of walls and debris and by not extending the walls above the 800
m/s layer. Previous modeling of the magnetic data suggested that the kiva walls
collapsed into the kiva, and this supported the proposed seismic model (Figure 2).
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Nisengard et al. 2006 LA-UR-05-6863
Figure 2. Model Constructed from LA 3841 Seismic Refraction Line 1.
Line 2’s first arrivals show three velocities, and the resulting model has these
layers parallel the surface topography. The one exception to this trend is at 22 m on the
line where the traveltimes are delayed in shots 20 to 24. The modeled excavation into the
800 m/s layer could either be geologic and related to erosion or to archaeological
excavations or looting activities (Figure 3). The traveltimes do not show an abrupt
change in the slopes as in the case of the structure identified in lines 1, 4, and 7 and
suggests that there are no vertical or near vertical walls associated with the excavation.
Figure 3. Model Constructed from LA 3841 Seismic Refraction Line 2.
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Nisengard et al. 2006 LA-UR-05-6863
There are two major areas of anomalous deviations in the traveltimes from the
three-layer model created from lines 3 and 6 in line 4’s data. The slowness around 27 m
is modeled as part of the same structure seen in lines 1 and 7. Line 4 intersects lines 1
and 7 at the structure (Figure 4). The second structure is centered at 11 m and is located
underneath the topographic high/roomblock area on the line. Unlike the anomalies in the
other lines that were associated with an anomalous slowness in the traveltimes, the
traveltime picks covering this area show an anomalous area of higher velocities. The
picks also show sudden jumps in the arrival times at the same position in multiple shots
(for instance, shots 42, 48, and 50 at ~ 16 m) that indicate sudden lateral changes in
velocity associated with vertically or near vertically sided structures.
Figure 4. Model Constructed from LA 3841 Seismic Refraction Line 4.
While the feature depicted in Figure 4 was modeled as a collapsed three-walled
structure, the actual shape and dimensions remain ambiguous and unresolved. It should
be noted that no other lines that crossed roomblock areas showed any strong indication of
anomalous subsurface structures. In particular, line 5, which intersects line 4 at 13 m,
does not appear to show the same velocity changes in this region (Figure 5). This may be
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Nisengard et al. 2006 LA-UR-05-6863
due to the first arrival picks for line 5 from noisy data (the most noisy data from the entire
survey), or from the possibility that the structure has a low point in the northing direction
at 13 m on line 4. If the feature is the remains of a structure, then line 5 could be
intersecting line 4 at a floor area. The GPR data also indicate the presence of rectilinear
structures in this area.
Figure 5. Model Constructed from LA 3841 Seismic Refraction Line 5.
LA 135290: The Los Alamos (LA) Airport Site
At the LA Airport site, LA 135290, the refraction data, acquired during SAGE
2002, was modeled using three layers. The 150 m/s layer represents a thin layer of loose
unconsolidated sediment, the 450 m/s layer represents a more compact but still
unconsolidated section of sediment, and the 930 m/s layer represents the Bandelier Tuff.
The area between 30 and 45 m on line 1, displays traveltimes that deviate from
the expected three-layer model seen in the 0 – 24 m region (Figure 6). The curves that
cross the 30 – 45 m region show two separate slowness events where the slopes of the
picks increase. This has been modeled as two excavations into the 930 m/s layer. The
excavations are thought to be Holocene stream channels cutting into the higher velocity
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Nisengard et al. 2006 LA-UR-05-6863
layer. This hypothesis was confirmed during trench excavations and analyses conducted
by LANL archaeologists and geomorphologists in 2004.
Figure 6. Model constructed from LA 135290 Seismic Refraction Line 1.
Line 2 has also been modeled with an excavation into the 930 m/s layer and is
also presumed to be the stream channel (Figure 7). In the model, the 930 m/s layer
appears to be terraced with the channel cutting into the thicker deposit from 17 – 24 m.
The shot points only extend to 24 m but the model was extended to 40 m to preserve the
proportionality that was used in line 1. The model was simply projected from 24 m to 40
m and the relationships between the layers in this range are not supported by any
refraction data. Unlike line 1, the travel-times in lines 2 and 3 have abrupt jumps in their
tails at their latest times that are not necessarily seen elsewhere in the picks (Figures 7
and 8). This is characteristic of changes in the velocity at very shallow depths. Some of
these jumps were reproduced by modeling rectangles of higher velocity features located
at within a meter of the surface. These rectangles represent cross-sectional blocks of
Bandelier Tuff and are in a similar position to the walls of the structures that were
mapped at the site. The resulting synthetic travel-times reproduce some of the roughness
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Nisengard et al. 2006 LA-UR-05-6863
seen in the later times of the observed travel-times. Small adjustments in the position
(0.25 m shifts) and dimensions of the blocks greatly affect the appearance and fit of the
synthetic travel-time curves. Further fine adjustment of the blocks could help to better fit
the synthetic to the observed travel-times.
Figure 7. Model constructed from LA 135290 Seismic Refraction Line 2.
Figure 8. Model constructed from LA 135290 Seismic Refraction Line 3.
