DIY Seismic;
It might be time for a refresher...
Passive seismic methods among a few others are borderline pseudoscience. That's why it's all proprietary!
The stuff that isn't proprietary, we can have a discussion about.
There are videos, books and many technical papers written on the subject. It's all been tested, proven, verified, peer reviewed, it's repeatable and anybody can do it.
- Geowizard
DIY Seismic
Moderator: chickenminer
-
- Mega Miner
- Posts: 1365
- Joined: Sun Dec 16, 2018 4:18 pm
- Has thanked: 559 times
- Been thanked: 459 times
Re: DIY Seismic
Depth to bedrock:
When investigating the shallow earth using any method, it should be understood that the earth is rarely uniform. Every measurement is an estimation. It's Geometry. We can do this!
When a seismic survey is done, sometimes the "source" of sound energy is from a small blast. Shotgun charges are frequently used and the terminology used is the "shot". The point where the shot occurs is called the "shot point".
Let's assume you setup a geophone 100 feet away from a shot point (called offset) as in the prior post. When the shot occurs, TWO waves are received at the geophone. The first is the direct wave across the surface and the second is the reflected wave from the assumed first interface, bedrock.
The prior example included a calculation of velocity and velocity was 200 feet per second.
The path of the reflected sound wave follows the course of a triangle. It moves down from the shot point to bedrock at an angle and returns at the same angle to the geophone.
Knowing the velocity is 200 feet per second and given a time of the reflected wave - let's say 1500 milliseconds, we can calculate the depth to bedrock.
In 1500 milliseconds, the wave travelled down to bedrock in 750 milliseconds and back to the surface in 750 milliseconds. At a speed of 200 feet per second, that makes the distance (D = speed/time) = 200ft/sec x .750 sec = 150 feet down and 150 feet up
The distance of the geophone is 100 feet from the shot point. The base of the triangle is 100 feet and two equal sides of the triangle are 150 feet.
We can bisect the triangle into two halves. Now, we have two right triangles. Each triangle has a 150 foot hypotenuse, a 50 foot adjacent side (1/2 of the base) and an unknown opposite side - "height" which is the depth to bedrock.
We can use the Pythagorean theorem to calculate the unknown side of a right triangle having sides A, B, and hypotenuse C.
Stick around, there's more!
- Geowizard
When investigating the shallow earth using any method, it should be understood that the earth is rarely uniform. Every measurement is an estimation. It's Geometry. We can do this!
When a seismic survey is done, sometimes the "source" of sound energy is from a small blast. Shotgun charges are frequently used and the terminology used is the "shot". The point where the shot occurs is called the "shot point".
Let's assume you setup a geophone 100 feet away from a shot point (called offset) as in the prior post. When the shot occurs, TWO waves are received at the geophone. The first is the direct wave across the surface and the second is the reflected wave from the assumed first interface, bedrock.
The prior example included a calculation of velocity and velocity was 200 feet per second.
The path of the reflected sound wave follows the course of a triangle. It moves down from the shot point to bedrock at an angle and returns at the same angle to the geophone.
Knowing the velocity is 200 feet per second and given a time of the reflected wave - let's say 1500 milliseconds, we can calculate the depth to bedrock.
In 1500 milliseconds, the wave travelled down to bedrock in 750 milliseconds and back to the surface in 750 milliseconds. At a speed of 200 feet per second, that makes the distance (D = speed/time) = 200ft/sec x .750 sec = 150 feet down and 150 feet up
The distance of the geophone is 100 feet from the shot point. The base of the triangle is 100 feet and two equal sides of the triangle are 150 feet.
We can bisect the triangle into two halves. Now, we have two right triangles. Each triangle has a 150 foot hypotenuse, a 50 foot adjacent side (1/2 of the base) and an unknown opposite side - "height" which is the depth to bedrock.
We can use the Pythagorean theorem to calculate the unknown side of a right triangle having sides A, B, and hypotenuse C.
Stick around, there's more!
- Geowizard