ANTH 3023: Lecture Notes: Geophysical Survey

Professor Kenneth L. Kvamme

Lecture Dates: W, Feb 4 & F, Feb 6

Contents:

  1. Geophysical Survey: Instrumentation
  2. Geophysical Survey Methods
  3. Geophysics & Other Remote Sensing Methods Locate Anomalies
  4. Magnetometry: Nature’s Gift to Archaeology
  5. Case Study: Double Ditch, ND 1
  6. Case Study: Double Ditch, ND 2
  7. Magnetometry: 7 Cultural Causes
  8. Magnetometry: People create fires
  9. Constructions Accumulate Topsoil
  10. Constructions Remove Topsoil
  11. People Use Iron Artifacts
  12. Electrical Resistance Survey
  13. Multiple-depth Resistivity
  14. Multi-depth Resistivity Results
  15. Ground Penetrating Radar (GPR)
  16. GPR: Time-Slicing 1
  17. GPR: Time-Slicing 2
  18. Principle 1: Use Multiple Methods
  19. Principle 2: Get the Big Picture!
  20. Principle 3: Cultural Features Exhibit “Patterned Geometries”
  21. Anomalies of Uncertain Source Should be Identified by Excavation
  22. Geophysics: The New Landscape Archaeology
  23. Case Study 1: Army City, Kansas (1917-21)
  24. Case Study 2: Data Fusion Through Computer Graphics

1. Geophysical Survey: Instrumentation

  • Magnetometry (passive)
  • Electrical Resistivity (active)
  • EM Conductivity (active)
  • GPR (active)

2. Geophysical Survey Methods

  • Survey is performed in grids measuring 10-30 m square
  • The number of measurements per square meter controls spatial resolution & detail—imagery pixel size
  • Marked tapes are used to guide the survey on the ground
  • Types of survey
    • Zigzag
    • Parallel

3. Geophysics & Other Remote Sensing Methods Locate Anomalies

  • ANOMALY:
    • 1. Measurements that are “different” from normal background measurements
    • 2. A geophysical “feature” that stands out
      • Anomalies may be cultural—caused by people—which are the targets of remote sensing
      • Anomalies can be natural—formed by environmental processes—such as coyote dens, tree throws, erosion

4. Magnetometry: Nature's Gift to Archaeology

  • Passive Method
  • Reveals features unseen
  • An astonishing number of natural & cultural processes affect soil magnetism, making magnetometry an ideal prospecting method

5. Case Study: Double Ditch, ND 1

  • Double Ditch is a Plains earthlodge village, apparently fortified with 2 fortification syustems seen on the surface. Occupied AD 1400s - 1780
  • Magnetometry conducted from 1997-2003

6. Case Study: Double Ditch, ND 2

  • Magnetometry reveals a northern Plains fortified village of unusual size
  • Two additional fortifications located, with bastions
  • Evidence of many houses, circular & rectangular
  • Evidence of borrow pits from which earth was taken for lodge coverings
  • Thousands of storage pits
  • Site to be renamed "Quadruple Ditch"

7. Magnetometry: 7 Cultural Causes

  • Topsoil is naturally more magnetic than subsoil
    • People magnetically enrich topsoil in settlements
    • People create fires (intentional & unintentional)
    • People make fired artifacts (ceramics, bricks)
    • Constructions accumulate topsoil
    • Constructions remove topsoil
    • Constructions import & utilize stone
    • People make & use iron artifacts

8. Magnetometry: People Create Fires (Cultural Causes Of Magnetic Variation)

  • Fires are:
    • intentional
    • accidental
  • Burned houses & hearth examples
  • Fired soil turns orange to red
  • Test Results
    • Excavations compared to
    • Magnetometry &
    • Fired Earth Weight

9. Constructions Accumulate Topsoil (Cultural Causes Of Magnetic Variation)

  • Effigy mound
  • Mounded soil around earthlodge
  • Raised berm next to ditch
  • Topsoil filled ditches
  • Topsoil filled storage pits

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10. Constructions Remove Topsoil (Cultural Causes Of Magnetic Variation)

  • Incisions in ground - truck tracks & trails
  • Ditches
  • Graves
  • View of magnetic profile compared to excavation profile

11. People Use Iron Artifacts (Cultural Causes Of Magnetic Variation)

Prairie Grove Battlefield, AR

  • Battlefield & other iron artifacts are highly magnetic

12. Electrical Resistance Survey

  • Active Method
  • Injects an electrical current into ground & records resistance to that current caused by subsurface deposits & materials
  • Sensitive to soil changes & particularly stone & brick

13. Multiple-depth Resistivity

  • Probe separation controls prospecting depth

14. Multi-depth Resistivity Results

  • Roman building, Wroxeter, England (AD 150-450)
    • At various depths below surface (.25 m .5 m .75 m 1 m 125 m 1.5 m ) different details of a Roman building may be seen

15. Ground Penetrating Radar (GPR)

  • Active Method: Sends microwaves into ground and records their reflections off buried soil changes & features
  • Consists of: control box (computer), power unit & antenna
  • Creates vertical profiles along transects
  • Typical profile: distance (horizontal) x time below surface (vertical)

16. GPR: Time-Slicing 1

First Mount Comfort Church (1840s-1863), Fayetteville, AR

  • 1. Record profiles every ½ meter across a site--these are vertical sections in ground
  • 2. Stack the recorded profiles side-by-side and “slice” through them at various times or “depths” below the surface--creates horizontal plan view maps

17. GPR: Time-Slicing 2

17th century garden at Sylvester Manor, Shelter Island, NY

  • Magnetometry shows: shallow garden features
  • GPR shows: same features, a cart/wagon track, and a buried pipeline that cannot be iron because it is not seen in the magnetometry

18. Geophysical Principle 1: Use Multiple Methods

EXAMPLE: Mount Comfort Church, Fayetteville, AR (1840s-1863)

  • Resistivity
  • Magnetometry
  • GPR

Each gives a different picture and new or complementary information

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19. Geophysical Principle 2: Get the Big Picture!

It is important to survey large, contiguous areas

  • Whistling Elk Village, SD (A.D. 1300)
  • Electrical resistance image, 1 m probe separation (depth), shows entire layout of a fortified settlement with numerous houses

20. Geophysical Principle 3: Cultural Features Exhibit “Patterned Geometries”

  • Linear, right-angle, and rectangular anomalies
    • Cultural, probably a building
  • Indistinct and unpatterned anomalies
    • Possibly cultural (wells, privies, storage pits)
    • Possibly natural (tree throws, badger dens)

21. Anomalies of Uncertain Source Should be Identified by Excavation

  • Excavated vs. magnetic data comparison show many similarities
  • Excavation of unidentified anomaly indicates a corn storage pit

22. Geophysics: The New Landscape Archaeology

  • Fort Clark Trading Post, ND
  • Sketch by William J. Hays, July 14, 1860, shows layout of the trading post & associated structures
  • Magnetometry results from 2001 reveal each feature in the sketch in some detail

23. Case Study 1: Army City, Kansas (1917-21)

A WWI commercial village associated with Camp Funston (now part of Fort Riley)

  • The site today is only a hay field

24. Case Study 1: Data Fusion Through Computer Graphics

  • Multiple sensors were employed: magnetometry, resistance, conductivity, magnetic susceptibility, GPR
  • Each data set reveals much
  • The 5 data sets can be overlaid in a multi-colored composite that simultaneously show the remote sensing results