Well Logging Probe on STM32F401 for Wells

Universal Borehole Logging Probe Based on STM32F401

The STM32F401 on a Black Pill board serves as the core for a water well logging probe. Its 84 MHz frequency and Cortex-M4 with FPU enable fast float operations for filtering. With 64 KB of RAM and 256 KB of Flash, it has sufficient capacity for the tasks. Three timers, three USARTs, I2C, and OneWire are key interfaces. The board fits inside a stainless steel tube housing with an outer diameter of 38 mm (inner diameter 32 mm), designed for wells with casing pipes from 114 mm.

Pin layout:

• PA4: Resistivity meter input
• PB3: Gamma counter input (edge-triggered interrupt)
• PB10: Resistivity meter power supply
• PB7–PB4: Multiplexer control for resistivity logging generator
• PB8 (SCL), PB9 (SDA): I2C for sensors
• PA2/PA3: UART2 for ultrasonic caliper
• PA9/PA10: UART1 for caliper
• PA8: OneWire for thermometer
• PA11/PA12: USB debugging

The housing size constraints complicate integration.

Logging Methods and Their Implementation

The probe combines gamma logging (GL), resistivity logging (RL), resistivity measurement (RM), thermometry, caliper logging, and magnetometry. Data is recorded relative to descent depth with noise filtering and real-time transmission to a PC.

Gamma logging with SiPM (silicon photomultiplier): Pulses are handled via interrupts on PB3. SiPM is preferred over PMTs—it requires no high voltage, is compact, and reliable.

Resistivity logging (RL): Generates a 70 Hz signal via a multiplexer (PB7–PB4). Measures voltage drop on current electrodes (A/B) and measuring electrodes (M/N). The current-based method detects casing leaks (peaks on diagrams) and thin layers (e.g., 0.5 m clay in limestone). Fast response without accuracy loss for hydrogeology.

Resistivity measurement (RM): Operates during RL pauses (300 ms out of a 1 s cycle). Input PA4, power PB10. Identifies filtration zones (e.g., 60–80 m in a Ø219 mm column).

Thermometry: DS18B20 via OneWire (PA8), polled at 1 Hz, resolution 0.01°C. Detects active filtration zones.

Synchronization and Data Recording

Cycle: 700 ms for RL (generation + measurement), 300 ms for RM. Descent depth is synchronized across all channels. Gamma via interrupts, thermometer periodically. Data with noise filters is transmitted to a logger and PC.

Example results:

• RM: Anomaly from 80 m (open borehole), filter column 60–80 m.
• RL (blue line): Peaks indicate holes or openings in the casing.

Key points:

• STM32F401 balances performance and size for compact probes.
• Combined RL/RM in one cycle simplifies depth correlation.
• SiPM for GL minimizes size and risks.
• Current electrode measurement speeds up layer and casing defect detection.
• Real-time recording with filters reduces post-processing.

Future Development Prospects

Integration of caliper (UART1), ultrasonic caliper (UART2), and video logging is delayed due to size constraints. Magnetometer detects filters and pipes. Lathe work for electrodes and sensor calibration are key steps. Housing errors required iterations.

— Editorial Team