Nye data – nye muligheder? Esben Auken, Anders V. Christiansen, Nikolaj Foged and Kurt Sørensen HydroGeophysics Group Department of Earth Sciences, University of Aarhus, Denmark

Indhold l Hvordan ser x-data ud? l Modelanalyser – motivationen l Problemerne med måling af vinkel – forurening af x-signalet l Inversion

Theoretical z-response 100 m 10 m 30 m 20 m 80 m 70 m 10 m Model z-component data

Theoretical z- and x-response l Late time x t -3 l Late time z t -2.5 l x-component signal level is 5 – 8 times smaller than z-component x- and z-component 100 m 10 m 30 m 20 m 80 m 70 m 10 m Model

Model Sensitivity Analysis – model 2 l White: z-component alone l Grey: x- and z-component l Black: x- and z-component, x with 60 kHz low-pass filter

Model Sensitivity Analysis – model 1 l White: z-component alone l Grey: x- and z-component l Black: x- and z-component, x with 60 kHz low-pass filter

Model Sensitivity Analysis – model 3 l White: z-component alone l Grey: x- and z-component l Black: x- and z-component, x with 60 kHz low-pass filter

Location of first gate – 11 or 18 micros? l White: z-component alone l Grey: x- and z-component l Black: x- and z-component, x with 60 kHz low-pass filter

The 3 Challenges for exploiting x-data 1.Signal to noise level Altitude dependency Natural background noise 2.Contamination of the x-data due to tilt of the Tx-frame 3.Timing of the instrumentation and measurement of the tilt

Altitude Dependency z-data x-data Altitude: 1 m Altitude: 1 m

Altitude Dependency z-data x-data Altitude: 1 m 18 m Altitude: 1 m 18 m

Altitude Dependency z-data x-data Altitude: 1 m 18 m 29 m – operating altitude Altitude: 1 m 18 m 29 m

The 3 Challenges for Exploiting x-data 1.Signal to noise level x-signal is 5 – 8 times smaller than z-signal x-signal decreases faster with altitude compared to the z-signal x-noise is 5 – 10 times larger than z-noise 2.Contamination of the x-data due to tilt of the Tx-frame 3.Timing of the instrumentation and measurement of the tilt

The 3 Challenges for Exploiting x-data 1.Signal to noise level x-signal is 5 – 8 times smaller than z-signal x-signal decreases faster with altitude compared to the z-signal x-noise is 5 – 10 times larger than z-noise 2.Contamination of the x-data due to tilt of the Tx-frame 3.Timing of the instrumentation and measurement of the tilt

z-component x-component Tilt – Normal Situation

Positive Tilt of the Tx-frame l z-response adds signal to the x-response l The x-response caluclated as: B x total = B x + sin(Tilt) B z z-contamination x-component

Contaminated x-response – Positive Tilt x-data Tilt: 0 deg

Contaminated x-response – Positive Tilt x-data 5 deg Tilt: 0 deg

Contaminated x-response – Positive Tilt x-data 10 deg 5 deg Tilt: 0 deg 9 usefull gates

Negative Tilt of the Tx-frame l z-response subtracts from the x-response z-contamination x-component

Contaminated x-response – Negative Tilt x-data Tilt: 0 deg

Contaminated x-response – Negative Tilt x-data Tilt: 0 deg -5 deg

Contaminated x-response – Negative Tilt x-data Tilt: 0 deg -5 deg -10 deg usefull gates

The 3 Challenges for Exploiting x-data 1.Signal to noise level x-signal is 5 – 8 times smaller than z-signal x-signal decreases faster with altitude compared to the z-signal x-noise is 5 – 10 times larger than z-noise 2.Contamination of the x-data due to tilt of the Tx-frame The tilt of the frame adds or subtracts signal form the x-response Addition of signal is preferred from subtraction of signal Tilt must treated as an inversion parameter - it can not be measured with the desired accuracy Does (probably) not need a priori information for the tilt 3.Timing of the instrumentation and measurement of the tilt

The 3 Challenges for Exploiting x-data 1.Signal to noise level x-signal is 5 – 8 times smaller than z-signal x-signal decreases faster with altitude compared to the z-signal x-noise is 5 – 10 times larger than z-noise 2.Contamination of the x-data due to tilt of the Tx-frame The tilt of the frame adds or subtracts signal form the x-response Addition of signal is preferred from subtraction of signal Tilt must treated as an inversion parameter - it can not be measured with the desired accuracy Does (probably) not need a priori information for the tilt 3.Timing of the instrumentation and measurement of the tilt

Timing of the Front Gate x-data converted to apparent resistivity Front Gate in 7.8 µs Front Gate preventing the primary signal to saturate the receiver – opens for the signal just before the first time-gate 100 m 10 m 30 m 20 m 80 m 70 m 10 m Model

Timing of the Front Gate Front Gate in 8.8 µs 300 nSec shift corresponds to approx. 3 % shift of the first gate x-data converted to apparent resistivity Front Gate in 7.8 µs 100 m 10 m 30 m 20 m 80 m 70 m 10 m Model Front Gate preventing the primary signal to saturate the receiver – opens for the signal just before the first time-gate

The 3 Challenges for Exploiting x-data 1.Signal to noise level x-signal is 5 – 8 times smaller than z-signal x-signal decreases faster with altitude compared to the z-signal x-noise is 5 – 10 times larger than z-noise 2.Contamination of the x-data due to tilt of the Tx-frame The tilt of the frame adds or subtracts signal form the x-response Addition of signal is preferred from subtraction of signal Tilt must treated as an inversion parameter as it can not be measured with the desired accuracy Does probably not need a priori information from the device 3.Timing of the instrumentation and measurement of the tilt The timing is accurate within 200 nanosec - OK! Accurate measurement of the tilt while flying

Inversion Methodology l LCI – soundings

Inversion Methodology l LCI – soundings

Inversion Methodology l LCI – soundings

Inversion Methodology l LCI – soundings

Inversion Methodology l Damped least squares scheme, modeling of system transfer function Low pass filters, front gate, turn-on and turn-off exponential ramps Flight altitude and tilt are constrained parameters l The model is parameterized or smooth 1D

Data

Diskussion og konklusion l x-data tilfører information om den overfladenære geologi l Ved to-moment-system kompenserer x-data for, at første gate er i 16 microsek. l Ved ét-moment-system (første gate i 10 microsek.) tilfører x-data opløslighed i betragetligt omfang. l x-signalet kan anvendes, hvis ikke modstanden er for høj Kræver at flyvehøjden er omkring 30 m (eller lavere) Kræver nøjagtig måling af vinkel og kalibrering af instrumenterne l Giver ikke ekstra arbejde i felten, men giver ekstra processerings- og tolkningsarbejde l Stadigvæk på forskningsstadiet