measured porosity values. Thermal neutron measurements also may be influenced by the

presence of hydrogen or chlorine bound in the lattice structures of clay minerals, micas, and

other hydrogen/chlorine bearing minerals. A thermal epithermal neutron tool is a dual

compensated, dual porosity tool that detects interacted neutrons in the thermal and (higher

energy) epithermal ranges. Because of the higher (epithermal) neutron energy levels, the

impacts of this tool include significantly improved neutron counting statistics in the

epithermal range, and less affected porosity values by neutron attenuators. The vertical

resolution of the neutron tool is approximately 2 feet with processing enhancement possible to

1 foot. The depth of investigation of the tool is a function of porosity and typically is in the

range of 10 to 12 inches.

Limitations associated with nuclear logging methods are related to the correction of

the logs for borehole parameters in the absence of tool compensation, the need to handle and

operate devices containing radioactive source material in an underground environment, and

the effects of radiation moderators in the borehole environment. Additionally, formation

porosities derived from gamma gamma and neutron measurements are dependent on

knowledge of the formation matrix density (not bulk density) which in many cases is

estimated in the absence of physical measurements. In formations where the matrix density is

significantly different from the response density utilized by the tool operator, the calculated

porosity may be in error. This is known as the matrix effect.

Sonic Methods

The sonic log is a recording of the transit time of an acoustic pulse through a

formation between a series of acoustic transmitters and receivers in a sonic probe as a

function of depth in a borehole. Application of the sonic tool in a borehole is analogous to

the surface seismic geophysical technique. Many of the tools commonly used for engineering

or ground water investigations are simple devices capable of providing detailed compressional

and shear wave velocity measurements. Multiple transmitter receiver sonic tools are available

for larger scale applications and provide greater vertical resolution of the formation and

enhanced delineation of the sonic waveform at later arrival times along the wavetrain. Full

wavetrain recording allows for extraction of information from the deeper sections of the

waveform such as the delineation of stonely wave arrivals. The measured interval travel

times ( sec/ft) are functions of the transmitter to receiver distances and the competence of the

measured formation. Computations using sonic interval transit times (compressional and

shear) are used to calculate porosity, formation dynamic elastic moduli, compressibilities

(bulk, rock), poisson's ratio, tensile strength, fracture pressure, and minimum horizontal stress.

Sonic log measurements are commonly obtained in open, fluid filled boreholes,

although cased hole measurements are used to evaluate cement bond integrity. In instances

where the cement bond between the formation and casing is adequately high, the sonic log

may be used to evaluate formation properties through the casing (Keys and MacCary, 1971).

November 1992

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