Minerals: Why it is important to Ground Balance and What to Look for to Determine When to Rebalance Your Metal Detector

Ground balancing a metal detector is essentially causing your machine to “ignore” various levels of mineralization and magnetic properties inherent in most soils.  This helps you as a detectorist to achieve the maximum depth that a given machine can accomplish. 

Mineralization in any given country or city lot can vary dramatically, sometimes even across just a few feet, and this directly affects how your detector responds and how you set your ground balance. Mineralization is basically the amount and type of naturally occurring or human-introduced metals and chemicals in the soil that influence conductivity and magnetic reactivity.

Here are several main factors that change mineralization in a city lot:

1. Natural Soil Composition

Even in cities, soil layers may include:

• Iron-rich clays (red clay is usually highly mineralized)
• Volcanic or igneous rock fragments (magnetic)
• Sandy soils (low mineralization and easy to detect in)
• Limestone base (very mild mineralization)

Iron and magnetic particles force the detector to ground balance higher, reducing depth. Sandy and limestone soils require very little ground balancing and give greater depth.

2. Fill Dirt or Soil Imported During Construction or even later in fill dirt.

Many city lots have had fill dirt hauled in, which is  comprised of:

• Road project leftovers
• Soil stripped from another location
• Industrial fill (worst case—slag, ash, cinders)

The subsequent effect is that the fill dirt often brings in unknown mineral content, slag, rusted debris, or ash. A lot can have mild dirt in one area and hot ground just a few feet awa,y where fill was dumped.

3. Old House Fire or Burn Pile Soil

Burned wood yields charcoal and ash
Old houses were primarily heated with:

• Coal
• Wood

Each of these produce either ash or cinders which often contain microscopic iron, and burned soil becomes magnetically reactive.

Result: The ground becomes “hot” and detectors become chatty, losing depth unless ground balance is done VERY carefully.

4. Yard Treatments & Chemical Leaching

Over decades, people have applied:

• Fertilizers (increases conductivity of soil moisture)
• Lime (raises pH, changes signal response)
• Iron lawn supplements (literally adds iron dust)

Effect: Chemicals change soil electrical conductivity, which affects VDI stability and causes detectors to misread targets.

5. Moisture Level and Recent Weather

Wet soil conducts better and often increases detection depth.
But heavy rain draws iron from soil and nails, raising mineral response in the days following the rain.

Dry soil = less conductivity which makes it harder to pick up deep coins
Wet soil = more conductivity but more creates falsing from iron halos

6. Metallic Debris in the Soil

City lots often contain:

• Nail beds from past siding or roofing jobs
• Old clotheslines (lots of dropped hardware)
• Rust flakes from tin roofs
• Fence wire
• House remodel scrap buried instead of hauled off

Even if decayed, rust creates a mineral halo, fooling detectors.

A Halo is not the  artifact itself — it’s the oxidized soil around it brought about by the object’s interaction with water and salts and leaching molecules into the surrounding soil matrix.

7. Proximity to Concrete, Brick, or Stone

Concrete and brick often contain:

• Iron oxide
• Limestone aggregate
• Cinders
• Magnetic sand

Old hand-made bricks especially contain iron.

Effect: You will see more false signals around the:

• Foundation
• Walkways
• Old clothesline poles

8. Previous Industrial Use

If the lot was ever:

• A blacksmith location
• A machine shop
• A storage yard
• A rail spur
• A burn dump
• A corrugated tin-roof building site

Expect severe iron contamination.

How Mineralization Affects Ground Balance

• Low-mineral soil yields Lower ground balance numbers, quiet detector, maximum depth.
• High-mineral soil yields  Higher ground balance numbers, noisy detector, depth reduction.
• Ground too positive creates  detector falses in iron as high tones.
• Ground too negative creates detector responses that clips out deep non-ferrous signals.

Practical Tips for Metal Detecting City Yards

When you start a yard:

First, do a noise cancel  if your machine is capable to eliminate EMI

Severe interference: In areas with extremely high EMI, noise cancelling may only lessen the effect, and some noise might still be audible. 

Different interference sources: The noise cancel function primarily addresses air-based EMI. It may not be as effective against ground mineralization or other sources of interference that only become apparent once the coil is moving close to the ground. 

Detector quality: Some detectors are engineered with better noise immunity than others, meaning the effectiveness of the noise cancelling feature can vary significantly between models. 

Second, Ground balance away from metal signals in an all metal mode if it is available.

