# Presentation: 671885

## GPR applied to San Marcos Pueblo, NM, Archaeological Site 

- Laura Russon

## Outline:

- What is GPR?
- Dewow Filtering
- Midden 2 site
- GPR scatter and reflection
- Anomaly east of Mission Complex

## Ground Penetrating Radar

- A.P. Annan, GPR notes

**Notes:**
Ground Penetrating Radar are radio waves transmitted and received by the Noggin 250.  The noggin emits a GPR pulse that penetrates the soil and can reflect or scatter when the electrical properties of the soil change, especially the dielectric constant, K.

The transmitter and receiver are close together.

GPR operates between 1 and 1000 MHz.  In this range, velocity and attenuation are almost frequency independent.  

Conductivity causes attenuation.

## Dewow Filtering

**Dewow Filtering**

- A.P. Annan, GPR notes

**Notes:**
The center frequency transmitted by the Noggin 250 is 250 MHz.  

The GPR pulse sent out is truncated.
Fourier transform turns out a wide band of frequencies, with 250 at its center and 250 as approximate bandwidth for short pulses like this.

However, while the bulk of the frequencies are in that range, there may still be some very low frequency waves transmitted. 

Wave propagation is more complicated at low frequencies, less that 1 MHz.
They are subject to:
1- Induction Phenomena
	When conductivity current is much greater than displacement current, induction occurs based on Faradays law of Induction.

2-Dispersion
	when attenuation and velocity vary with frequency.

3- May cause dynamic range limitations.

 A high amplitude low frequency wave could cause some dynamic range limitations.  Low frequency waves are also associated with induction phenomena.  These waves are subject to dispersion as well.

Dispersion:  

## N

- Average Amplitude Time Slice: 0 to 9.94 ns

- X position in meters

- Y position in meters

- μV

**N**

**Notes:**
This is the potential kiva site.  
This data was migrated using a ground velocity of 0.1 meters/nanosecond.  I did try 0.15 m/ns which is the text book value for soil, but 0.1 gave better result.  

This time slice averages the received signal amplitude, which is in μV, through the time section.  9.94 nanoseconds, which is two way time, corresponds to 0.5 meter depth.

You can begin to see two pseudo circular amplitude lows.  (click to bring circles)  
The higher amplitude green area correlates to the midden 2 site as mapped by UNM. (click to bring midden).
The true shape of the right potential kiva is somewhat obscured by the midden to the north.  As we go to deeper time slices, the effects of the midden decrease and the circular anomalies have a better shape.

## N

- Average Amplitude Time Slice: 9.94 to 19.87 ns

- X position in meters

- Y position in meters

- μV

**N**

## N

- Average Amplitude Time Slice: 19.87 to 29.81 ns

- X position in meters

- Y position in meters

- μV

**N**

## Line 1

- S

- N

- Line 2

- N

- S

- S

**Notes:**
These two cross sections correspond to line 1 and 2 from the previous slide.  I used automatic gain control on the data, with a time window of 0.1 and a maximum value of 500.
Average signal is computed over a time window and then the data point at the center of the window is amplified or attenuated by the ratio of the desired output value to the average signal amplitude.
The window width is in units of pulsewidth based on the nominal antenna frequency.

You can see the characteristic sediment of the area on the right, with parabolas and reflectors.  On the left there is a clearly defined column of parabolas, which depict a break in the character sediment in a vertical line.  (click to get lines)
Because of the vertical character of the break, it appears that a cylindrical hole was dug into the sediment.

I took some smaller time slices to see how deep I could resolve the circular anomaly, and could resolve it to about 1.5 meters.  This does not mean it is the bottom of the hole, of the floor of the potential kiva.  It may go deeper, but the radar does not penetrate deep enough to see the bottom.  Therefore, 1.5 meters is a minimum depth.

## Fresnel type reflection

- Normalized cross-sectional area

- A.P. Annan, GPR notes

**Notes:**
An important question to answer is why we see a distinction between GPR data in the potential kivas versus the surrounding material.  This figure helps explain the theory.

The image is for spherical objects.  Normalized cross sectional area is the scattering cross section for the target.  For a small sphere, the amount of radar scattered is small.  For a large sphere, the response approaches a planar target.  Fresnel reflection coefficients describe the amplitude of reflected and transmitted energy at planar boundaries, and when the target becomes larger and flatter, the radar reflects as in fresnel reflection.

When the dimensions of the object are small compared to the excitation wavelength, the

## Midden 2

- Top Slice

- Second Slice

- Potential Kivas

**Notes:**
You’ve seen these before.  They are the top two time slices potential kiva area.  You can see the higher amplitude scatters in the midden, and the lower amplitude areas within the circular anomalies.  Based on the depth cross sections, it appears that the character sediment of the area has been disturbed and cut into vertically.  The cylindrical holes will have filled in over time with fine grained erosional sediment.  The fine grained filler has low amplitude scatter because the particles are very tiny.  The character sediment has larger clumps in it, which was also evident from the depth sections.  The midden area would contain any kind of waste from the pueblo, including broken pottery, human and animal remains such as teeth, bones, antlers, and food.  These objects have much larger dimensions than the fine grained filler sediment, and thus have a much higher amplitude radar scatter.  It is the fine grained erosional sediments that make the circular anomalies stand out so well.

## Mission

**Average Amplitude: 0 to 9.94 ns**

**Mission**
**Complex**

**Room Block **
**12**

**Notes:**
We found something interesting between the mission complex and room block 12.  Note the rectilinear amplitude low.  This would be something that would merit further investigation.

## Conclusions

- GPR data revealed two cylindrical anomalies characterized by fine grained erosional sediment.
- Minimum depth of cylindrical anomalies is 1.5 m.
- A kiva is cylindrical...