larger version
bandpass filtered version
larger version
bandpass filtered version
Test of .22 calibre seismic source
Sept 9,1998
Purpose
Shot pipes have long been used in small scale seismic investigations. Their simplistic nature, high frequency source characteristics, portability and low expense make them attractive.
On the other hand, most all ammunition for these sources is subject to the strict control of firearm regulations. To the environmental geophysicist this means there are severe restrictions on where they can be used (legally). In addition, the explosive nature of shot pipes invariably alters the soil around the shot point. This makes repeat measurements difficult and time-dependent work near impossible with shotpipes.
For work on the scale of tens of meters very small charges can be used which help with both of these difficulties. .22 calibre blank shells are available from most hardware stores for unrestricted use in nail guns. Their very low yeild means lower amplitude seismic waves but also less disturbance to the soil around the test site and hence the possibility of higher repeatability.
A test of a .22 calibre shot pipe source for small scale environmental seismic experiments was conducted to address the following questions:
Method
The test was conducted on the lawn in front of the machine shop at LDEO. The soil consists of fill emplaced during the construction the adjacent parking lot - fine for a first look at these sources but not necessarily representative of a particular field environment. No rain had fallen for several days. There was a variable wind and the close proximity of trees to the test sight made the ground noisy. With the combination of inhomogeneous soil and high background noise, these tests can be considered "worst case scenerios".
Twenty Mark Products 50Hz geophones were placed in a linear array with a spacing of 1.5 meters. The first phone was 1.5 meters from the source. The last phone was 30 meters from the source. The phones were not buried. Waveforms were recorded on a Geometrics Strataview-R system with a sampling period of 0.250 ms for 0.512 seconds (2048 samples). No acquisition filters were used.
Results
Five raw records sections are shown below. The first two compare the .22 calibre shot pipe fired into a dry hole and into a lined water-filled hole. Following this is a comparison of the .22 source, the 12-gauge shot pipe and a stack of five hammer blows for comparison. All records are scaled by trace. Beneath each record section there are links to a larger .gif version of the image and to a bandpass filtered version. The filtered version have had a 100 to 250 Hz bandpass filter applied to bring out the first arrival compressional wave.
Dry hole vs. water filled cavity
The .22 calibre shot pipe was placed in an 8" hole which was then backfilled with sand. The shots did not disrupt the backfill when fired. They were audible as quiet "thumps". Despite the low yeild of the .22 blank, the surface wave signals remained well above the noise level across the array (left figure). Likewise the direct compressional wave is clearly visible though beyond 20 meters picking the first arrival is difficult without filtering.
| .22 calibre shotpipe, dry hole | .22 calibre shotpipe, water filled cavity |
|---|---|
larger version bandpass filtered version |
larger version bandpass filtered version |
In addition to the dry hole, the .22 shot pipe was fired into a hole filled with water (right figure). A hole, slightly larger than that used for the dry test, was made. A latex balloon was placed in the hole and filled with water until it filled the cavity. Then the shot pipe was placed in the balloon and the balloon was sealed at the top around the pipe. A small quantity of sand was then mounded over this seal. We had hoped the water would provide better coupling into the soil. It is obvious from the record that this was not the case. The signal-to-noise ratio is much lower for the water coupled shot. The surface wave deteriorates significantly over the last 15 meters of the array and the compressional wave is nearly absent. It should be kept in mind however that this is a single record under questionable conditions. Perhaps other factors, such as firing beneath the water table or a different method of water coupling could provide different results.
Repeatability of source
The shot pipe was fired into the dry hole twice. It is clear that the shot pipe method as used in this test is far from repeatable. This figure compares the P wave arrival at a distance of 17 meters from each of these otherwise identical shots. The larger signal is from the second shot. Not only are the amplitudes different but the first breaks differ by ~1 ms. These differences may suggest different coupling due to repeated explosions in the hole. Digging a new hole for the pipe would eliminate this but would introduce a new suite of problems. Alternatively, these differences could be due to natural variability of the shot pipe. Slightly variations in the quantity of powder could alter the amount of time required to break the rupture disk. Errors could also be attributed to the trigger pulse from the shot pipe.
.22 calibre, 12-gauge, hammer&plate
No source is ideal for all applications. The .22 calibre source will only be useful in those situations where it meets or exceeds the usefulness of other methods. The .22 calibre source was compared to a traditional hammer & plate and to a shot pipe modified to accept 12-gauge shotgun blanks.
| .22 calibre shotpipe | 12-gauge shotpipe | hammer on plate source |
|---|---|---|
larger version bandpass filtered version |
 larger version bandpass filtered version |
 larger version bandpass filtered version |
The strength of phases from each source were measured roughly by maximum amplitude on a given trace as measured in counts. P waves recorded from the 12-gauge source were 15-20 times larger than those from the .22 calibre source. P wave signals from the hammer and plate record were 2-3 times larger (NOTE: this record was a stack of five blows so the amplitude on the record is actually 10-15 times larger than the .22 calibre source). Surface wave amplitudes were compared using the same method. The 12-gauge source created surface waves with 20 times the amplitude while individual blows from the hammer can be expected to create signals 4 times larger than the .22 source. Surface wave generation is very sensitive to the source depth so these numbers should be taken as only rough comparisons.
The triggering system on the shot pipes relied on a microswitch which was closed by the falling mass through the pipe. The records above show this method results in a trigger delay of over 10 ms. This delay can be attributed to the distance the falling mass must travel between the switch and the shell. It also reflects the time required for the burning power to build up enough pressure to rupture the pvc disk in the pipe. This likely explains why the .22 and 12-gauge sources have different delays.
Frequency content
Frequency spectrums were created for each of the sources. The full waveform was used for the spectrums. The vertical axis is linear but each spectrum is scaled independently to account for different amplitudes. The closest geophone was 1.5 meters from the sources (left figure). At this range there is no separation between phases. The wet and dry .22 calibre shots are similar as one might expect. Both have a peak at 110 Hz. The 12-gauge source and the hammer have noticeably lower frequency content.
| 1.5 m from source | top to bottom: |
15.0 m from source |
|---|---|---|
 |
.22, dry .22, wet 12-gauge hammer |
 |
At a range of 15 meters, the full waveform spectrums are more similar. It is not surprising that the higher frequencies of the .22 calibre source have been attenuated at this distance; high attenuation is typical of inhomogeneous backfilled earth.
| P wave spectrum - .22, 12-g, hammer |
|---|
 |
 |
 |
