Geomagnetic Methods
3.9 Applications and case histories
3.9.6 Acid tar lagoon survey
FEET CENTIMETRES
1 2
100
4 6
200 400 600
10 20
800 1000 40 60
2000 80 100
3000 4000 150
DISTANCE FROM MAGNETOMETER
nT
2 3 4 5 10 20 30 40 50 100 200 300 400 500
200 1 POUND OF IR
ON 1 KILOGRAM OF IR
ON BA
R MA GNET (5000 cgs U
NITS) 100 POUNDS
OF IR ON
100 KILO
GRAMS OF IR
ON 6 INCH
PIPELINE 12 IN
CH PI PELINE 1 T
ON OF
IR ON
Figure 3.65 Minimum detectable anomaly amplitudes for different types of ordnance at various depths of burial. Note that the distances cited are those between the sensor and the target, not the depth below ground of the target. From Breiner (1981), by permission.
mapping (Chapter 11) is also useful to help to define boundaries and internal zones. Cross-sections through landfills can be helpfully achieved using electrical resistivity tomography (Chapter 7).
An example is given of an integrated geophysical investigation undertaken over a closed landfill site in the UK. The three geophys- ical methods mentioned above were used and each demonstrated different aspects of the former landfill. Whereas the brief from the client was to identify the landfill boundary, it proved possible to not only do this but also to differentiate different zones within the land- fill on the basis of the electrical and magnetic anomalies measured.
The total magnetic field intensity anomaly map of part of the site is shown in Figure 3.66A. The interpreted outline of the landfill is shown along with the internal division boundaries identified from the geophysical survey. It was thought that different types of waste materials had been dumped through the active period of operation of the site, giving rise to distinct geophysical zonation. The mag- netic anomalies indicated areas with large numbers of buried steel drums. The ground conductivity anomaly map for the same area is shown in Figure 3.66B, and one resistivity cross-section is illustrated in Figure 3.66C. Differences in the apparent conductivity anomaly maps using a Geonics EM31 ground conductivity meter indicated possible lateral migration of leachate southwards at shallow depths.
The areas of buried drums also coincide with areas with elevated apparent conductivities. The corresponding zones are also clearly seen in the resistivity section.
Apparent conductivity (mS/m)
Key
Interpreted landfill boundary Landfill division boundary (A , A , , A B) Main concentrations of metal (drums) Uncertain boundary of metal concentrations Shallow anomaly
Anomaly at depth Large target Small target X
B A
A A
X X X
X X
X XX X
X
XX XXX X XXXX
X XXX
X X X X
X
R8 R6
R7
R6 R5 R4 R3
R2 R1
X
Key
Interpreted landfill boundary Landfill division boundary (A , A , A , B) Zone of non magnetic and high conductivity Zone of high conductivity
A3
B
A1
A2
R8 R6
R7
R5 R4 R3
R6
R1
R2 R
?
?
?
?
? (B)
(A) 400
Path (um) Path (µm)
375 350 325 300
450425400375350450425400375350 450425400375350450425400375350
425 450 475
400 375
350 325
300 425 450 475
400 375
350 325
300 425 450 475
500 525
400 375 350 325
300 425 450 475 500 525
Magnetic field strength (nT)
R9 R8 R7
R1
Resistivity (Ωm)
R7 intersection point R8 intersection point R9 intersection point
Outer Zone of B
A A B
S N
Elevation
(C)
52.0 50.0 48.0 46.0 44.0 42.0 40.0 38.0 36.0 34.0
0.29 0.8E 2.6 23.6 70.6 211 632
60.0 100 34.0
0.0
7.9
Figure 3.66 (A) Magnetic anomaly map over a closed former landfill with (B) the corresponding apparent conductivity anomaly map from a Geonics EM31 (vertical dipole), and (C) an electrical resistivity section along the profile indicated in A and B. The interpreted outer boundary of the landfill is shown along with internal divisions within the landfill based on the geophysical anomaly patterns from the three different techniques. [C]
(A)
Waterproof boxes containing cabling &
logging unit / batteries
Winch cables
Geodimeter ‘Active’
Autolock prism
Radio modem
and antenna 1.98 m forward
offset
Geometrics G858 Antennas in vertical gradient mode
Winch cables Catamaran hulls
Small car battery to power radio modern
(B)
Figure 3.67 Details of the Geometrics MagCat platform: (a) instrument deployment offset diagram, and (b) the equipment in use. A vertical gradiometer (two white sensors) is mounted on the dual surf boards with an active prism at the rear (for position fixing) and telemetry aerial (for remote communication with the shore station). From Reynolds (2002), by permission. [C]
spatial sampling interval of<0.2 m along each profile. The whole winch assemblies were moved along each opposite shore to provide a line interval of 2.5 m.
Measurements of total magnetic field intensity at each sensor were captured (e.g. Figure 3.68A) and the resulting vertical gradi- ent obtained. Maps of the magnetic parameters provided a spatial perspective across the lagoon from which it was possible to iden- tify the locations of drum graves and, in many cases, individual drums, as well as, importantly, areas without any drums present.
Each magnetic profile was modelled using commercially available software in order to locate the position of the magnetic targets along each transect and its depth below the lagoon surface. As well as a vi- sual inspection of each magnetic anomaly identified, commercially- available software for the identification of UneXploded Ordnance
(UXO) was also used to help to define the location of the magnetic targets (see next section). The resulting output was a map (Figure 3.68B) showing the interpreted locations of the drums and exclu- sion zones around them in which there was a risk of encountering a drum (or drums) in case any subsequent intrusive work was to be undertaken. It was also noted during this project that the magnetic signature associated with the drums was also strongly influenced by the amount of degradation of the metal through corrosion, with the weakest signatures being associated with the most corroded drums. This aspect and that of the orientation of each drum with respect to the Earth’s magnetic field will result in a variety of dif- ferent anomaly characteristics. The implications of these factors has been considered by Emerson et al. (1992) and Furness (2002, 2007).
Location of short wavelength, high amplitude anomaly.
Location of short wavelength, low amplitude anomaly.
Location of long wavelength, high amplitude anomaly.
Location of long wavelength, low amplitude anomaly.
Drum grave
50 m 50 m
(A) (B)
N
N
Figure 3.68 (A) Magnetic anomaly map (bottom sensor) from the magnetic gradiometer and (B) the corresponding map of interpreted zones of influence of steel drums and drum graves. From Reynolds (2002), by permission. [C]
In addition to the magnetic survey, a seismic refraction investi- gation was also undertaken (see Chapter 5, Section 5.5.5). Where both magnetometry and seismic refraction data were obtained over the same transects, it was possible to integrate the analysis. From this it was possible to identify the locations of the drums and to see where within the tar they may be located. One such integrated profile is shown in Figure 3.69. It was clear that the drums tended to be located in the tar that had the lightest viscosity (as evidenced by the lowest seismic P-wave velocity values).