1. Front Yard Geophysics in Northern Austin: Are the Presence of
Karstic Features Underestimated in Austin Chalk?
Mustafa Saribudak
Environmental Geophysics Associates, Austin, Texas
Interpretation of NP Data Interpretation of GPR Data
All the NP data collected along the front yards of the houses across
Two GPR profiles (G1 and G2) were processed and interpreted below. First pair of the
Introduction N profiles are chosen between stations zero (0) and 25 feet where both data sets
the study area are plotted on a site map and is shown below. The
Unusual road cracks and several road patches exist on a circle road indicate subsiding subsurface soil layers. Locations of these subsidence anomalies
NP data points are shown with a red/yellow circle and the NP values
located in north of Austin. A google map of the area shows the are correlative with the locations of the repair patches in the northern part of the site.
are shown with white numbers. The NP map indicates zero and/or
peculiar geometry of the cracks and some of the asphalt patches. The 0 GPR Data on Profile G1 2 5 Ft S
background values in the south-eastern part of the site. Negative N
first question comes to mind is that what is causing the cracks and the
values appear to show further west on either side of the front yards.
second is if their existence somewhat related to the road patches. Ft
However, a very significant positive NP anomaly (~115 mV) is
Subsidence
observed on the eastern flank of a front yard where the street makes
A chat with a neighbor indicated that the cracks have been there as
a turn to the north.
long as she remembers (~20 years), and the patches on the road
were due to the “sinkhole repairs.” The last repair, according to the
neighbor, was approximately done two years ago. This circle road and -3 10 7 N
6 9
its vicinity is located in the geological formation of the Austin Chalk. -5 6
0 GPR Data on Profile G2 25 Ft
3 44 10
1 102
39
-5
6 11 L2 Ft
Ft S ub
-3 13 5 s IVoid n c e
de
Geology
. -5
-11
-1 3
-6 Subsidence
-12
The Austin Chalk is an upper Cretaceous geologic formation in the -13
-15 -13 15
-
Coast region of the United States. It is named after type section -17 -23
3 -16
outcrops near Austin, Texas. The Austin Chalk consists of -22
-1 -16
-12 L1
-22 -18 -6
recrystalized, fossiliferous, interbedded chalks and marls. The Austin -13
-15 -6
-4
Chalk crops out in a wide belt almost through the center of the City of -11 0
-11
Austin, extending from south of Onion Creek all the way past Site map showing presence of unusual fractures and repair 0
-14
-10
0
0 Second pair of the GPR profiles are chosen between stations 175 and 200 feet
Pflugerville. Interstate Highway 35 is constructed on the high ground patches (red/yellow stars) on the road. Patches were due to
where both data sets indicate subsiding subsurface soil layers. Locations of these
-7
sinkhole repairs. --5
that represents the Austin Chalk escarpment. The study area is -3
-4 subsidence GPR anomalies are correlative with the locations of the repair patches
shown with a red/yellow star on the geological map of Austin. Site
-2 -1 in the southern part of the site.
N 175 GPR Data on Profile G1 200 Ft S
Geophysical Surveys-Field Survey Design
To determine what is causing the cracks and/or to understand the
Interpretation of Resistivity and NP F
Subsidence
extent of the sinkhole(s), resistivity, natural potential (NP) and ground Data t
penetrating radar (GPR) surveys were performed at the study area. The resistivity imaging and NP data taken along line L1 are shown
below. The resistivity profile indicate horizontal layers of resistivity
First, as a reconnaissance work, I collected NP data along the either ranging between 20 and 200 Ohm-meter, which correspond to marl
side of the front yards of the houses encircling the road (see blue lines). and limestone units, respectively. The NP data also show no
The station spacing was held 10 feet but where anomalous NP values significant anomalies along the ALONG L1
profile. GPR Data on Profile G2
RESISTIVITY DATA
are observed, the data was collected with a tighter spacing of 5 feet . W E
The total number of the NP data point was 58. I collected the NP data Ft
after a reasonable good rain in the vicinity of the site. And all the NP Subsidence
Horizontal marl and
values were tied to a near-by base station, and diurnally corrected. The limestone layers of
NP unit is measured in miliVolt (mV). The rule of the thumb in the NP Austin Chalk
interpretation is that sinkholes create negative whereas caves filled with
mV
air form positive NP anomalies.
Geology map of the Austin area showing
the study area in the Austin Chalk. NP DATA ALONG L1 Conclusions
Secondly, I performed GPR surveys along the road crossing the Areas of negative NP (yellow-dashed line), positive NP (white-dashed line)
asphalt patches on the road. North-south GPR profiles of G1 and G2 N
are approximately defined on a site map below. The significant high NP
are shown with a yellow colored line. Note that I did run few E-W GPR G2 Feet anomaly location and resistivity anomaly indicating subsidence are also
G1
lines on the road; however, they did not indicate any significant shown with a green circle on the eastern part of the circled road.
anomalies. For this reason, I will not discuss the results from the E-W The resistivity imaging and NP data taken along profile L2 are given
Locations of GPR N
profiles. Lengths of the both profiles were 200 feet. The depth of below. The resistivity data show a significant subsidence between Road repair
patches
stations 20 and 50 feet. The resistivity layers range between 7 and anomalies are
exploration with the GPR survey was about 8 feet. Area with positive
NP 180 Ohm-meter, which may be indicative of clay and weathered shown with blue NP
Area with
Resistivity limestone. The NP data also indicate a quite significant NP anomaly lines and their Area with
Finally, based on the NP results, I chose two resistivity locations and L2 negative NP positive
(positive) over the resistivity anomaly. subsidence
collected resistivity data along the lines L1 and L2. These profiles are GPR NP values
N S directions with
shown with a red colored line. The location of the profile R1 was Repair
L1 white arrows. Cave
chosen along NP values that did not show any anomaly whereas the patches S ubside nce location
In conclusions.
location of the profile R2 showed extremely significant NP values. Resistivity data presence of all
Although I wanted to collect two more resistivity profiles across some Into a cave along L2
these geophysical
negative NP anomalies but it was not possible logistically due to
mV anomalies indicate
moving traffic in and out of drive ways. Source of this NP
Subsidence
Site map showing the NP (blue ), resistivity NP data along L2 subsidence, wi t h P direction
anomaly is likely A re a ve N
(red ) and GPR (yellow ) profiles. NP Anomaly sinkholes and ga ti defined by
For further information due to a cave
caves in the
ne
v a lue
s GPR
Please contact ega@pdq.net for more information on this project and subsurface and
Ft
related karstic projects over the Edwards Aquifer. More information their and their tectonic interactions could be responsible for the peculiar
can be obtained at www.egatx.com. geometry of the surface fractures that is present at the site.