At its simplest it is apparent that an aerial produces the strongest response when it is as close as possible to the buried conductor and in correct alignment with it, i.e. at right-angles to it and directly above it.
This shows the difference in response as the receiver traverses over the line through positions 1, 2 and 3. The response at position 2 is greatest not only because of the proximity of the aerial to the line, but also because of the full alignment of the aerial core with the field direction, so that it is linked with a higher proportion of the field. Because the response is related to the orientation of the aerial core relative to field direction it is also related to line direction. So a horizontal aerial gives two pieces of information - line position and its direction. This makes a horizontal antenna preferable for general sweeping, locating and tracing work.