UTAM 2004Travis Crosby

UTAM 2004Travis CrosbyVery Low Frequency EM Surveys for the Purpose of Augmenting Near-Surface Seismic Studies

UTAM 2004Travis Crosby Introduction Instrument Theory Geophysical Results Other Instrument Applications Future Work

UTAM 2004Travis CrosbyFor Single Method Surveys: Instrument may record excessive noise. Ground may not provide sufficient contrast. Overlapping anomalies may hinder interpretation.

UTAM 2004Travis CrosbyFor Single Method Surveys: Instrument may record excessive noise. Ground may not provide sufficient contrast. Overlapping anomalies may hinder interpretation.Reconciliation of multiple data sets oftenprovides a more true interpretational picture.

UTAM 2004Travis CrosbyVLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ).

UTAM 2004Travis CrosbyVLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with predictable magnetic and electrical components.

UTAM 2004Travis CrosbyVLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field.

UTAM 2004Travis CrosbyVLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. Instrument measures anomalous response to the induced current.Surface Locationof AnomalySecondaryEM FieldVertical AnomalyHz/Hx

UTAM 2004Travis CrosbyVLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. Instrument measures anomalous response to the induced current. Karous & Hjelt filter applied to each data point (Dn) to convert complicated anomalies into simple peaks.Surface Locationof AnomalySecondaryEM FieldVertical AnomalyHz/Hx

UTAM 2004Travis CrosbyVLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. Instrument measures anomalous response to the induced current. Karous & Hjelt filter applied to each data point (Dn) to convert complicated anomalies into simple peaks.Filtered Data n = - 0.102 Dn-3 + 0.059 Dn-2 0.561 Dn-1 + 0 Dn + 0.561 Dn+1 - 0.059 Dn+2 + 0.102 Dn+3 Where:Dn = Hz / Hx, as measured by the instrument

UTAM 2004Travis CrosbyVLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. Instrument measures anomalous response to the induced current. Karous & Hjelt filter applied to each data point (Dn) to convert complicated anomalies into simple peaks. By increasing filter spacing (Dn-2, Dn, Dn+2,), information about deeper apparent depths can be obtained, and used to produce cross-section plots of current density.

UTAM 2004Travis CrosbyVLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. Instrument measures anomalous response to the induced current. Karous & Hjelt filter applied to each data point (Dn) to convert complicated anomalies into simple peaks. By increasing filter spacing (Dn-2, Dn, Dn+2,), information about deeper apparent depths can be obtained, and used to produce cross-section plots of current density. VLF used to look for bodies of low electrical resistance, i.e. vertical fractures and ore deposits up to depths of 300 m.

UTAM 2004Travis CrosbySeismic RefractionVLF 595 m profile length 120 Geophones 5 m Spacing 41 Shot Points 15 m Spacing Source used 550 lb EWG First arrival P-wave measured 595 m profile length 3 m station spacing Instrument used Abem Wadi Frequency used 25.1 kHz (Tx located in North Dakota)

UTAM 2004Travis CrosbySite Location20 kmN

UTAM 2004Travis CrosbyLine LocationNLegendVLF Detected FaultSeismic Detected FaultMapped Fault, dottedwhere inferredSeismic or VLF LineWater Well600 m

UTAM 2004Travis CrosbySeismic TomogramRay Path Density

UTAM 2004Travis CrosbySeismic TomogramVLF Data (Karous & Hjelt Filtered)SENW?

UTAM 2004Travis CrosbySite InterpretationN600 mLegendVLF Detected FaultSeismic Detected FaultMapped Fault, dottedwhere inferredSeismic or VLF LineWater Well

UTAM 2004Travis Crosby??N600 mTest ProfileLegendVLF Detected FaultSeismic Detected FaultMapped Fault, dottedwhere inferredSeismic or VLF LineWater Well

UTAM 2004Travis CrosbyWater WellStreamVLF Data (Karous & Hjelt Filtered)SENW

UTAM 2004Travis CrosbyN??500 mSite InterpretationLegendVLF Detected FaultSeismic Detected FaultMapped Fault, dottedwhere inferredSeismic or VLF LineWater Well

UTAM 2004Travis CrosbyOther VLF Studies: Ore Deposits VLF Survey area 90 km northeast of Yellowknife, NW Territories, Canada. Original defined strike limits Ag-Pb-Au banded sulfide lens was 160 m. VLF survey complementing other data sets suggested a greater strike length of 400 m.Data from: FugroAirborne Surveys

UTAM 2004Travis CrosbyFuture Work To augment seismic refraction tomography studies by conducting smaller scale, higher resolution VLF to detect normal and antithetic faults bounding larger colluvial wedge structures.

UTAM 2004Travis CrosbyFuture Work To augment seismic refraction tomography studies by conducting smaller scale, higher resolution VLF to detect normal and antithetic faults bounding larger colluvial wedge structures. Multi-electrode, high-resolution, 2-D, DC resistivity imaging of colluvial wedges for comparison with seismic tomograms.

UTAM 2004Travis Crosby

Very Low Frequency is otherwise referred to as VLFPresentation outline.Listed are some of the pitfalls of relying on only one method to solve geophysical problems.The solution is to collect more than one data set and compare/contrast their results.VLF was used to augment seismic data in a recent study to located faults and fracture zones. Discussed over the next several slides is how VLF works and what it is measuring.Parameters of the seismic/VLF survey to be discussed over the next several slides.Survey site was located near Fivemile Pass between Utah and Tooele Counties.Proposed line location and anticipated mapped faults and fracture zones according to the geology map of Fivemile Pass.This slide was not included in the presentation, and should be ignored.Results of the seismic refraction tomography/VLF survey. The center normal dip-slip fault was detected with seismic, which did not detect the two end faults. This could be due to the fact that they are at the very edge of the imaging aperture with very few rays transecting. These faults could also be strike-slip faults. The center fault might be inactive and have resealed with calcite veins, thus rendering it invisible to VLF. The geology of the area is entirely limestone.Between the two methods applied, all anticipated faults were detected. By using only one method, the problem would not have been completely solved.A second ground truth VLF line was performed near a well that was an excellent water producer. Other wells drilled in the area provided little water return. Why?It should be noted that the profile was collected about 150 meters away from the well. A strong anomaly occurs very near the projection of the well., thus possibly explaining why this well was successful. Several additional anomalies are also detected.Site interpretation for the ground truth VLF line.VLF has also been used to look for mineral deposits with excellent success.Possible future work.