LA 35001: The Pueblo Canyon Site
LA 35001 was first identified by Charles Steen during the 1970’s and has been
revisited by archaeologists several times over the past 30 years. The site is located on a
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Nisengard et al. 2006 LA-UR-05-6863
small ridge along the south side of Pueblo Canyon in Los Alamos in a piñon/juniper
woodland. LA 35001 is a circular to oval shaped pueblo with a partially enclosed plaza,
and within the 10 by 10 meter plaza is a circular depression, which has been interpreted
as a kiva (Hoagland et al. 2000; LANL 1999).
The 25 by 30 meter roomblock mound, consisting of shaped and unshaped tuff
blocks, is between 1 and 2 meters high (Hoagland et al. 2000). The plaza is partially
enclosed and entrance to the area is from the east and/or west sides. There is a
rectangular alignment of rocks extending out from the south side of the pueblo, which
may represent the remains of a small water control device. Three small shallow
depressions within the roomblock indicate that this site has been subject to looting. No
archaeological testing has been conducted to date at the site.
In July 2001, archaeologists and geophysicists visited LA 35001 and subjected
one of the roomblocks and the plaza to ground penetrating radar, seismic refraction, and
electromagnetic surveys. Seismic refraction data at LA 35001 is comprised of 5 lines:
two 50 m lines trending generally north to south, and three 25 m lines trending generally
east/west. Each seismic profile was acquired with 48 vertical component (recording only
vertical ground motion) geophones, spaced at 0.5 m intervals. Shots were deployed at 5
m intervals along the cable, including the two end points, for a total of 32 shot points.
The three velocity layers for the Pueblo Canyon site, LA 35001, are layers of
sediment with increasing velocity. The alluvial terrace geology of this site is somewhat
different from the weathered tuff at the other two sites. The 150 m/s layer is the loose
unconsolidated sediment, the 350 m/s layer is the more compacted sediment layer, and
the 800 m/s layer is probably an older sediment rather than Bandelier tuff. Line 1’s data
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Nisengard et al. 2006 LA-UR-05-6863
depicts two excavation areas (Figure 9). The excavation centered at 13-14 m is narrower
and shallower than the larger excavation centered at 35 m. The smaller excavation
coincides with a known looter’s pit. The larger excavation appears in both lines 1 and 3,
and the excavation position and dimension mirrors the topographic depression (Figure 9).
The size, shape, and depth of the larger excavation suggest that it is an Ancestral Pueblo
kiva.
Figure 9. Model constructed from LA 35001 Seismic Refraction Line 1.
An excavation in the higher velocity sediment is also seen in line 4. The position
of line 4 precludes the excavation from being related to the large circular feature seen in
lines 1 and 3 (Figure 10). However, the feature does line up with the small excavation
modeled in line 1 and could be another cross section of the stream channel. The presence
of a cluster of juniper bushes in the area of these smaller excavations provide support for
the theory that they represent a stream channel.
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Nisengard et al. 2006 LA-UR-05-6863
Figure 10. Fence Diagram of the LA 35001 Seismic Lines with Subterranean Features Visible.
Geophysics and Archaeology: A Eye to Future Research
Like many federal and state agencies, LANL is required to monitor and protect all
Register eligible archaeological sites and those site whose Register eligibility has not yet
been determined. Some of these sites may not meet the significance standards of the
NHPA, but they remain management roles because there is no cost effective or non-
destructive way to evaluate their significance (Isaacson et al. 1999). The large number of
sites that await evaluations and must be protected severely constrains future land use
options.
Geophysical techniques offer efficient, non-invasive, economical, and effective
ways to examine the presence of large-scale subsurface archaeological features at a site,
making it easier to make Register eligibility determinations. This is of particular
importance at pueblo roomblocks or plaza pueblos where locating possible subsurface
communal structures can be vital to developing an excavation strategy (Nisengard et al
2004). It is also helpful at homestead sites where eligibility may be decided on the basis
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Nisengard et al. 2006 LA-UR-05-6863
of the detection of a privy at a site. Yet, practical application has found that some
geophysical applications do not perform well enough to be useful (Nisengard et al. 2004).
Seismic refraction surveys, like those discussed in this paper, have produced the most
consistent, effective, and promising results of all geophysical techniques applied at
archaeological sites on the Pajarito Plateau. These surveys are also economical and most
sites can be characterized in less than one day. Seismic refraction on the Plateau has
been most successful at identifying structures excavated into the Bandelier tuff.
Geophysicists and archaeologists continue to work together to understand subsurface
deposits associated with Ancestral Pueblo sites and the local geology to fine-tune their
techniques.
Acknowledgements. The authors wish to thank the SAGE students and faculty for their hard work and dedication to the goals of the research presented here. We thank Rich Lundin and Carothers Environmental, LLC for inviting us to contribute a paper to their symposium. We also thank Los Alamos National Laboratory, Bandelier National Monument, and the University of Texas at Dallas for allowing the authors to participate in research studies and the 71st annual Society for American Archaeology meetings. Hector Hinojosa provided editorial comments on the abstract and expedited the review process.
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