Seismic Geophone Calculator

What is a Seismic Geophone Calculator?

A Seismic Geophone Calculator helps geotechnical engineers and geophysicists determine subsurface layer depth using seismic refraction survey data. By measuring the crossover distance where refracted waves overtake direct waves, along with the seismic velocities of each layer, the calculator computes the depth to the refracting interface using the two-layer refraction formula. For related seismic analysis, check our Seismometer Calculator for earthquake magnitude calculations.

How to Use the Seismic Geophone Calculator

Select the variable you want to solve for from the dropdown, then enter the known values. The calculator instantly computes the result. Ensure that the lower layer velocity (v₂) is greater than the upper layer velocity (v₁) for valid refraction survey data.

Seismic Refraction Formula

For a two-layer earth model with a faster lower layer, the depth to the refracting interface is calculated using:

t = (d/2) × √((v₂ - v₁) / (v₂ + v₁))

Where:

t = Depth to the refracting interface (m)
d = Crossover distance where direct and refracted waves arrive simultaneously (m)
v₁ = Upper-layer seismic velocity (m/s)
v₂ = Lower-layer seismic velocity (m/s)

The relationship assumes horizontal, uniform layers with a step increase in velocity. The calculator also supports inverse solves for crossover distance, upper layer velocity, and lower layer velocity.

Typical Seismic Velocities

Loose dry soil: 200-500 m/s
Saturated soil or dense clay: 500-1,500 m/s
Weathered rock: 1,500-3,000 m/s
Sound bedrock (granite, limestone): 3,000-6,000 m/s

Applications

Construction Site Investigation: Determining depth to bedrock for foundation design and excavation planning.
Water Table Mapping: Locating the saturated zone by detecting the velocity contrast between dry and saturated soil.
Mining Exploration: Estimating overburden thickness for open-pit or strip mining operations.
Road and Dam Engineering: Profiling subsurface layers to assess bearing capacity and stability.

Frequently Asked Questions

How do geophones detect underground vibrations?

A geophone is a small spring-mounted magnet inside a coil. When the ground moves, the case shakes with it but the suspended mass lags, generating a tiny voltage proportional to ground velocity. An array of geophones wired to a seismograph records seismic wave arrivals at each station, which is then analyzed to image the subsurface.

What is the crossover distance in seismic refraction?

The crossover (intersection) distance is where the direct wave and the refracted wave arrive at a geophone at the same time. Beyond this point, the faster refracted wave arrives first because its longer path through a faster medium overtakes the slower direct path. This distance, combined with the two layer velocities, is used to calculate overburden depth.

What is seismic refraction used for?

It is used to determine depth to bedrock, map the water table, estimate overburden thickness for mining, and characterize subsurface conditions before construction. The method is fast and inexpensive compared to drilling and works well for shallow targets up to a few tens of meters.

How deep can a seismic refraction survey measure?

The maximum depth is roughly one-third to one-fifth of the total geophone spread length. A typical 120 m spread can resolve layers to about 25-40 m depth. Deeper investigations require longer spreads and a larger energy source such as a weight drop or small explosive charge.

What are common mistakes in seismic refraction surveys?

Common mistakes include assuming the lower layer is always faster (seismic refraction only works when deeper layers have higher velocities), using too short a geophone spread (the spread must extend well beyond the crossover distance), and ignoring lateral velocity variations (the method assumes horizontal, uniform layers).

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