Geomagnetic observatory data were analysed to obtain the spectral amplitudes and phases of the 27-day variation and its harmonics over a network of mostly European observatories, but extended to global scale by including some suitably distributed geomagnetic stations on other continents. The spectral amplitudes, estimated by different methods, were subjected to spherical harmonic analysis by expanding the geomagnetic potential function into a series of spherical harmonics up to the 2nd order and by calculating the corresponding coefficients. The contours of the H and Z components were then calculated to analyse their latitude-longitude dependence over European area. The moduli of electromagnetic response functions estimated from global data for individual harmonics were compared with the results obtained previously for European data only. The distribution of deviations of (Z/H) ratios defined by differences between the actual data and the theoretical fits were investigated. Using the theoretical basis of electromagnetic induction in stratified conductors, we estimated the depths of a perfect substitute conductor at those depths from characteristics of the 27-, 13-, and 9-day variations.