Using Induction Tools to Identify Magnetic Formations and to Determine
Relative Magnetic Susceptibility and Dielectric Constant
Thomas Barber, Schlumberger Wireline and Testing Barbara Anderson, Schlumberger-Doll
Research Gordon Mowat, Schlumberger GeoQuest
Abstract: Modern induction tools measure the quadrature, or X-signals,
from the formation as an aid in skin-effect correction. Although the models
on which skin-effect corrections are based assume negligible formation
magnetic susceptibility, many sedimentary rocks contain magnetic minerals
that produce weak paramagnetism or diamagnetism. R-signal and X-signal
measurements are both a product of the magnetic permeability and the conductivity,
and the effect of very thick paramagnetic or diamagnetic formation layers
should be negligible. However, thinly layered magnetic formations can cause
significant perturbations on the X-signal, as has been observed on logs
from these tools.
X-signal logs contain significant information about the formation, which
can be extracted through a detailed analysis of the cause of observed perturbations.
In addition to indicating skin effect, the X-signal is also a measure of
the mutual balance of the tool. Commercial induction arrays are arranged
so that the direct mutual coupling from transmitters to receivers is canceled,
with the magnitude of total cancellation being on the order of 10,000 mS/m.
With a mutual-balanced array, magnetic layers can produce mutual imbalances
on the order of tens of millisiemens per meter. This imbalance is manifested
as high-spatial- frequency excursions on X-signal logs as the tool crosses
bed boundaries between layers containing magnetic materials. This magnetic
effect on the X-signal has been studied by modeling formations with magnetic
susceptibilities based on logs, cores, and tabulated data on rock magnetic
susceptibility. A simple iterative technique for deriving the relative
magnetic susceptibility of formation layers has been developed that gives
values for susceptibility that correlate well with core measurements. Uses
of susceptibility information include detection of iron-containing layers
and as an additional correlational tool.
Another electromagnetic parameter that can affect the induction X-signal
is the dielectric constant. Although dielectric constants are normally
negligible at induction frequencies, some minerals can produce very large
dielectric effects. The effects of high dielectric constant on the X-signal
logs are distinguishable from magnetic effects, and modeling can also be
used to estimate the dielectric constant.
Technical Note
Gamma Ray Log Response in Shaly Sands
Keith W. Katahara, ARCO Exploration and Production Technology
Abstract: This Note addresses the use of gamma ray logs for deducing shale content in shaly sands and reviews implications of some old but poorly appreciated results on gamma ray response. These results imply that a porous clean sand has a different gamma ray response than a nonporous formation with the same grain composition. A gamma ray response that is proportional to radioactivity per unit volume should not be used because it is significantly different from the more correct response that is proportional to radioactivity per unit mass. When shale composition is constant, the simple linear gamma ray shale index works well for typical shaly sands regardless of how the shale is distributed within the formation. Shale distribution effects alone cannot account for some of the curved gamma ray-versus-shale volume relations that have been observed.
Hydrostratigraphic Characterization of a Coastal Aquiferby Geophysical
Log Analysis, Cape Cod National Seashore, Massachusetts
Roger H. Morin, US Geological Survey, Daniel W. Urish, University of
Rhode Island, Department of Civil and Environmental Engineering
Abstract: The Cape Cod National Seashore comprises part of Provincetown,
Massachusetts, which lies at the northern tip of Cape Cod. The hydrologic
regime in this area consists of unconsolidated sand-and- gravel deposits
that constitute a highly permeable aquifer within which is a freshwater
lens floating on denser sea water. A network of wells was installed into
this aquifer to monitor a leachate plume emanating from the Provincetown
landfill. Wells were located along orthogonal transects perpendicular to
and parallel to the general groundwater flow path from the landfill to
the seashore approximately 1,000 m to the southeast. Temperature, epithermal
neutron, natural gamma, and electromagnetic induction logs were obtained
in five wells to depths ranging from 23 to 37 m. These logs identify the
primary contamination and show that its movement is controlled by and confined
within a dominant hydrostratigraphic unit about 2 to 5 m thick that exhibits
low porosity, large representative grain size, and high relative permeability.
A relation is also found between the temperature-gradient logs and water
quality, with the gradient traces serving as effective delineators of the
contaminant plume in wells nearest the landfill.
Contamination is not detectable in the well nearest the seashore and farthest
from the landfill, and the induction log from this well clearly identifies
the freshwater/seawater transition zone at a depth of about 18 m. The geophysical
logs provide fundamental information concerning the spatial distribution
of aquifer properties near the landfill and lend valuable insight into
how these properties influence the migration of the leachate plume to the
sea.
Ichnofacies Recognition in Turbidites/HemiturbiditesUsing Enhanced
FMS Images: Examples from ODP Leg 129
A. R. M. Salimullah and D. A. V. Stow, University of Southampton, Geology
Department, United Kingdom
Abstract: The correlation of characteristic trace or pattern in volcaniclastic turbidites with the deep marine ichnofacies is a highly complex and uncertain task, and even more complex in boreholes where core recovery is poor and there is a high degree of disturbance and fragmentation. In this study of ODP cores from Leg 129 in west central Pacific basins, FMS1 Formation MicroScanner images provided a continuous picture in the form of resistivity maps of the borehole walls. The sequential enhancement of these FMS images shows various mottling patterns caused by bioturbation. Detailed study of these enhanced FMS images has led to