Geophysics foundations:    
Quick overview:  
Outline of gradient magnetics

Introduction

Most modern instruments are capable of collecting magnetic field data using two sensors for gradient surveys. The vertical gradient is measured using two sensors at (typically) 2 and 3 metres above the ground. Horizontal gradient surveys can be conducted if the sensors can be mounted some distance apart on a frame.

Gradient measurements

When buried objects are the target, geophysical surveys must usually detect features with high magnetic susceptibility and/or high electrical conductivity. Some objects will be magnetic, but others have negligible magnetic susceptibility (such as aluminum or some forms of stainless steel). When the buried targets are expected to be magnetic, measurements of variations in the strength of Earth's magnetic field often produce excellent results. However, because of the many ferrous objects and electrical sources of magnetic fields under and around industrial sites, total field anomaly maps may be too complicated to interpret, or subtle variations may be overwhelmed by larger features. A gradient survey is often a better choice under these conditions because the magnetic field gradient varies more rapidly than total field strength and it, therefore, provides higher spatial resolution. This is illustrated by the interactive figures below.

In addition to higher spacial resolution, temporal variations are automatically eliminated because the measured parameter is a difference of two total field measurements. Therefore, the base station measurements and subsequent data corrections normally required for total field surveys are not required. If the goal is to map variations in geological materials, then more subtle trends in magnetic response must be observed.

Data acquisition

Data acquisition of total field magnetics or gradient magnetics is very rapid. For finding buried objects, simple anomaly detection is often adequate since the depth of burial and quantitative estimates of physical properties may be unimportant. Under these conditions, rapid acquisition of spatially dense data sets is usually required, and results are often presented with minimal processing. The most important survey design consideration is to avoid spatial aliasing. For small 3D targets such as buried drums or other objects, a tightly spaced grid would be required; while for 2D targets such as buried utilities, data spacing along profile lines would likely be much tighter than spacing between lines. This assumes that lines can be placed perpendicular to target orientation.