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Stratagem EH4 Field Evaluation of Data Quality. Field techniques for determination of the source and nature of low-quality data acquisition with the Stratagem EH4. It is important to remember that all poor quality data has a cause – some we can fix and some we cannot fix.
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Stratagem EH4 Field Evaluation of Data Quality. • Field techniques for determination of the source and nature of low-quality data acquisition with the Stratagem EH4. • It is important to remember that all poor quality data has a cause – some we can fix and some we cannot fix.
Every case of low-quality data has a cause. The job of the operator is to do three things. • Recognize there is a problem • Incoherent noise • Signal saturation in one or more channels • Loss of signal in one or more channels • Determine its cause or source • Parallel test • Review the nature of spectral amplitudes, resistivity curves, and phase • Fix the problem if possible
Cause of low-quality data: Environmental noise • Power-line noise • Noise sources in the acquisition frequency band • Civilian or military transmissions such as VLF • Local electrical noise such as power generators, electrified animal fences, water pumps, etc. • Noise outside the acquisition range such as nearby AM radio transmissions.
Cause of low-quality data: Setup problems 1 • Improper set up of instrument • Ex not 90-degrees from Hy and Ey not 90-degrees from Hx (Hy and Hx switched positions)
Set up problem 3: Hy is accidentally pointing in X direction and Hx is pointing in Y direction causing Ex to be parallel with Hy and Ey to be parallel with Hx. Notice Ex coherent with Hx and Ey coherent with Hy.
Hy perpendicular to Ey and Hx perpendicular to Ex. Note poor data quality.
Solution to improper set up of sensors • In field stop data acquisition and fix sensor orientation, then rerun the station • In IMAGEM software in office. • Go to OPTIONS • Select “Change Channel” • Change channels so that Ex and Hx are processed together and Ey is • processed with Hy. • In DATA ANALYSIS rerun all the time series for the affected station • Recreate new impedance file for the station by editing in 1D ANALYSIS
Same data as previous slide. Sounding curve with proper orientation of sensors – very 1-D environment. Hy perpendicular to Ex and Hx perpendicular to Ey. Good data quality.
Cause of low-quality data: Setup problems 2 • Disconnected ground stake at AFE receiver • Magnetic coils over top of metal conductor • Electrodes with high contact resistance • Disconnected electrodes or loose connectors on cables
Cause of low-quality data: broken instrument – first step. • First step is to do parallel test to determine if problem is instrument or environment. • In parallel test Ex is parallel with Ey and Hx is parallel with Hy, but both magnetic sensors are parallel with both electric dipoles. • Must run parallel test in “Scalar” mode (problem below). • Possible problems with parallel test that can give false results: • High magnetic field gradient makes it very difficult to get good results unless Hx to Hy alignment is perfect. • Contact resistance different from one electrode to another: solution is to put both buffered electrodes on same metal stake. • High frequency saturation in range of 1 MHz can result in different paths in the electronics of the instrument (Problem #1 below) causing spikes.
Cause of low-quality data: broken instrument – bad ground connection • Things that can be done in the field • Check ground for poor connection with Ohm meter. • Ground connection on back of EH4 console should be the same as ground stake at AFE. • Connect Ohm meter between back of EH4 console and AFE ground stake. The reading should not be more than a few Ohms. If reading is high possible open connection somewhere between ground stake and console.
Problem # 1: Nearby AM radio transmitter in range of 1M Hz • Electric-field spiking caused by nearby AM radio signal. • Solution: • Use shorter E-field dipole • Do survey at different time of day • Do survey farther away from radio transmitter antenna • Change orientation of communication cable to minimize coupling of the radio signal
RF Interference Spikes caused by nearby AM radio signal with a transmitter signal approximately 1M Hz. Notice single uncorrelated spike on right for Ey, and multiple E spikes incoherent with H field measurements. Poor results from a parallel test with Ex parallel to Ey and Hx parallel to Hy (E dipoles perpendicular to H coils)
RF Radio transmitter causing spikes in E field incoherent with H-field. Example two of parallel test results next to an AM band commercial radio station.
Radio Frequency Interference Causing E-field spikes in low-pass incoherent with H fields. Example three of saturation caused by nearby AM radio transmitter antenna.
Problem #2 • Saturation by high 50 or 60 Hz power line harmonics. • Solution: • Move station farther away from power lines. • Avoid metal fences or underground metal pipes • Rotate station orientation to minimize 50 Hz signal
Sounding curves showing measurement overpowered by 60 Hz harmonics. The split in the sounding curve is greater than a 45-degree slope, when viewed on an equal log-log scale. The data is suspected to be biased and not reflecting geology.
60 Hz harmonics seen in spectral amplitudes. Notice strong 60 Hz peak. Also notice high Hx:Hy coherency and high Ex: Ey coherency showing this is a parallel test. In high frequency no coherency because signal is too low.
Time series showing powerful 60 Hz signal and harmonics. Notice gain setting of only “1” and “2”
Problem #3: Saturation of high-frequency E or H fields caused by improper gain setting. Can also occur in low frequency but not as commom. • Caused by arrangement of gain amplifiers and power line notch filters.
Straight-line portions of the time series in Hy and Ex caused by saturation of 60 Hz harmonics even though in high frequency. Gain set to 8, which is the highest gain before filters. Must reduce strength of power line noise. Solution: use lower gain (4), use higher gain (10) to get the 60 Hz filter, use shorter dipole length.
Reduce gain to avoid power line saturation. Notice that at a gain setting of 4 and 4 the straight line saturations disappear.
Set to gain 10 to bring in 60 Hz powerline filter. No saturation with gain of 10. This is because the gain settings of 10 and greater are after the power line filters. The gain settings of 8 and less are before the power line filters.
No X-direction sounding curve because power line saturation in Ex and Hy reduced coherency below set coherency limit.
With gain set properly both X and Y direction soundings are measured. This is in tensor mode and split in curve is a result of geologic structure, not noise.
Problem #4 • View parallel test result in tensor mode • Solution: Change to scalar mode.
Parallel Test in Scalar Mode – Must use scalar mode to view parallel test
Same parallel test viewed in tensor mode. Cannot interpret the test.
Standard survey mode. Note random Hx:Hy and Ex:Ey coherency showing this is not a parallel test. Also notice strong transmitter frequencies showing transmitter is close to receiver.
Natural field dead-zone • From approximately 500 Hz to 4 kHz there is no natural field signal. It must be complemented by the EH4 transmitter. The following screens show the spectral amplitudes and impedances for both using and not using the transmitter in a low signal area.
Spectral amplitudes in weak signal area. Note lack of signal between 1 and 4 kHz and above 30 kHz
Weak natural field. Note dead zone between 1 kHz and 4 kHz and above 30 kHz
Interference from VLF military transmitter. Notice high noise level even at E gain of 1 caused by 20 kHz VLF.
Note spectral peak at 20 kHz in amplitude curve. Other peaks are transmitter frequencies
Solution for high noise source • For VLF transmitter most of the energy is in electric fields so the electric dipoles can be shorted to prevent saturation of the Analog-to-Digital converter. Magnetic dipoles cannot be shorted but they are less of a problem with radio transmissions.
Other sources of poor data quality in the field. • Local sources • Portable power generator (heating coffee) • Electrified animal fences • Large military transmitters (VLF) • Near-field E source (depress phase and increased resistivity) • Near-field H source (increased phase and decreased resistivity)
