An Electrostratigraphic Study of the Formations in the Coastal Area of Yunlin Hsien, Westcentral Taiwan

A geoelectrical survey was conducted in the coastal area ofYunlin Hsien, \Vestcentral Taiwan, for the study of electrostratigraph)r as well as seawa­ ter intrusion. The direct current resistivity method was used, and forty-six vertical electrical soundings with Schlumberger array were carried out in the study area. The sounding data were interpreted using the 1-D inversion method. The results indicate that the shallow part of the study area can be divided into three electrostratigraphic units. They are designatd, from top to bottom, the A, B and C formations. The A-formation is a combination of thin layers of medium resistivity and is correlated with the layers of soil and fine sand on the top. . The B-formation is characterized by a thick layer or layers of low re­ sistivity and is correlated with the layers of clay, mud and fine sand with saline groundwater. The C-formation is characterized by high resistivity and is correlated with the layers of pebble, sand and clay with fresh groundwater. The parameters in Archie's equation are evaluated with the resistivity interpreted from the VES data, and the results are a=0.858 and m=l.367 . The critical resistivities of salt}' strata are also evaluated. There is seawater intrusion in the study area, but it is locally distrib­ uted within a small area and is near to or on the surface of the ground. The intrusion was caused by the flooding of seawater during typhoons, storms and storm surges. The main saline groundwater body is the formation wa­ ter in the B-formation, which is not caused by seawater intrusion but is the connate water in the stratum sedimented in marine environments.


INTRODUCTION
The study area is situated on the coast of Yunlin Hsien, westcentral Taiwan ( Figure I). Formerly, most of the area was rice and sugarcane fields. In the last thirty years many fish ponds were constructed, especially in the area near to the coastline . The surface ot· the study area is low and tlat, \\1ith more than sixty percent of the area being }()\Ver than five meters in altitt1de. Hence, several artit' icial channels were CC.)nstructed for drain age. Because thei�e was n()t enough surface t� 1�esh \\1ater, man)' \:Vater \\Iells were constructed for irrigation and fish re.ari ng, and me) st of these wells are 90 to 150 rneters i.11 depth. The litho lc)gic columns of the wells indicate that the strata bel()ng to the allu\1ium and are structurally u11disturbed. The tipper part of the strata (0 to 60 meters in depth) is mainly composed ot' layers ()f. clay, sandy mud and f'ine sand. The lower part (60 t() 150 meters in depth) is mainly Cl)ffi posed of layers C)f pebble, sand '1nd clay·. The total thickness of� the la)rers of pe. bble and s'tnd is about thirty meters, which is the main confined aquifer in the study area4 Because of overpumping, the piezometric surt'ace has declined and was lower than the mean sea lev·el in 1nost areas. According to the nieasurements from 1962, the 1()\\i'est piezometric Sttrface of minus 5 meters (below the n1ean sea lev·el ) appeared around the Wundi village on the southern boundary of the study area. At that tirne, no sea\.\1ater intrusion \\1as f()Und. The measured data indicated that the piezo1ne. tric surt' ace declined continU()Usly in the t' ollowing years, the lo\\iest values being minus 13 1neters� minus 20 meters an(l minus 25 meters in the years 1973 , 1977 and 1986 respectively (Liu, 1986� Ts<:tC) and We:1ng, 1984). The ove11Jumping not only caused the lo\vering (1t' the piez<)111etric su1·t· ace of · ground\vater but �tlso caused subsidence in the stud)' ar·ea. The surt' ctce of the ground in so111e places was lower than the high tide level. Hence banks were constructed along the sides ot' channels to pre\1ent f1ooding by sea\vater at high tide. But seaw·ate1· t1ooding has occurred several times as a result of typhoons, stl1rms and storm surges.
The region of high salt content, the cause ot· the high salinity ot· pumped water. and the seawater intrusio11 are studied from the point view of ' electrostratigrapl1y·.

M.ETHOD
The direct current resistivity 111ethc)d was tised in this study to investigate the resistiv·ities ot ' the strata. The Schlumbe1�ge1� electrode configuratio11 �1as used in vertical electric soundings (YES). In each sounding, tl1e current electrodes \Vere spread out step by step f1�0111 2 meters to the 1naximurn spacing \Vi th 1 ()spacings per logarithmic c1·cle. The inaximum spacings of the soundings range from 320 to 8()0 meters depending on location, most of them being greater than 480 mete.rs. The apparent resistivity curves 'A1ere plotted on double logarithmic paper in the field for inspecting the quc:1l ities <)f raw data. If a distc)rted datum appeared, the measure ment \\las repeated 01· the position ()f' the current electrodes was changed to impt�ove the quality· of the datum.
The YES data \Vere interpreted with the 1-D inversion method, since the strata are struc turally undisturbed and can be regarded as a 1-D structt1re. The computer program for 1-D inversion used in this study was developed by the geoelectrical research team at the Institute <)f Geophysics at National Central University. The t · orward part is based on the method of digital line<:1r filtering of' C()nvcllt1tion (Ghosh, · 1971;Kc) 1979� O'Neill andMerrick, 1984 ), and the inv·erse part is based ()n the second order Marqtta1·dt method (Jupp andVozot ' f, 1975: Tong, 1988).

RESULTS
Fo1·ty-six vertical elect14i.c S()Undings ( \l"ES) were carried out in the study area, the loca tions of ' the YES are shO\\ltl in Figure l.

