# Presentation: 281257

## Are thermal effects responsible for micron-level motions recorded at deep- and shallow-braced monuments in a short-baseline network at Yucca Mountain, Nevada?

**Emma Hill, Jim Davis, Pedro Elosegui, Brian Wernicke, Eric Malikowski, and Nathan Niemi**

**Station REPO**

## Introduction

**Introduction**

**Baseline lengths**
- SLI4-SLID (Slide Mtn): 0 mREPO-REP2: ~10 mREPO-REP3: ~100 mREPO-REP4: ~1000 m
- (Similar instrumentation)

**Blue dots = BARGEN sites**

**Southern Nevada**
- Desert environment
- ‘Low’ tectonic rates

## Data Processing

**Data Processing**

**Data processed using GAMIT:**
- Fixed orbits (IGS final)
- No TZD estimation
- L1-only position estimates
- Only look at baselines

**Site-specific effects:**
- Phase errors
- Un-modeled physical motions

- Photo by Beth Bartel

## Time Series - East

**Time Series - East**

**Short baseline**
**Annual cycles:**
**0.03-0.54 mm**
**RMS:**
**0.06-0.20 mm**

**Time series have been offset for illustration**
**RMS calculated about model of seasonal cycle**

**Zero baseline (ZBL)**
**RMS:**
**0.03 mm**

## Time Series - North

**Time Series - North**

**Short baseline**
**Annual cycles:**
**0.02-0.19 mm**
**RMS:**
**0.06-0.23 mm**

**Zero baseline (ZBL)**
**RMS:**
**0.03 mm**

## Time Series - Radial

**Time Series - Radial**

**Short baseline**
**Annual cycles:**
**0.10-0.40 mm**
**RMS:**
**0.12-0.73 mm**

**Zero baseline (ZBL)**
**RMS:**
**0.08 mm**

## Temperature Data

**Temperature Data**

**Temperature data was obtained from the Beatty weather station (~25 km NW of Yucca Mountain)**

## Longer-period Signals

**Longer-period Signals**

**“****Longer-period” signals (quasi-periodic) = Gaussian-filtered time series**

**REP2-REPO ****(~10 m baseline)**
- For illustration, the north component time series has been reversed (i.e. figure shows REPO-REP2 for the north)

## Longer-period Signals

**Longer-period Signals**

**REP2-REPO**
**REP4-REPO**

**Cross-correlation between temperature and GPS time series**

**The GPS seasonal cycles might lag those of the temperature data, but it is hard to detect using this method.**

**Correlation coefficients:**
**East = 0.74-0.98****North = 0.45-0.93****Radial = 0.55-0.76**

## Monte Carlo Analysis

**Monte Carlo Analysis**

**Add noise to GPS and temperature time series**** **
**Gaussian filter to get long- and short-period signals**** **
**Cross-correlation as before**** **
**Record peak correlation and corresponding time step**** **
**Repeat 5000 times**

## Longer-period Signals

**Longer-period Signals**

**Monte Carlo analysis (cross-correlation between temperature and GPS)**

**Indicates a lag (15-30 days) for many baselines in the horizontal component...**

**EAST: Similar results for all other baselines to REP4 (no lag for shorter baselines).**

**NORTH: Similar results for all other baselines to REPO (no correlation for other baselines).**

## Longer-period Signals

**Longer-period Signals**

**... ****but we do not see a lag for the radial component.**
**The temperature ‘lags’ the GPS by >50 days.**
**Although there is a correlation for the radial, it looks like we are comparing two periodic signals that do not appear to be related. **

## Shorter-period Signals

**Shorter-period Signals**

**“****Shorter-period” signals = residuals from Gaussian-filtered time series**

**REP2-REPO ****(~10 m baseline)**

## Shorter-period Signals

**Shorter-period Signals**

**REP2-REPO (~10m)**
**REP3-REPO (~90 m)**

- Both regular cross-correlation...

- ... and Monte Carlo technique indicate **no lag time for short-period signals**

**Highest correlation (0.67) for the east component and baselines to REP2 (shallow-braced monument)**

## Thermal Expansion

**Thermal Expansion**

**Monument (shorter-period?)**   - Different leg lengths and orientation    - REP2 different type of pipe

**Cliff / Bedrock (longer-period?)****	**    - *Dong et al.* [2002] estimate ~45 day lag     - Differential effects from orientation of ridgeline?

**Upper ground layers (shorter-period?)**        - Deep versus shallow-braced monuments

**Red = longest leg****Green = shortest leg**

**Several processes occurring at different time-scales?**

**Steep cliff**

**Gradual slope**

**Something else?**

## Baseline-dependent Noise

**Baseline-dependent Noise**

**Orbits? **~0.002 mm over 1 km (assuming 5 cm accuracy of IGS final orbits)
**Troposphere?**
**Ionosphere?**
**Multipath?**

**(and what is causing the seasonal cycles in the radial?)**

## Tropospheric Delay

**Tropospheric Delay**

**REP4-REPO**

**When TZD parameters are estimated:**
**Time series for horizontal components are very similar.**
**But seasonal cycles in the radial component are reduced by ~50% for the longer baselines.**

**No TZD estimation**
**With TZD estimation**

**A mean has been removed from both time series**

## Ionospheric Delay

**Ionospheric Delay**

**When LC is used:**
**Time series are considerably noisier and have visible receiver change offsets.**** **
**Seasonal signals remain. **

**REP4-REPO**

**L1-only**
**LC**

**A mean has been removed from both time series**

**Differences between L1- and L2-only**
**(Receiver changes at REP4, Jan and Nov 2007 (NetRS to 4000 SSI to NetRS)**

**REP4-REPO**

## Elevation-Angle Dependence

**Elevation-Angle Dependence**

**Multipath?**
**Antenna differences?**

**Time series from results using different elevation cutoff angles are offset.**
**Largest effect in radial component.**

**REP3-REP2 (~10m)**
**REP4-REP2 (~900 m)**

**Y-axes have different scales**

## Conclusions

**Conclusions**

- The sites appear to be very stable (RMS 0.06-0.73 mm).  However, the time series do show both seasonal (annual amplitude 0.03-0.54 mm) and shorter-period signals.
- We suspect the horizontal seasonal signals may be related to bedrock thermal expansion (they are correlated with temperature, but with a lag time of ~15-30 days), but this is not the case for the radial component (instead atmosphere/multipath?).
- Shorter-period signals are correlated with temperature, mainly for the east component and particularly for REP2 (the short-braced monument).  We suspect this could be thermal expansion of the monument or upper ground layers (or both).

## Thanks!

**Thanks!**

## Rates

**Rates**

**REP3-REP2 (~90 m) (north)**
- -0.07 ± 0.01 mm/yr

**REP3-REPO (~100 m) (east) **
- 0.06 ± 0.01 mm/yr

**REP4-REP3 (~1 km) (north)**
- -0.24 ± 0.01 mm/yr

## Elevation-Angle Dependence

**Elevation-Angle Dependence**

**Reduction in annual amplitude for the radial component with higher elevation angle cutoffs.**

**Mean annual amplitude**