Sampling Interval: MWD Time Sampling, Wireline Depth Sampling, and Log Resolution in WCSB Wells

The sampling interval is the spacing, in either depth or time, between successive measurements taken by a sensor as it records a log. It is one of the most consequential acquisition parameters in formation evaluation because it sets the finest detail a log can resolve: features thinner than the sampling interval are blurred or missed entirely, while an unnecessarily fine interval inflates data volume and, for some tools, slows logging or strains telemetry. The convention differs by measurement method. For wireline measurements, where a tool is pulled up the hole on cable at a controlled speed and a depth wheel tracks position, the sampling interval is most commonly a depth, traditionally every 0.1524 m (half a foot) or finer at 0.0254 m (one inch) for high-resolution imaging and dipmeter tools. For measurement-while-drilling (MWD) and logging-while-drilling (LWD) logs, where sensors sit in the bottomhole assembly and record as the bit advances, the sampling interval is most commonly a time, because the tool samples on its own clock and the data are later merged with surface depth based on the rate of penetration. A sensor sampling every 10 seconds while the bit drills at 30 m/hr lays down a reading roughly every 0.083 m, but if the rate of penetration doubles, the same time interval yields one reading every 0.167 m and the effective depth resolution halves. This depth-versus-time distinction is the source of many log-quality issues in fast-drilled Western Canadian Sedimentary Basin horizontals, where high rate of penetration in the Montney or Viking can stretch LWD samples far enough apart to smear thin laminations and bed boundaries critical to geosteering. The sampling interval is distinct from but related to the vertical resolution of the physics: a density tool with a 0.3 m source-detector aperture cannot truly resolve a 0.1 m bed no matter how finely it samples, so oversampling beyond the tool's physical resolution adds data without adding information, while undersampling below it throws information away. Memory capacity, telemetry bandwidth, and logging economics all push back against fine intervals. Real-time mud-pulse MWD telemetry runs at only a few bits per second, so operators transmit a coarse subset in real time for geosteering and recover the densely sampled data from downhole memory after the bit trips out. Choosing the sampling interval is therefore a deliberate trade among the geological detail required, the tool's intrinsic resolution, the available bandwidth or memory, and the cost of the data, and it is set in the logging program before the job alongside cable speed or expected rate of penetration.

Sampling Interval and Geosteering Quality

In a WCSB horizontal the LWD gamma ray and resistivity logs steer the bit inside a target window often only 2 to 4 m thick. If the driller pushes rate of penetration to 60 or 80 m/hr to save rig time, a fixed time-based sample rate spreads readings far enough apart that a bed boundary crossing arrives blurred and late, and the geosteerer reacts after the bit has already left the window. Operators counter this by tightening the LWD sample interval, capping ROP through the landing and lateral, or upgrading to higher-bandwidth telemetry so denser real-time data reach surface.

Wireline Depth Sampling for Thin Beds

Evaluating a laminated Cardium or Viking sand with shale partings demands fine depth sampling. A standard half-foot (0.1524 m) interval may average a 0.2 m clean sand with adjacent shale and understate net pay, so the analyst orders one-inch (0.0254 m) sampling on a high-resolution microresistivity or borehole-image tool. The finer interval, paired with a tool whose physical resolution actually supports it, resolves individual laminae and lets the petrophysicist count net sand that a coarser log would have homogenised into a single low-quality average.

Related Terms

Sampling interval sits at the centre of logging acquisition. Measurement-while-drilling records on a time basis and depends on rate of penetration to convert samples to depth. Wireline logging samples on a depth basis governed by cable speed. Gamma ray log response is one of the measurements whose bed-boundary sharpness the interval controls, and vertical resolution is the companion concept describing the finest bed a tool can physically distinguish regardless of how densely it samples.

Real-World WCSB Scenario: LWD Geosteering a Montney Lateral near Dawson Creek

An operator drilling a 2,800 m Montney lateral near Dawson Creek geosteers on LWD gamma and resistivity to stay in a 3 m siltstone target. Early in the lateral the directional driller runs at 70 m/hr, and with the LWD tool sampling every 15 seconds the effective spacing stretches to about 0.29 m, nearly a tenth of the target thickness, so a subtle gamma rollover signalling the bit drifting toward the bounding shale arrives blurred and the well briefly exits the zone.

The geosteering team caps ROP at 35 m/hr and shortens the real-time sample interval, halving the effective depth spacing to roughly 0.15 m, which sharpens the bed-boundary signal enough to keep the remaining 2,400 m inside the target. The tighter sampling cost a few extra hours of drilling time, perhaps CAD 25,000, but raised in-zone footage from 78 to 94 percent, a swing worth far more in deliverability over the well's life.