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Ground Penetrating Radar

A Ground Penetrating Radar Survey of the Lynden Cemetery

Bernard Housen, Geology Dept., Western Washington University, Bellingham, WA 98225

Dec 11, 2009


Introduction and methods

     In order to document the presence and locations of burials in the Lynden Cemetery, and to verify the lack of any interment in cemetery plots that do not have grave markers (and thus are available for future burials), a ground penetrating radar (GPR) survey was conducted. The data were collected in May 2008 by a group of students in the Western Washington University Geology Department, as part of a class project. For this survey, the four large portions of the Lynden Cemetery (Areas A-D) were studied. A Geophysical Survey Systems International (GSSI) SIR-3000 GPR unit, with a 200 MHz antenna, all mounted on a custom-built radar cart, was utilized. Survey lines were run in a N-S direction, with two to three survey lines covering each row of cemetery lots. The locations, survey line designation, and the direction (south to north, or north to south) run for each line are marked on the maps that accompany this report. The data were analyzed using GSSI software (Radan v 6.1), the radar transects were produced and converted into Acrobat .pdf files. Copies of both the original radar data files, and the .pdf images of each transect, are included in a digital appendix to this report.

     For ease of interpretation, and illustration of the results used to produce the conclusions for this report, one transect file for each row of cemetery lots was annotated with the Lot and plot locations. It should be noted that because the radar transects were collected in one continuous run along the north-south length of each row of plots in each area, and that during the radar transect small deviations in the path of the GPR antenna (due to simply meandering students, or due to need to avoid an obstacle along the transect line), many of the radar transects may have small errors in their location. Examination of the data, and observations made while the survey was being conducted, indicate that the extent of this possible location error may be approximately the width of a single cemetery plot (approximately 3 feet). Thus in some cases, there is a slight mismatch between the location of radar anomalies used to interpret the interment status of an individual cemetery plot, and the plot that is indicated on these example figures. In addition, some of the illustrations (see for example Area A lot 1) will depict the plots covered by the individual radar line, but in some cases near the margins of each area where the lot shape is irregular no GPR data may be available for all the plots in that lot- in this example plot C was not covered by the example line used for these illustrations. Because the remainder of the lot was covered by an adjacent GPR line, the interpretation of the plot’s status was determined by examination of the other (adjacent) GPR line.

     Based on the results of the GPR survey, the data were used to classify each cemetery plot into several categories to reflect the status of the plots. These categories are:
  1. Cemetery plot with marker, GPR data confirms interment. Criteria used to place a plot in this category include the presence of a strong hyperbola-shaped radar reflection, especially in the case of a more recent burial, or a more weakly-defined hyperbola that may be the result of either a very old burial, or of an urn, or an infant burial. Generally, it was assumed that in the majority of cases where a cemetery plot has a marker, the presence of buried remains is highly probable- thus even a weakly defined radar anomaly whose location coincides with a cemetery marker was used as a basis to indicate confirmation of an interment for the survey.

  2. Cemetery plot with no marker, GPR data shows is clear of interment. Criteria used to place a plot in this category are the absence of any clearly-defined radar anomalies, and the presence of laterally continuous radar reflections that indicate the original soil and sediment layering is preserved beneath the ground surface.

  3. Cemetery plot with marker, GPR data does not indicate interment. Criteria used to place a plot in this category are the absence of any clearly-defined radar anomalies, despite the presence of a marker indicating interment for the plot. In many cases, the photo inventory and transcription of the cemetery markers indicated that most plots in this category are family markers with incomplete inscriptions (lacking date of death) for one or more of the marked plots. A subset of this category, Cemetery plot with monument, GPR data does not indicate interment, is used for the case of a family plot monument that does not have an individual interment associated with the monument’s location.

  4. Cemetery plot with no marker, GPR data indicates (suggests) interment. Criteria used to place a plot in this category include the presence of a clearly-defined radar anomaly (used to indicate an interment is highly probable), or a weaker and less defined radar anomaly (used to suggest an interment is possible). In some cases, it is possible that the suggested interments in this category may be the result of tree roots or large root casts in the subsurface.

The depth of penetration of the GPR surveys is determined by the radar velocity of the subsurface materials- this velocity is a function of a physical property known as the dielectric constant. For these surveys a value for dry silt/sand (in keeping with the nature of the subsurface of the cemetery site) was used. The 200 MHz radar penetrated to a depth of ~ 20 to 30 feet, although clearly defined data is only found in the upper 10 to 15 feet. The depth of the GPR anomalies defined above was also utilized in the interpretation of the radar data in terms of cemetery plot status. Given the common practice of interment in a 6 foot deep grave, radar anomalies from 2 to 5 feet below the surface were considered to be possible anomalies to be evaluated for this survey. Anomalies deeper than 8 feet were not considered to represent probable interments.

In order to illustrate how the GPR data were interpreted and used to place all of the cemetery plots within one of these categories, several examples of the data are presented below.

