the vector e t is defined to be zero or at least to have its minimum value the effect of synthesizing e r by measurement of the source field instead of measuring e r by a sensor nominally aligned in a radial direction is to remove the effect of errors due to alignment category 1 above alignment errors are likely to dominate the accuracy of the measurement and trying to obtain alignment accuracy to even u00b1 2 u00b0 is time consuming by this method the sensor axes e x and e y can be emplaced quickly with nominal orientation achieved by a handheld compass and the spacing between sensors determined quickly and accurately by a simple tape measure or light based distance finder a further benefit of synthesizing measurement axes e r and e t is that by definition e t contains no bsem signal and so can serve as an interference and noise cancelling channel this method is particularly powerful because the interference e i and noise fields e n are not aligned with either e r or e t but are present in both channels in a coherent manner thus a standard coherency based cancellation algorithm can be used to reduce noise and interference in the bsem measurement channel e r based on the information content represented by the data e t a benefit of reducing noise and or interference in the primary measurement channel e r is that the power that is transmitted by the source can be reduced it is expected that the voltage applied in order to transmit the field at depth may be no more than 100 volts and in some instances no more than 50 volts and even no more.