Investigation Methods and Procedures - Geotechnical Engineering Investigation - page 64

and lakes. The ESP technique provides clear images in low-conductivity materials such as
sand, freshwater, or rock, and poor results in high-conductivity materials such as wet clay
because the penetration depth is limited by the strong attenuation of the signal.

Magnetometer Surveys
Applications
Magnetometer surveys are used for the detection of magnetic ore bodies or rocks that are
strongly magnetic, such as the crystalline types as differentiated from sedimentary types.
They are seldom used for engineering studies.

Theoretical Basis
Many rocks contain small but significant quantities of ferromagnetic materials which vary
with rock type. The weak magnetization modifies the Earth's magnetic field to an extent
that can be detected by sensitive instruments.

Operational Procedure
Magnetometers provide the measurements and, when towed behind aircraft, they can
cover large areas and provide appreciable data in a relatively short time.

Data Presentation
Contour maps are prepared showing lines of equal value that are qualitatively evaluated
to locate anomalies indicative of ore bodies or rock-type changes.

Gravimeter Surveys
Applications
In their normal geologic application, gravimetric or microgravity surveys, are used for the
detection of major subsurface structures such as faults, domes, anticlines, and intrusions.
Gravimetric surveys have been used in engineering studies to detect cavities in limestone
and the location of old mine shafts (Ghatge, 1993). Modern instruments are extremely sen-
sitive, however, and the requirement for the precise determination of surface elevations
may cause the application of the method to be relatively costly.

Theoretical Basis
Major geologic structures impose a disturbance on the Earth's gravitational field. The part
of the difference between the measured gravity and theoretical gravity, which is purely a
result of lateral variations in material density, is known as the Bouguer anomaly. Other fac-
tors affecting gravity are latitude, altitude, and topography, and have to be require con-
sidered during gravitational measurements to obtain the quantity representing the
Bouguer anomaly.
Gravimeters consist of spring-supported pendulums similar in design to a long-period
seismograph ( Section 11.2.3).

Data Presentation
Isogal maps are prepared showing contours of similar values given in milligals (mgal) to
illustrate the gravity anomalies. (Note: 1 mgal 0.001 gal; 1 gal acceleration due to grav-

ity 1 cm/s 2 .)