     Example one shows some well-defined and poorly defined radar anomalies. This example is from Area A, Lot 95. The map and photo survey indicate that markers are present for plots A, B, E, and F for this lot. Plots A and B are marked by a family (couple) headstone (Peter Bonsen, 1894-1980 and Margaret Bonsen, 1900-1978). Both of these burials have very distinct radar anomalies, and are good examples of the strong radar reflections that are common in graves that are less than ~60 years old. Plot E has a similarly well-defined radar anomaly, and is also a recent burial (Mary Bonsen, 1904-1994). Plot F has a much weaker and less pronounced, but still apparent, radar anomaly. This burial (Christian Bonsen, 1897-1958) may be an urn or other cremains, or did not include a vault. Plots C and D lack even weak radar anomalies, and so are interpreted to be clear of any interments.


Plot A Cemetery plot with marker, GPR data confirms interment
Plot B Cemetery plot with marker, GPR data confirms interment
Plot C Cemetery plot with no marker, GPR data shows is clear of interment
Plot D Cemetery plot with no marker, GPR data shows is clear of interment
Plot E Cemetery plot with marker, GPR data confirms interment
Plot F Cemetery plot with marker, GPR data confirms interment

     Example two, plots with small, and old, interments that have weak radar anomalies. In this example, from Area A Lot 140, plot C is marked “Infant Knapp”, the GPR data show a clear, but very weak, radar anomaly. Plots A and B have weak but noticeable radar anomalies- both plots have relatively old burials (plot B, Eva G. Thompson, 1856-1926 and plot A, Rhoda A. Lanpher, 1832-1912). It has been generally found that older burials often- but not always- have weak radar anomalies such as these. Plots D-F lack any even weakly defined anomalies, and have a set of continuous radar reflectors that indicate preservation of the original soil/sediment strata beneath the surface.


Plot A Cemetery plot with marker, GPR data confirms interment
Plot B Cemetery plot with marker, GPR data confirms interment
Plot C Cemetery plot with marker, GPR data confirms interment
Plot D Cemetery plot with no marker, GPR data shows is clear of interment
Plot E Cemetery plot with no marker, GPR data shows is clear of interment
Plot F Cemetery plot with no marker, GPR data shows is clear of interment

     Example three, showing two family (couple) markers each with one confirmed and one not-indicated interment. In this example, from Area C, Lot 11, plot A lacks a distinct radar anomaly, while plot B has a very clear and well-defined radar anomaly. The marker- Flack, with Edwin F (1912-1997) on the left (plot A) and G. Jane (1918- ) on the right suggests that only plot B has an interment. The other pronounced radar anomaly occurs in the alley south of Plot E- as with plots A and B, there is a family marker with one complete inscription (Jose Ibarra Barajas, 1921-1995) and one incomplete inscription (Maria Ibarra Barajas, 1924- . Plots C and D in this example lack any clear radar anomalies and are therefore interpreted to be clear of interments.


Plot A Cemetery plot with marker, GPR data does not indicate interment
Plot B Cemetery plot with marker, GPR data confirms interment
Plot C Cemetery plot without marker, GPR data indicates interment
Plot D Cemetery plot without marker, GPR data indicates interment
Plot E Cemetery plot with marker, GPR data does not indicate interment
Alley alley plot with marker, GPR data confirms interment

     Example four, showing a weak radar anomaly from an old burial, and the radar anomaly produced by the roots of a large fir tree. This example is from Area D, Lot 97; plot F has a very weak radar anomaly, with a marker (William H. Batty, died 1923) that indicates an old (and possibly child or infant) burial. The remainder of Lot 97 has a large fir tree- the roots of this tree produced the weak, but noticeable anomaly in plots A and B. Because the nature of the anomalies produced by large tree roots can be very similar to the weak radar anomalies produced by older and/or very small burials, it is not always possible to distinguish between these two sources of radar anomalies. In many cases, the weak anomalies used to suggest the presence of an interment in a plot without any marker (category 4 above) may be due to the presence of old tree roots, rather than an unmarked interment.


Plot A Cemetery plot with no marker, GPR data shows is clear of interment
Plot B Cemetery plot with no marker, GPR data shows is clear of interment
Plot C Cemetery plot with no marker, GPR data shows is clear of interment
Plot D Cemetery plot with no marker, GPR data shows is clear of interment
Plot E Cemetery plot with no marker, GPR data shows is clear of interment
Plot F Cemetery plot with marker, GPR data confirms interment

     Overall, the results of the GPR survey provided a set of high-quality data. While there is some uncertainty regarding the interpretation of weak radar anomalies produced by small or old burials, the similarity of these weak radar anomalies produced by large tree roots and these small and/or old burials, and in the position of the radar transect’s data relative to the lots and plots present in each row of the cemetery, the results of this survey can be interpreted with a high degree of confidence. Of concern to the cemetery, radar anomalies that indicate an interment occurs in unmarked cemetery plots should be investigated to determine, if possible, the identity of the person buried in these unmarked plots. Radar anomalies that are weaker, and thus interpreted to suggest the possibility of the presence of an interment in an unmarked cemetery plot, should be investigated to verify the status of these plots. A soil probe or thin auger could be used to physically test for the presence, or absence, of a burial in these plots. For the cemetery plots that lack markers, and that lacked any significant radar anomalies, the results of this survey indicate these cemetery plots are clear of any interment and are thus available for future use.

     The remainder of this report consists of example GPR data for each cemetery lot, showing all of the cemetery plots, with the interpreted category of each plot in that lot noted below the illustration. These results, as well as the results of all of the radar transects in the survey, were used to produce the survey result maps that accompany this report.



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