Princeton Geoscience, Inc. provides borehole geophysical logging services to our clients and consulting industry colleagues, bringing improved understanding of the subsurface conditions important to decision-making at project sites. The services are used in environmental site remediation, geotechnical engineering and water supply. The logging process involves deploying an array of downhole instruments to record properties of the soil and rock adjacent to the borehole and of the fluid within the borehole. These data are recorded in digital format, and used in concert to define and interpret features, including:

Bedding orientation

Fracture depths/orientations

Borehole diameter

Casing depth

Water-producing zones

Vertical cross-flows

Bedding Orientation: Bedding orientation is important at fractured bedrock sites in the Newark Basin, where the principal flow zones of the Leaky, Multi-unit Aquifer System (LMAS) are oriented parallel to bedding. For example, dip angle should be known to within about one degree, or horizontal projections will be inaccurate over distances relevant to site remediation projects (e.g., hundreds of feet or more). It is frequently difficult to ascertain dip with this level of accuracy using

traditional methods. Geologic map coverage may be inadequate and visually evident markers absent from rock cores (Newark Basin rocks include intervals of apparently featureless mudstone).

The Natural Gamma log, which measures the natural radiation of the geologic materials adjacent to the well casing and screened or open-hole interval, is particularly useful for correlating rock or sediment types from well to well. In Example 1 (below), overall log character and the three marker lines placed on the logs support the conclusion that individual rock units are encountered about nine feet deeper in Well B than in Well A. Based on known ground elevations and depths to marker

units at three or more locations, bedding orientation (strike and dip) can be accurately determined.

Fracture Depths / Orientations: The depths at which fractures occur is commonly evaluated based upon rig performance during drilling, or by inspection of bedrock cores. However, drilling observation provides no information about fracture orientation and unless oriented cores are collected (rarely done and costly), only dip magnitude (not dip direction) can be determined by coring. And incomplete sample recovery is common with coring, especially in the fractured zones typically of interest in evaluating contaminant migration and control measures.

Example 2 (following page) is a plot of the Caliper, Acoustic Televiewer (ATV) and Optical Televiewer (OTV) logs recorded from a well installed in calcareous mudstone. The Caliper log gages borehole diameter, and can be used to interpret depths at which fractures intersect the borehole. A fracture is evident on the caliper log in Example 2 at a depth of 51 feet, based on a sharp increase in borehole diameter. However, conditions other than fracturing, such as lithology changes and drilling effects, can cause borehole diameter enlargement. Additional methods to assess fracture occurrence can therefore be useful, and the ATV and OTV logs serve this purpose very well.

The ATV and OTV logs are presented as vertically “unrolled” images of the borehole wall, oriented to North (0°). In this

Borehole Geophysical Logging: Tools and Techniques for Systematic Characterization

and Remediation of Complex Sites

projection, planar fractures or bedding units intersecting the borehole wall show as sinusoidal traces, with amplitudes proportional to dip magnitude, and their lowest limbs positioned toward the direction of dip. The OTV is based on photographic images, and can be used in water– or air-filled portions of the borehole, but water must be clear. The ATV can only be used in water-filled portions of the borehole, but can provide useful data in turbid water intervals where the OTV cannot be used.

In Example 2, the ATV and OTV logs confirm the presence of the fracture at 51 feet and identify two additional fractures. Compositional layering within the basalt is evident on the OTV log. A structural projection is made reflecting the log analyst’s interpretation, and strike and dip are provided in notes for each interpreted planar feature.

Water-producing Zones and Cross-Flows: Understanding fracture occurrence is important, but it’s also necessary to identify which fractures are hydraulically active within a borehole. This can

be difficult to deduce from traditional methods such as drilling observations and coring. Fluid Temperature and Resistivity logs and Heat-Pulse Flowmeter (HPFM) logs can help with this assessment.

In Example 3 (below), a hydraulically active fracture is evident at a depth just below 42 feet, based on the Caliper log and the strong deflection in Fluid Resistivity at this depth, with water of different and nearly uniform resistivities above and below the fracture. The resistivity change occurs because water of different chemical quality enters or

exits the well at this fracture, but it cannot be determined whether inflow or outflow takes place.

In Example 4 (next page), HPFM testing at depths between fractures measures the direction (up or down) and magnitude (in gallons per minute) of flow within the well under non-pumping conditions. Interpretation of this record indicates:

No vertical flow at top of well

Inflow starts at fracture at 23’ depth, below which downward flow of 0.25 gpm is indicated

More inflow from fracture at 45’ (downflow increases to 0.37 gpm)

Outflow starts at fracture at 65.5’ (downflow decreases to 0.12 gpm)

No vertical flow below exit zone fractures at 83’ and 86’

Broader-scale conclusions and implications for remediation based on the OTV and HPFM logs in Example 4 include:

The observed downward flow indicates a downward hydraulic gradient exists in the aquifer units intersecting the wellbore

If shallow contamination is present, the well should be abandoned or lined (e.g., with a Flute® liner) to prevent vertical cross-contamination of aquifer sub-units

Conductive fractures are present at depths of 23’, 45’, 65’ and 83’, mostly parallel to the shallow-dipping bedding planes. Tracking contaminant extent based on bedding orientation (i.e., along strike direction) is likely appropriate.

Most of the water is exchanged between fractures at 23’ and 65’. These could be appropriate target intervals if this well were to be constructed as a monitoring well.

Princeton Geoscience, Inc.’s Logging Services: The attached chart (next page) shows specific applications for logging services which Princeton Geoscience can provide, indicating which tools are useful for particular applications and the borehole conditions required. Effective interpretation almost always involves looking at results from more than one log type. Fortunately, many probes are capable of logging multiple parameters during a single logging run.

Princeton Geoscience owns equipment manufactured by Mount Sopris Instrument Company (MSI), a pioneer and world leader in the design and manufacture of “slimline” borehole logging equipment used in the water resources, mining, environmental and geotechnical engineering fields. Data acquisition is accomplished using MSI’s MATRIX Logger and software in conjunction with a field computer running Microsoft Windows 7.

Our MX Series winch holds 700 meters of single conductor cable, and most applications in the environmental, geotechnical and water supply fields are within reach. In addition, the winch and other logging equipment are semi-portable and can be run with power from a small generator, allowing access over difficult terrain or to remote locations.

Princeton Geoscience utilizes MSI’s time-tested Poly-Series logging probes. We own and have received MSI training in the use of probes for logging the following parameters of common interest in environmental and water supply investigations: Caliper (3-arm), Natural Gamma, Spontaneous Potential, Normal Resistivity (8”-16”-32”-64”), Single Point Resistance, Fluid Resistivity, Fluid Temperature, HPFM. In addition, we routinely provide ATV and OTV logging, using probes rented from a local vendor with whom we have a well-established business relationship. And we can conduct other specialized logging (e.g., narrow-diameter natural gamma) using probes rented from MSI.

Logs are prepared in a variety of formats, according to client and project needs. In addition to MSI’s Logger Suite, Princeton Geoscience utilizes WellCAD software. WellCAD is developed and distributed by MSI’s affiliate, Advanced Logic Technology (ALT), who also are designers and manufacturers of the Optical- and Acoustic Televiewer probes sold and rented by MSI.

WellCAD is a widely used and powerful software package that enables robust presentation of logging data, and output in many formats convenient for use in other programs. Princeton Geoscience’s WellCAD license includes the Reader, Crossplot, Browser, Image and Deviation modules, enabling full, accurate and efficient use of a full range of logging tools.