scholarly journals Airborne electromagnetic footprints in 1D earths

Geophysics ◽  
2006 ◽  
Vol 71 (2) ◽  
pp. G63-G72 ◽  
Author(s):  
James E. Reid ◽  
Andreas Pfaffling ◽  
Julian Vrbancich
Keyword(s):  
Sea Ice ◽  
Frequency Domain ◽  
Half Space ◽  
Field Data ◽  
Inductive Limit ◽  
Thin Sheet ◽  
Ice Thickness ◽  
Sea Ice Thickness ◽  
Flight Height ◽  
Airborne Electromagnetic

Existing estimates of footprint size for airborne electromagnetic (AEM) systems have been based largely on the inductive limit of the response. We present calculations of frequency-domain, AEM-footprint sizes in infinite-horizontal, thin-sheet, and half-space models for the case of finite frequency and conductivity. In a half-space the original definition of the footprint is extended to be the side length of the cube with its top centered below the transmitter that contains the induced currents responsible for 90% of the secondary field measured at the receiver. For a horizontal, coplanar helicopter frequency-domain system, the in-phase footprint for induction numbers less than 0.4 (thin sheet) or less than 0.6 (half-space) increases from around 3.7 times the flight height at the inductive limit to more than 10 times the flight height. For a vertical-coaxial system the half-space footprint exceeds nine times the flight height for induction numbers less than 0.09. For all models, geometries, and frequencies, the quadrature footprint is approximately half to two-thirds that of the in-phase footprint. These footprint estimates are supported by 3D model calculations that suggest resistive targets must be separated by the footprint dimension for their individual anomalies to be resolved completely. Analysis of frequency-domain AEM field data acquired for antarctic sea-ice thickness measurements supports the existence of a smaller footprint for the quadrature component in comparison with the in-phase, but the effect is relatively weak. In-phase and quadrature footprints estimated by comparing AEM to drillhole data are considerably smaller than footprints from 1D and 3D calculations. However, we consider the footprints estimated directly from field data unreliable since they are based on a drillhole data set that did not adequately define the true, 3D, sea-ice thickness distribution around the AEM flight line.

Post-processing calibration of frequency-domain electromagnetic data for sea ice thickness measurements

2004 ◽  
Vol 2004 (1) ◽  
pp. 1-4
Author(s):  
James Reid ◽  
John Bishop ◽  
Angus Munro ◽  
Andi Pfaffling ◽  
Kazu Tateyama ◽  
...  
Keyword(s):  
Sea Ice ◽  
Frequency Domain ◽  
Ice Thickness ◽  
Post Processing ◽  
Sea Ice Thickness ◽  
Electromagnetic Data ◽  
Thickness Measurements

scholarly journals Direct helicopter EM — Sea-ice thickness inversion assessed with synthetic and field data

Geophysics ◽  
2007 ◽  
Vol 72 (4) ◽  
pp. F127-F137 ◽  
Author(s):  
Andreas Pfaffling ◽  
Christian Haas ◽  
James E. Reid
Keyword(s):  
Sea Ice ◽  
Field Data ◽  
Flying Height ◽  
Small Scale ◽  
Ice Thickness ◽  
Systematic Bias ◽  
Sea Ice Thickness ◽  
Instrumental Noise ◽  
Precision And Accuracy ◽  
Better Than

Accuracy and precision of helicopter electromagnetic (HEM) sounding are the essential parameters for HEM sea-ice thickness profiling. For sea-ice thickness research, the quality of HEM ice thickness estimates must be better than [Formula: see text] to detect potential climatologic thickness changes. We introduce and assess a direct, 1D HEM data inversion algorithm for estimating sea-ice thickness. For synthetic quality assessment, an analytically determined HEM sea-ice thickness sensitivity is used to derive precision and accuracy. Precision is related directly to random, instrumental noise, although accuracy is defined by systematic bias arising from the data processing algorithm. For the in-phase component of the HEM response, sensitivity increases with frequency and coil spacing, but decreases with flying height. For small-scale HEM instruments used in sea-ice thickness surveys, instrumental noise must not exceed [Formula: see text] to reach ice thickness precision of [Formula: see text] at 15-m nominal flying height. Comparable precision is yielded at 30-m height for conventional exploration HEM systems with bigger coil spacings. Accuracy losses caused by approximations made for the direct inversion are negligible for brackish water and remain better than [Formula: see text] for saline water. Synthetic precision and accuracy estimates are verified with drill-hole validated field data from East Antarctica, where HEM-derived level-ice thickness agrees with drilling results to within 4%, or [Formula: see text].