Swing and check how stable it is

If it is still unstable:

2. Lower sensitivity a bit
3. If needed, switch to 5–10 kHz if your detector allows (handles iron better)
4. Use a small coil to reduce signal blending from multiple targets

Visual/physical clues

As you dig holes (or flaps or whatever) observe the ground for different clues which will indicate soil modification of fill dirt, burn piles, fertilization, clays, cinders, etc..

• Note:  The very first visual clue you may observe on arrival is the difference in the height of the lawn compared to the sidewalk or curb.  You can use as a rule of thumb that if the soil is higher than the sidewalk or curb by several inches, then it is highly likely that sometime in the past fill dirt has been brought in.  You can almost always see this in the difference of fill dirt color and texture vs old soil below once you open the ground.  And remember that every inch of fill dirt will reduce the deepest signal reduction by the same amount.  Keep this in mind when looking for lots to detect.  If the soil is at or below the sidewalk, you will find more old coins than if there is a 3 or 4 inch difference.  Granted the fill dirt had to be brought in  and it is possible that the original site the fill dirt came from *might* be from an old home site, but in my experience that is seldom the case.  I almost always choose lots with soil that is at or below sidewalk level.

Observe cinders or ash in soil as shown by clinker bits, charcoal

Red/brown clay will likely show as a seam showing in edges, holes, or flowerbeds

In the case of Imported fill (texture suddenly sandy/gravelly or layered), Ground balance and VDI behavior can change every few feet.  While I am on the VDI behavior, and especially for newer folks out there, let’s talk about what VDI is all about. 

How many of us have seen in any forum the question “What were the VDI Numbers?”.  In reality…..whatever they were, it was ONLY for that time, that place, and the existing conditions.  Here’s why…. Your coil on the ground is comprised of two long loops of copper wire. One is the transmitter coil and the other is the receiver coil.  The transmitter coil sends an electromagnetic wave into the ground and when any metallic receives that signal there is an eddy current formed and it is that which the receiver coils “sees” and the electronics of your detector does the magic work of figuring out the difference between what the ground (mineralization) background is and the signal received.  The Processor of your machine analyzes the information and assigns a VDI number based upon that analysis.  What can and does affect that is the Halo Effect (with ferrous it is called Falsing) we all hear about, when salts build up around the target and create the effect of making the target appear both larger and more conductive.  Years ago it was believed that the molecules of the actual target leached out into the surrounding soil, but it is my belief that it is the creation of salts around the target due to chemical differences in the soil and metal.  The salts have a conductive property to them and creates this effect.

The orientation of the target in relation to the coils also affects what the VDI number is.  I swing a Minelab Manticore, and in the instructions, they advise to swing your coil over a target several times to cause the detector to increase confidence of what the target VDI is because of this.  By following this procedure, look at it this way.  A single pass is like a snapshot of the properties.  Swinging over it from several different directions (left to right, top to bottom) provides a more complete view o the shape, size, and conductivity/ferrous characteristics.  That is actually an enormous selling point for me as a user with the 2D display of the Manticore…..As the coil passes over the 2D map on the screen of that machine develops a “Target Trace” and consistent VDI number IF it is a solid nonferrous object. 

Depth is another factor that changes VDI readings.  A smaller target deeper in the ground and at an angle has less eddy current that say an aluminum can or even a flat lying target at equal depth.  Effectively it will produce a lower VDI number due to the effect of mineralization/target interference with the return coil signal and the angle of the target has less surface area to reflect effectively.

Mineralization along with the above stated factors is what makes VDI numbers not absolutes.  It is my contention that if you depend on rock solid VDI numbers, you are missing a lot of what you seek.

My Final Thought on this: an axiom known in the metal detecting community is that you will never find it all.  In this topic of discussion, you can ground balance a machine every 3 or four feet to insure that you are getting the absolute best depth, of you can make a general assumption that unless you encounter a dramatic difference in observed soil composition (determined by the observations you make), the numbers should remain kind of the same.  On any given city lot, I will normally ground balance in the front yard and keep the GB the same, and in the back yard, I will be more attentive and I will re-ground balance in response to soil composition changes or in particular to any burn pile areas that were generally in one spot over and over through the years.  I can genuinely say that burn piles are almost always an area likely to be within a 10 ft square area of a property.  Also, I will generally do more ground balancing in the back yard than the front yard with the reasoning being that people did more things that would moderate the soil, like the burn piles, working on cars, chemical spillage, etc. .

 
And in the decades I have detected, I too have been overly dependent on VDI numbers, and there have been times I should have ground balanced again and again.  I am quite sure I left some amazing metallic targets for the rest of you.  Happy Hunting.