GPR Ground Penetrating Radar

• Ground Penetrating Radar (GPR) is a tool archaeologists willuse more and more for excavations in the 21st century. GPR allows archaeologists to survey archaeological sites in a short amount of time with precision and without digging.

Basic Principles

• Uses radio waves to create picture of underground before excavation

• Picture is used to locate any variations in the sub-surfaces

Basic Mechanism

• Electrode magnetic waves emitted into the ground and time measured for wave to be reflected and received

• When wave hits areas of change in soil, it is hit back to receiver antenna

• Changes in soil can include objects buried underneath the surface

• Greater surface difference = Stronger signal

• Strong signal has large amplitude

• Weak signal has small amplitude

• Amplitude wavelength and time are used to create image of what is underground

RDPRelative Dielectric Permitivity

• “RDP is ability of a material to store and allow passage of electromagnetic energy when a field is imposed upon it.”

• RDP of a substance = electrical permitivity / vacuum

CK

V

K = RDP

C = Speed of light

V = Velocity of radar energy

GPR Depth Determination

D = (5.9t)/sqrt of (Er)

D = depth of target (in)

t = wave travel time (nanosec)

5.9 = a constant incorporating speed of light and unit conversions

Er = dielectric constant of subsurface material

Air 1

Pure water 81

Fresh water (ice) 4

Granite (dry) 5

Clay (saturated) 8-12

Sand (dry) 4-6

Sand (saturated) 30

Dielectric constants for common materials

Example

How deep below the surface is an object that is detected at 46 ns in dry sand (Er = 4)?

Answer:

D = (5.9 x 46) / sqrt (4)

D = 135.7 in

Therefore the object is about 11 ft underground

Equipment

• Consists of:

1. control unit

2. display unit

3. transmitting and receiving unit

Case Studies

• Cave of Letters

• Bethesaida, Israel

• Forum Novum

Cave of Letters

• Site located in Nahal, Hever (a canyon by the Dead Sea)

• GPR was used to map possible excavation sites in the Cave of Letters

Artifacts Found

• Coins

• Clothes

• Letters mentioning Jewish leader Bar-Kokhba who had fought against the Romans around the middle first century

Bethesaida, Israel

• Located in Northern Israel near the sea of Galilee

• Was an active fishing community 2000 years ago

• Site where Jesus healed a blind man and walked on water

Overview Movie

• http://www.joeinfo.com/gpr/overview.MOV

Bethsaida Excavation

• GPR has tested land for cave locations

• Artifacts found:1. coins (dated as far back as 1st and 2nd Century)

2. child’s sandal

3. fabric

4. pottery

5. human bones and remains

Antenna Movie

• http://www.joeinfo.com/gpr/antenna.MOV

• Recently, Dr. Harry Jol, professor of geography at the University ofWisconsin - Eau Claire, used GPR to map an area by the city gates of Bethsaida

Vescovio, Italy

• Romaneque church of Santa Maria

• Remains of marketplace, living quarters, storage areas, hallways and door entrances reconstructed visually

Forum Novum

• Remains of marketplace, living quarters, storage areas, hallways and door entrances reconstructed visually

• Believed to be Roman marketplace which initially began construction in the 1st century BC and flourished through the 4th century AD as the Roman town of the Forum Novum

Excavation

• British School of Archaeology in Rome have conducted excavation sites

• Sites include:

1. an amphitheatre

2. a bath complex

3. the villa

4. a tomb complex

5. crypts from the 8th to 9th centuries located behind the church

GPR reading from Forum Novum

Wrap up…

• GPR usage has become more wide-spread

• Now used in construction

• Average person can now purchase GPR equipment