Inversion of airborne electromagnetic survey data for sea‐ice keel shape

Geophysics ◽  
1991 ◽  
Vol 56 (12) ◽  
pp. 1986-1991 ◽  
Author(s):  
Guimin Liu ◽  
Austin Kovacs ◽  
Alex Becker
Keyword(s):  
Sea Ice ◽  
Survey Data ◽  
Inductive Limit ◽  
Sea Water ◽  
Ice Thickness ◽  
Inversion Method ◽  
Perfect Conductor ◽  
Arctic Seas ◽  
Worst Case ◽  
Airborne Electromagnetic

It is possible to interpret conventional airborne electromagnetic (EM) data acquired over ice‐covered Arctic seas to obtain values of the sea ice thickness and, where needed, the actual sea ice keel geometry. To do so, we require high‐frequency (inductive limit) data that allows us to assume that the ice is virtually transparent to the EM fields while the sea water forms a perfect conductor. Practically, a 100 kHz operating frequency is needed, but data acquired at a lower frequency can be scaled to obtain the required inductive limit anomaly. The data inversion is done by linking Occam’s inversion method to a rapid numerical, two‐dimensional, forward solution for the ice keel problem. A most useful feature of the adopted inversion scheme is the minimization of the roughness or the mean square slope of the keel boundary. In some cases, where the keel might be bounded by steeply dipping walls, this constraint may result in a less accurate solution than might be obtained with a conventional technique. In most cases the advantage in stability that it provides outweighs the possible loss of accuracy that it may occasion. Tests on synthetic data show a possible worst case ice thickness error of about 15 percent. The results of inversion tests for two sets of survey data acquired near Prudhoe Bay, Alaska, also indicate an accuracy of this order of magnitude. While some portion of the inversion error must be ascribed to the roughness constraint and is therefore inherent to the inversion technique used, the remainder must be ascribed to the instrumentation and is probably remediable.

Shipborne electromagnetic measurements of Antarctic sea‐ice thickness

Geophysics ◽  
2003 ◽  
Vol 68 (5) ◽  
pp. 1537-1546 ◽  
Author(s):  
James E. Reid ◽  
Anthony P. Worby ◽  
Julian Vrbancich ◽  
Angus I. S. Munro
Keyword(s):  
Sea Ice ◽  
Half Space ◽  
Single Frequency ◽  
Ice Thickness ◽  
Sea Ice Thickness ◽  
Model Calculations ◽  
Space Model ◽  
Apparent Conductivity ◽  
Antarctic Sea Ice ◽  
Processing Module

We present a study of Antarctic sea‐ice thickness estimates made using a shipborne Geonics EM31 electromagnetic (EM) instrument, based on both 1D and 3D models. Apparent conductivities measured in the vertical coplanar (VCP) geometry are shown to be the measured quantity most sensitive to changes in the height of the instrument above seawater. An analysis of the effect of instrument orientation on the measured VCP apparent conductivity shows that the effects of pitch and roll on the calculated sea‐ice thickness can be neglected except in the case of very thin sea ice. Because only a single (quadrature) component of the magnetic field is measured at a single frequency, interpretation of shipborne EM31 data must necessarily be based on very simple models. For a typical sea‐ice bulk conductivity of ∼60 mS/m, a uniform half‐space model representing conductive seawater is appropriate for interpretation of VCP EM31 measurements over level sea ice up to ∼2.5 m thick. For thicker, more conductive sea ice, the interpretation model must account for the effect of the finite sea‐ice conductivity. Simultaneous acquisition of EM data at several frequencies and/or transmitter–receiver geometries permits interpretation of the data in terms of multilayered models. A synthetic example shows that 1D inversion of single‐frequency in‐phase and quadrature data from two transmitter–receiver geometries can yield reliable estimates of sea‐ice thickness even when the ice contains thin, highly conductive brine layers. Our 3D numerical model calculations show that smoothing the measured response over the system footprint means that the sea‐ice thickness recovered over multidimensional sea‐ice structures via half‐space inversion of apparent conductivity data yields a highly smoothed image of the actual keel relief. The dependence of footprint size on the height of the system above seawater results in the interpreted sea‐ice thicknesses being dependent on the deployment height of the instrument. Sea‐ice thickness data acquired using an EM31 equipped with a hardware processing module can be transformed to apparent conductivity and then inverted assuming a conductive half‐space model. For EM system heights >4.5 m above seawater, corresponding to large altitude and/or thick sea ice, inversion assuming a conductive half‐space model yields an improved estimate of the true sea‐ice thickness compared to that obtained using the processing module. However, the noise level in the estimated depth to seawater is relatively large (±0.1 m) in comparison with typical Antarctic sea‐ice thicknesses, and thickness estimates made using the shipborne system may be significantly in error over thin ice.

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