Magnetic Profiling

Outline | Detail | Results |

OUTLINE

Magnetic profiling is a passive method that involves measurement of localised variations in the amplitude of the geomagnetic field resulting from buried ferrous targets, such as underground storage tanks and pipes, and variations in the magnetic susceptibility of near surface materials. The amplitude and shape of the anomaly caused by buried ferrous objects will depend on the target's shape, orientation and susceptibility. Modern magnetometers are capable of measuring the field to 1 part in 5 million (0.01nT), enabling detection of small scale ferrous targets, such as unexploded ordnance, and subtle features, such as those resulting from archaeological activity (e.g. walls/kilns). Up to 10 readings can be acquired per second enabling rapid reconnaissance of a site.

DETAIL

Magnetic profiling is a passive technique that involves measurement of the total amplitude of the Earth's magnetic field. Buried ferrous materials result in a localised reduction or increase in the amplitude of the field (depending on their orientation and shape) and the technique is therefore invaluable in the detection of targets such as underground storage tanks, pipes and ferrous waste. Magnetic surveys are carried out using a range of highly sensitive magnetometers which can measure the amplitude of the field to within as little as 1 part in 5 million (0.01nT). It is common in most surveys to additionally measure the vertical gradient of the field using a second sensor mounted above or below the first. Vertical gradient data is especially sensitive to near-surface magnetic disturbances and improves resolution of individual anomalies in a general area of magnetic anomalies.

Typically optically pumped caesium vapour magnetometers are used for high-resolution environmental and engineering surveys. These instruments have an inherent accuracy of 0.01nT in the Earth's magnetic field and an extremely high gradient tolerance. The latter feature enables them to be used successfully in culturally noisy settings as are commonly found in site investigation work. Fluxgate magnetometers are also used for small-scale detailed archaeological prospecting.

Advanced processing of magnetic data enables accurate location of the source of individual anomalies both laterally and with depth. To assist with the depth determination of ferro-metallic objects in the subsurface, 3D Euler Deconvolution is applied to the gridded magnetic data.

In addition to 3D Euler Deconvolution, the 3D analytic signal is also calculated for total magnetic field data. The analytic signal produces a maximum over the source of a magnetic anomaly regardless of the direction of magnetisation and consequently enables more accurate location of the magnetic targets.

RESULTS

The results of magnetic profiling surveys are presented as scaled colour-coded plots similar to those used in EM profiling (right). Anomalies will typically be either dipolar or monopolar depending on the orientation of the source, but may also coalesce to form clusters of irregular anomalies if the targets are closely spaced. This is apparent in the bottom right of the image over an area of buried scrap metal. Better definition of individual anomalies in a cluster is provided by vertical magnetic gradient data.

The plot, right, illustrates a profile over a single dipolar total magnetic field anomaly. Profiles can be used to provide an initial indication of the approximate depth of the source of the anomaly whilst dipole matching algorithms can be used to calculate approximate target volumes. The latter is particularly useful in the detection of buried UXO.

Microgravity Profiling

Outline | Detail | Results |

OUTLINE

Microgravity profiling is a passive technique that involves highly accurate (to within 1 part in 109 (1mgal)) measurement of relative changes in the Earth's gravitational field. Measurements are made using a microgravity meter, which essentially comprises a highly sensitive temperature stabilised spring balance. Subtle changes in gravity result from variations in the density of materials within the subsurface and the method can therefore be used to successfully locate voids or buried features such as underground storage tanks (USTs). The effects of tidal and instrument drift that would otherwise mask any subtle anomalies are overcome by repeat readings at a fixed base station throughout the survey. Accurate topographic levelling is carried out at each station in order to correct for the effects of terrain.

DETAIL

Microgravity profiling is a passive technique that involves highly accurate (to within 1 part in 109 (1mgal)) measurement of relative changes in the Earth's gravitational field. Subtle localised changes in gravity result from variations in the density of subsurface materials or the presence of buried engineering structures. As a consequence the method can be used to successfully locate voids (tunnels/mineshafts or solution features) or dense structures such as underground storage tanks (USTs) and dense mineralised (usually base metal) orebodies.

A variety of other factors affect the amplitude of the gravitational field measured at a particular location and time and in order to identify the often subtle anomalies that are of interest in engineering and environmental studies, careful correction of the raw data is required. The effects of instrument drift and earth tides are overcome by repeat readings at a fixed base station throughout the survey in order to build up a drift curve. Drift corrections are then calculated based on the closure error between the first and last base reading in each loop. Tidal effects can alternatively be calculated using computer based programs or published tables of tidal gravity corrections and removed from the data prior to the drift correction.In order to account for the increase in the acceleration due to gravity towards the poles a correction for latitude is applied using the 1967 International Gravity Formula. The latitude correction is made by subtracting the theoretical gravity, calculated for each station location, from the measured value. The Free Air Anomaly (FAA) takes account of the elevation of the station above the spheroid and requires accurate topographic levelling of all the survey stations. The Bouguer Anomaly is calculated from Microgravity meters essentially comprise highly sensitive temperature stabilised spring balances. The spring normally comprises either a fine metal (helically wound) or a quartz rod. GSI (UK) Ltd. commonly use both semi-automated digital (Edcon Super-G and Scintrex CG3 and CG3-M) as well as analogue (Lacoste-Romberg Model D & G) gravimeters.

RESULTS

The results of a microgravity survey are presented either as scaled single profiles or colour-coded maps and contour plots of the reduced data. Three levels of data reduction may be provided:

1. Bouguer corrected data: includes all reduction processes except the local terrain correction

2. Complete Bouguer corrected data: includes the local terrain correction

3. Residual Bouguer corrected data: includes removal of regional gravity trend through spatial filtering of the Bouguer/ Complete Bouguer corrected data.

Inverse modelling of individual profiles is carried out in order to fit the observed data to a geological model. Groundtruth information is included at this stage to help constrain the model and provide a more accurate fit. The results of modelling are displayed as profiles of the observed and calculated gravity anomaly against distance (top) above a density model of the sub-surface (bottom).