# Presentation: 153666

## Transient EM at the Cochiti 180 WellGeological and Sensitivity Analysis

- Morgan G. Jones
- UCSC
- SAGE 2005

## Cochiti ZeroTEM Overview

- Preliminary Processing and Results
*WinGLink*
- Brief interpretation
- Sensitivity Analysis: How precise is this model?
- General expectations for sensitivity variations
*EMMA*
- Layered model resolutions
- Conclusions

## The Modeling Process

- 1) WinGLink analysis software computes 15-layer Occam inversion (pink line)
- 	 from voltage decay data (red dots).

- 2) Morgan forward-computes a simple layered earth model (green line).

- 3) Dongle stolen; Morgan begs Jaimie for computer time.

## Occam Inversion + 1-D Layered Models

- NW

- SE

- 0

- 200

- 400

## Slide 5

## AeroMag Correlation

**AeroMag Correlation**

- 1000 m

- USGS 2003

**Notes:**
Aeromag correlation, usgs?
Recognition to George for spotting this little guy.

## EMMA Sensitivity Analysis (1)

**E**lectro**M**agnetic **M**odeling and **A**nalysis
- Computes a covariance matrix, or a measure of resolution of the data set, using the first order Taylor expansion of each model with respect to its parameters.
- Theoretically, we should see the following:
- Excellent resolution of the depth to the top of a conductor.
- Large resistivity values will be poorly resolved
- If a conductor overlies a more resistive layer, it will mask the resolution of the underlying layer.

- Of course, no model is perfect....

**Notes:**
Model function * model parameters = data  
This is ideal, since we want F(M)*M - D = 0
Since it’s not exact, you gots to expand it.

## EMMA Sensitivity Analysis (2)

- Results presented as a Standard Deviation Factor for each parameter in a layer:
- Resistivity, Thickness, and Depth resolution factors
- Good to Moderate resolution at 1 < STDF < 2
- Poorly or completely unresolved parameters at STDF > 2
- Auken et al., EMMA, 2003(?)

**Notes:**
Increasing transmitter current improves SNR in distinct ways for each parameter:
For a resistive layer, more amps gives better resolution of resistivity.
For conductors, increasing current improves thickness resolution.

## Resistivity Resolution

| Resolution Quality | STD Factor |
| --- | --- |
| Excellent to Moderate | 1 - 2 |
| Poor to unresolved | 2 - 99 |

**Notes:**
Conductor masking examples....
Note especially how resistors are just hard to resolve.  They don’t return much signal I guess.

## Thickness Resolution

**Notes:**
Good example of point (1), how big conductors have good resolution down to their surfaces.
Also note conductors masking lower layers.
- And note low STDFs for the shallow resistive body.... Next slide.

## Depth Resolution

**Notes:**
Again, very good resolution of the shallow resistor.  I interpret this to say that we know very accurately the body’s shape and location, but our constraints on its true resistivity values are much worse.

## Occam Inversion + 1-D Layered Models

- NW

- SE

**Notes:**
Reference figure for resolution plots.

## Conclusions

- Shallow TEM resistor is likely a geologic effect, possibly a mafic intrusive body.  The evidence?...
- Lateral position strikingly coincident with USGS magnetic high.
- Though the precise resistivities are unresolved, a massive resistor is present and could easily be consistent with the properties of igneous bodies.
- On this particular survey, ZeroTEM hit its resistivity resolving limit at approximately 300 m depth, though depth resolution for general features remained reliable below this line.

## Many thanks to

- All SAGE staff ...
- Especially Louise and George ...
- But not Bernie.
- My team.
- My hallmates for waking me up this morning.
- Cochiti Pueblo for the land.
- The Danes for EMMA.
- Zonge for the equipment and patience.
- Just kidding Bernie!  You’re the man!