Building Your Own Towed Magnetometer

Build Your Own Towed Magnetometer

Commercial proton magnetometers cost $3,000–$8,000. A $20 sensor chip and a $30 Arduino can get you surprisingly close — for the price of a tank fill.

$80–$150 Total Build Cost | ~10–100 nT DIY Sensitivity | RM3100 Key Sensor Chip | Arduino Nano Brains

Most people assume that finding a shipwreck's iron and steel from a moving boat requires professional salvage equipment. It doesn't, anymore. A proton magnetometer towed behind a slow-moving boat detects disturbances in the Earth's magnetic field caused by cannons, anchors, hull spikes, and engine blocks — even buried meters beneath the seafloor — and the detection range for a large wreck can extend hundreds of feet. What used to require a $3,000–$8,000 commercial unit can now be approximated with the PNI RM3100, a magneto-inductive sensor chip built for the scientific and aerospace market, paired with a $30 Arduino. It won't match a professional unit's 1-nanoTesla sensitivity, but at 10–100 nT it will reliably flag large iron and steel targets — which is exactly what a weekend wreck hunter is looking for.

What makes it realistic for a beginner is that an electrical engineer and hobbyist already did the hard part and published it for free. The RM3100-and-Arduino design was featured in a 2022 IEEE Spectrum build log, and it has since been picked up, extended, and re-documented by makers adding ESP32 WiFi logging, IoT dashboards, and full PVC tow-fish housings borrowed from the DIY ROV community's waterproofing playbook. Below are the build references, technical background, and community projects worth reading before you order parts.

THE FOUNDATIONAL BUILD

• IEEE Spectrum — Search for Buried Treasure With This DIY Magnetometer — spectrum.ieee.org/magnetometer — The source article. A homebuilt magnetometer using an RM3100 three-axis sensor board kept physically separated from the other components by about 30 centimeters to avoid the electronics affecting the measured field. Full circuit description, RM3100 wiring, and real-world test results — essential reading before ordering parts.

• GitHub — Brown Space Engineering, RM3100 Magnetometer — github.com/BrownSpaceEngineering/RM3100-Magnetometer — Sample code for I2C and SPI communication between an Arduino and the PNI RM3100 breakout board, with a quick-guide PDF covering wiring and usage, from the university team that evaluated the chip for satellite work.

• Hackaday.io — Playing With Ultra-Sensitive Magnetometer RM3100 — hackaday.io — A documented ESP32 build that connects the RM3100 sensor board to an ESP32 controller and streams measured magnetic field values to ThingSpeak IoT for visualization, with notes on tuning the chip's cycle-count setting for the best noise-to-sensitivity tradeoff.

TECHNICAL & ACADEMIC BACKGROUND

• University of Liverpool — Magnetometer on a Chip — liverpool.ac.uk — Academic evaluation of the RM3100 for marine survey use, including noise floor analysis and comparison to proton magnetometer performance.

• HamSCI — PSWS Ground Magnetometer Installation Manual — hamsci.org/mag_install — A separate but closely related RM3100 build using a PVC housing and Raspberry Pi, with guidance that the remote sensor board should be placed away from sources of electromagnetic interference while the computer and support board stay inside — directly applicable to keeping a tow-fish build accurate.

• Sten Odenwald — Exploring Space Weather with DIY Magnetometers — A book presenting several simple DIY approaches to registering geomagnetic field changes, including a chapter specifically on the RM3100-plus-Arduino approach that inspired several maker builds.

COMMERCIAL BENCHMARKS

• JW Fishers — Boat Towed Magnetometers — jwfishers.com — The professional benchmark: Proton 4 and Proton 5 specs, detection ranges, and accessory options, useful for understanding what real commercial sensitivity looks like before you calibrate expectations for a DIY build.

COMMUNITY BUILDS & FORUMS

• Hackaday — Magnetometer Projects — hackaday.io — A running collection of community builds, including fluxgate magnetometers, HMC5883L compass-based detectors, and paired-sensor gradiometer designs.

• IEEE Xplore — DIY Magnetometer: Search for Buried Treasure for Less Than $100 — ieeexplore.ieee.org — The peer-reviewed writeup behind the Spectrum article, with the author's own account of building a crude but functional fluxgate magnetometer after reading World War II-era journal articles on the magnetometers used to locate submerged submarines — useful context for why this approach works at all.

A weekend, under $150 in parts, and an Arduino sketch already written by someone else is enough to start narrowing down a survey area — just don't expect to find a single musket ball with it. That's what the dive afterward is for.