Airborne Electromagnetic Sea Ice Thickness Sounding in Shallow, Brackish Water Environments of the Caspian and Baltic Seas

Ice engineering projects often rely on the knowledge of ice thickness in shallow, brackish water like in the Baltic and Caspian Seas. By means of field data and model results, the paper shows that helicopter-borne electromagnetic induction measurements using frequencies of 3.68 and 112 kHz can yield accurate thickness estimates with salinities as low as 3 ppt. The higher frequency yields the strongest EM signals. In addition, in shallow water the higher frequency is less sensitive to the sea floor signal, and can thus be used in water depths as shallow as 4 to 6 m, depending on flying altitude. Because the low frequency signal is very sensitive on shallow water depth, a combination of both signals will allow the retrieval of both ice thickness and water depth. INTRODUCTION Sea ice thickness is one of the most important parameters for ice engineering problems and climate studies. Apart from ice profiling sonar (IPS) measurements, electromagnetic induction (EM) thickness sounding has become an accurate and efficient method for thickness profiling (see description of method below) and can be operated on the ice, from ships (Haas, 1998; Haas et al., 1999), or from structures like lighthouses or oil rigs (Haas and Jochmann, 2003). However, EM sounding is most powerful when operated from helicopters (Kovacs and Holladay, 1990; Prinsenberg and Holladay 1993, Haas, 2004) or fixed-wing aircrafts (Multala et al., 1996). The accuracy of EM thickness sounding increases with the salinity of the water below the ice (see below and references above). Unfortunately, in many key regions for ice engineering activities the salinity of the water is very low, like in the brackish waters of the Baltic and Caspian Seas. Haas (2004) has shown that accurate EM measurements are still possible down to sea water salinities of 3 ppt. In addition, there are large regions with very shallow water less than 10 m deep. Because the lowfrequency electromagnetic fields generated by EM instruments penetrate into the water (see below), they can also reach the sea floor and contribute to the measured secondary field strength. This could result in overestimates of ice thickness if the water depth is unknown or if the wrong signal frequencies are used (see below). On the other hand, induction in the sea floor enables to determine both ice thickness and water depth (Kovacs et al., 1987). Here we show an example of EM thickness measurements in the shallow water of the Bay of Bothnia in the northernmost Baltic Sea using a small helicopter EM sensor, and some model results. They demonstrate the particularities of EM sounding in brackish water and that accurate ice thickness measurements are still possible with very low salinities of 3 ppt. In addition, it will be shown by means of model calculation and the data that EM ice thickness measurements in shallow water less than 10 m deep are possible with instruments using high frequencies. METHODS The measurements discussed here have been performed with a helicopter EM sensor on February 5, 2004, in the Bay of Bothnia south of the Finish island of Hailuoto (Figure 1). They were carried out in the framework of an international, 1.5 month long ice thickness monitoring program funded by the Ice Ridging Information for Shipping (IRIS) project of the European Union. The specific profile is discussed here because it crosses the 10 m isobath noted in the Bathymetric charts of the Baltic Sea Research Institute (IOW, Warnemünde, Germany). According to a Finish Hydrographic Office sea chart, water depths are only slowly increasing westwards to 20 m, and range between 10 and 4.4 m to the east. Figure 1: Map of the flight track in the Bay of Bothnia (red), with the 10 m isobath (grey) and the island of Hailuoto in the right top. Helicopter EM thickness sounding An EM system consists of an assembly of coils for the transmission and reception of low-frequency EM fields, and a laser altimeter. The EM components are sensitive to the sensors height above the conductive sea water surface, while the sensors altitude above the ice or snow surface is determined with the laser altimeter. Over sea ice, the water surface coincides with the ice underside. Therefore, the difference of the height measurements of both components corresponds to the ice-plus-snow or total thickness (Figure 2; Haas, 1998). We used a small, lightweight, helicopter-borne EM Bird, 3.5 m long and weighing 100 kg. It was suspended 20 m below the helicopter and towed at heights of 10 to 20 m above the ice surface. The EM bird operates at frequencies of f1 = 3.68 and f2 = 112 kHz, with a coil spacing of 2.7 and 2.1 m, respectively. Signal generation, reception, and processing are fully digital, maximising signal-to-noise ratio. The EM system is calibrated by means of internal calibration coils with a known response. EM sampling frequency is 10 Hz, corresponding to a measurement point spacing of approximately 3 to 4 m. Measurements are interrupted every 15 to 20 minutes by ascents to high altitude, to monitor electrical system drift. For the thickness computation shown in this paper we used the in-phase and quadrature components (real and imaginary parts) of f1 and the inphase component of f2. Thickness was retrieved form each channel individually by means of a one-dimensional inversion of EM Bird height above the water surface (see below). Ice thickness is computed by subtracting the laser height measurement over sea ice from the inverted height above the water surface which coincides with the ice underside (Figure 2; Haas, 1998). Comparison with drill-hole data shows that the EM derived ice thicknesses agree well within ±0.1 m over level ice. However, the accuracy is worse over ridges. Because the low-frequency EM field is diffusive, its strength represents the average thickness of an area of 3.7 times the instruments altitude above the ice surface (Kovacs et al., 1995; Reid et al., 2005). Due to this “footprint”, the maximum ridge thickness can be underestimated by as much as 50% in the worst cases, depending on the geometry and consolidation of the ridge keel (Haas and Jochmann, 2003). EM modelling The EM signal over any underground can be exactly calculated if the conductivity distribution is known (Kovacs et al., 1987; Ward and Hohmann, 1988). Here, we use a one-dimensional model with horizontal layers of air, ice, water, and seafloor. The EM response is given as relative secondary EM field strength, the quotient of secondary and primary magnetic field strength Hs/Hp. We have performed two different model runs, where the conductivities of the ice and water have been set to 0 and 340 mS/m, respectively, representing typical conditions in the Bay of Bothnia and Caspian Sea: First, as the ice thickness measurement is essentially a measurement of bird height above the water surface (Figure 2), we calculate the EM signal just for a simple model of varying bird height above an infinitely thick layer of water. In this case, ice thickness is set to zero and due to the infinitely thick water layer, the seafloor is not sensed by the EM measurement. Second, we investigate the effect of shallow water on the EM signal, with an ice thickness of 0 m and an infinitely thick layer of sediment or rock with a conductivity of 10 mS/m, representative of the sea floor. It should be noted that this is a worst case scenario, as seafloor conductivities can be much higher in most cases, and therefore the seafloor signal would be more similar to the seawater signal. With the model computations, the full seafloor response can be computed individually for each channel. In the EM sounding community, it is also common to calculate an approximate penetration depth which is called the skin depth, dependent on signal frequency f and the layers