Geophysical Fieldcraft: Mastering Discrimination and Pinpointing with Digital Detectors

Operating a metal detector effectively is a skill that bridges the gap between technology and technique. While the hardware provides the raw data, the user’s “fieldcraft”—the methodical application of the device—determines the success of the survey. A metal detector is essentially a remote sensing device, and like any scientific instrument, it requires calibration and proper handling to yield valid results.

The PalliPartners 970B Metal Detector offers a suite of digital tools designed to interpret the subsurface environment. However, these tools rely on the operator maintaining a consistent geometric relationship between the search coil and the ground. Variations in sweep speed, height, and angle can introduce variables that the internal algorithms cannot fully compensate for. This article explores the best practices for utilizing discrimination modes and the physics-based logic of the pinpointing function.

The Geometry of the Electromagnetic Field

The search coil projects an electromagnetic field that is roughly the shape of a semi-circle or cone extending into the ground. To ensure complete coverage, the operator must sweep the coil parallel to the ground surface. A common error, “pendulum swinging,” involves lifting the coil at the end of each sweep. This varies the detection depth and can cause the detector to miss targets or misinterpret ground mineralization as a signal.

Keeping the coil level and close to the ground (about 1 inch) ensures maximum coupling between the magnetic field and potential targets. The “All Metal” mode is the baseline for this survey phase. It operates without filters, providing the maximum depth and sensitivity. In this mode, the detector acts as a raw motion sensor; it requires the coil to be in motion relative to the target to induce the eddy currents necessary for detection. If the coil stops moving, the signal disappears. This is a fundamental characteristic of VLF motion discrimination circuits.

Logic: Discrimination and Notch Filtering

Once a target is detected, the challenge becomes identification. This is where “Discrimination” (Disc) and “Notch” modes apply specific filters to the signal. Discrimination works like a high-pass filter, progressively ignoring targets with lower conductivity/phase shift (like iron) while accepting higher ones. This is useful for ignoring rusty nails while hunting for coins.

“Notch” mode offers higher granularity. It allows the user to selectively reject or accept specific segments of the conductivity spectrum. For example, a user might choose to notch out the specific range associated with aluminum pull-tabs while keeping the ranges for iron (relics) and silver (coins) active. The PalliPartners interface facilitates this by allowing users to toggle specific target ID segments on the LCD. However, users must be aware that targets with similar conductivity (e.g., a gold ring and a pull-tab) often share the same phase shift signature. Over-aggressive discrimination can lead to missed valuables.

Precision: The Physics of Pinpointing

Locating the exact position of a target before digging is crucial to minimize ground disturbance and potential damage to the find. The “Pinpoint” (PP) mode functions differently from the motion modes. When PP is engaged, the detector switches to a “static” or “non-motion” DC induction mode.

In this state, the detector does not filter out the ground signal in the same way, nor does it require coil motion to generate a sound. Instead, the audio pitch and volume modulate directly based on the signal intensity. As the center of the coil (the “hot spot”) approaches the target, the signal strength peaks. By moving the coil slowly in an “X” pattern over the suspected area, the operator can identify the point of highest intensity, which corresponds to the target’s vertical axis. This static mode effectively measures the proximity of the metal to the center of the coil’s windings, allowing for surgical recovery of the object using the included shovel.

Industry Implications: The Citizen Scientist

The proliferation of affordable, high-accuracy detectors is transforming hobbyists into citizen scientists. Detectorists are increasingly responsible for recovering historical artifacts and cleaning environmental metallic waste. The shift towards digital displays and intuitive interfaces lowers the barrier to entry, allowing more individuals to engage in this form of geophysical exploration. As technology continues to miniaturize professional-grade DSP features, the line between recreational detectors and professional survey tools continues to blur.