Progressing from 1D to 2D and 3D near-surface airborne electromagnetic mapping with a multisensor, airborne sea-ice explorer

Abstract

The polar ocean's sea ice cover is an unconventional and challenging geophysical target. Helicopter-electromagnetic (HEM) sea-ice thickness mapping is currently limited to 1D interpretation due to traditional procedures and systems. These systems are mainly sensitive to layered structures, ideally set for the widespread flat (level) ice type. Because deformed sea ice (e.g., pressure ridges) is 3D and usually also heterogeneous, ice thickness errors up to 50% can be observed for pressure ridges using 1D approximations for the interpretation of HEM data. We researched a new generation multisensor, airborne sea ice explorer (MAiSIE) to overcome these limitations. Three-dimensional finite-element modeling enabled us to determine that more than one frequency is needed, ideally in the range 1-8 kHz, to improve thickness estimates of grounded sea-ice pressure ridges that are typical of 3D sea ice structures.With the MAiSIE system, we found a new electromagnetic concept based on one multifrequency transmitter loop and a 3C receiver coil triplet with active digital bucking. The relatively small weight of the EM components freed enough payload to include additional scientific sensors, including a cross-track lidar scanner and high-accuracy inertial-navigation system combined with dual-antenna differential GPS. Integrating the 3D ice-surface topography obtained from the lidar with the EM data at frequencies from 500 Hz to 8 kHz in x-, y-, and z-directions, significantly increased the accuracy of sea-ice pressure-ridge geometry derived from HEM data. Initial test flight results over open water showed the proof-of-concept with acceptable sensor drift and receiver sensitivity. Noise levels were relatively high (20-250 parts-per-million) due to unwanted interference, leaving room for optimization. The 20 ppm noise level at 4.1 kHz is sufficient to map level ice thickness with 10 cm precision for sensor altitudes below 13 m.

Not extinct yet: innovations in frequency domain HEM triggered by sea ice studies

“…This system, MAiSIE, is introduced and described in detail in Pfaffhuber et al (2012a). In brief, MAiSIE is a small (3.5 m length) and broadband HEM system nominally operating from 500 Hz to 8 kHz.…”

“…To avoid the usual strong transmitter drift effects, we implemented an active current feedback system that keeps the actual transmitter current constant. Remaining changes in amplitude are handled by real time processing based on the actual, rather than nominal, transmitter moment (Pfaffhuber et al, 2012a). The maximum current supplied by the amplifier in the 500 Hz -4 kHz frequency range is 16 A.…”

“…In this work we discuss three discrete system innovations and improvements implemented in a system conceived in 2011, the Multi-method Airborne Sea Ice Explorer (MAiSIE). We provide detailed specifications of this device in Pfaffhuber et al (2012a) and have presented first field data in Pfaffhuber and Hendricks (2012). The three improvements are in operation in the current version of MAiSIE.…”

“…Acquiring sea ice thickness in the climatologically desired accuracy and precision (+/À 5 cm) posed a new challenge to HEM that illustrated the limitations of purpose-built, but traditional, systems (Pfaffling and Reid, 2009) and triggered new developments (Pfaffhuber et al, 2012a). In this work we discuss three discrete system innovations and improvements implemented in a system conceived in 2011, the Multi-method Airborne Sea Ice Explorer (MAiSIE).…”

Developments in frequency domain AEM: Tackling drift and noise with a ferrite-core, receiver triplet.

“…The measured data agrees well with forward modelled responses assuming 4 S/m ocean water conductivity (typical for this area and season). High altitude noise tests revealed a standard deviation of 20 ppm for the 4.1 kHz in-phase channel, leading to a derived ice thickness uncertainty of 10 cm at a system altitude of 13 m. The lower frequencies, however, showed higher noise levels and noise reducing measures are subject of our ongoing research (Pfaffhuber et al 2012). The actual signal to noise ratio (SNR) for the simultaneously transmitted frequencies is around 3-4 orders of magnitude with respect to background noise for airborne and groundborne data, respectively ( Figure 4).…”