Types of Apparent Resistivity Curves and Interpretative Results
Every apparent resisti,1ity curve (or VES curve) measured in the study area has one or two minima and an ascending segment on the right-hand branch. The dominant types of the curves are KH, HA and HKH types which implies that the . strata contain one or two conductive layers and a resistive bottom layer. Stations 17, 21 and 25 are representative examples; the curves and interpretative results of these Stations are . shown in Figure 2. The interpretative results indicate that the bottom layer is more resistive than its overlying layer. At Station 17, the . re are two conductive layers: the shallower one is minor, and is 2.2 meters thick, being between the depths of 0.5 and 2.7 meters; the deeper one is major, and is 43.7 meters thick, being between the de . pths of 31.6 and 75.3 meters. At Station 21, there is one conductive layer between 8.9 and 32.4 meters in depth, which is correlated with the major conductive layer of Station 17. At Station 25, there is one conductive layer between 3.3 and 6.7 meters in depth, which is corre lated with the minor conductive layer of Station 17.
The interpretative results of all the VES measured in the study area indicate that the shal l0Vt1 formation (0 to 100 meters in depth) c. an be divided into four to seven layers by resistivity. These la)'ers include one or t\vo conductive layers and a resistive bottom layer. The bottom layer has a resistivity ranging mainly from 40 to 70 ohm-m. Except for the northwestern part of the study area, the. bottom layer is overlain by a thick conductive layer. The conductive • layer is about I 0 to 55 meters thick and has a resistivity of between 0.9 and 12 ohm-m. In some places, there is a thin conductive layer several meters thick on or near to the ground surface.

Resistivity Distribution
The interpretative results of the VES indicate that the resistivities of the strata vary with position. The resistivities of the strata at twelve different depths are show in Figure 3. The most remarkable feature in Figure 3 is the distribution of the low resistivity region (with a resistivity of lower than 12 ohm-m), as shown by the shaded regions in Figure 3. The low resisti,1ity regions are mainly distributed between depths of 5 and 60 meters, and as shown in Figure 3, the central part does not connect \\lith the �ea. The area of the cross-section of· the low resistivity region is small at depths less than 10 meters, and expands with depth, reaching the maximum at about 30 meters deep and then shrinking. In a few isolated small areas this occurs at about 60 meters in depth and disappears at a depth of 100 meters.  .

Electrostratigraphic Units and Resistivity Profiles
:           The B-formation is characterized by a thick layer or layers of low resistivity. The thick ness of the B-formation ranges from ] 0 to 55 meters and the resisti\1ity is lo\ver than 1 . 2 ohm m (predominantly lower than 5 ohm-m). The B-formation is absent in the northwestern part of the study area (: Figures 4 and 6). A transition zone of resistivity ranging from 12 to 16 ohm-m exists be . tween the areas \V'ith and without the B-formation, as denoted with B' in Figures 4 and 6. T. he lithologic units corre. Iated to the B-formation are the layers of clay·, mud and fine sand.

\J VES Station
The C-t� ormation is characterized by higher resistivity ranging mainly from 40 to 70 ohm m. The lithologic units correlated to the C-formation are the layers of pebble, sand and clay.
The C-formation is also correlated with the main confined aquifer in the study area.

Parameters for Archie's Law?
Archie's law is a satisfactory· expression t�or the resistivity of a \\1ater-bearing rock (�Nabighian, 1988). It is an empiric.al equation, for a \\'ater-saturated stratum, and is \\1ritte11 as �1here p and p H . are the resistivities of the stratum and the formation water respectively, </J is the porosity of the stratum, cz and n1 are parameters of the stratum.
The v·alues of' a and 111 in Archie's equation have not been determined in the study area. Owing to the lack of resisti \i'ities measured directly on samples or by well loggings, the resistivities of strata interpreted from the YES data were used to evaluate the values of a and m. The data t' or the evaluation are listed in Table 1. *The 1·esisti vity of formation water is the re\1erse of the conductivity measured by Liu (1986).
Except for well \\'5 where a lack <)f. water resisti\'it)' pre\1ented it, ten sets of· data \\/ere collected. The \'alues of · a and tn were e\1aluated by· power regression analysis. The results are a=0.858 and rn= 1.367, and the corre. lation coet ' t ' icient is 0.987. The estin1ated resistivities ()f the strata and the e1·ro1·s percentages �tre listed in the right-hand column c>t· Table 1.
Based on Archie's law, the resisti\1it)' of a stratum can be estimated it· the resistivity of the formation water, the porosity and the parameters c1 and 111 are knoV\1n. The resistivities of· strata evaluated using Archie's eqL1ation t' or v·arious salinities ot' f' c.)rmation wate1· are listed in Table  2. Alternative.ly, the 1·esisti\1ity and the salinity of t · ormation wate1· can be e\1aluated if the. resisti'v'it)' of the stratun1 is known.

The Values of a and m for Archie's Equation
Normall;1, the resistivities of strata used t , or determining the v·alues C)f ll and n1 in Archie's equation are measured {)n r9ck samples or b)1 'A'ell loggings. Usually, a lot of-samples are needed for the determination if the samples are small in din1ension with respect to the st1·atum.
The resistivity inte-rpreted from the VES data is a representative value for a stratum of wide extension and of a thickness similar in dimension to the stratum considered in Archie's law. Therefore, the resistivity interpreted from VES data may be regarded as the mean resistiv ity of many samples.
The ''alues calculated for a and m are 0.858 and 1.367 respectively. They approximate to the ,,alues 0.88 and 1.37 for weakly-cemented detrital rocks suggested by Nabighian ( 1988),. and are within the ranges described by Keller and Frischknecht (1966). The correlation coeffi cie . nt is 0.987 and the errors are lower than 6o/o. The results are reasonable and acceptable.

Criterion of Salt)1 Stratum
Saline groundwater is a term referring to any groundwater containing more than 1000 total dissolved solids (TDS). Electrical conductivity is often used to estimate the TDS in water for classification of water quality. An approximate relationship for most natural water in the range 100 to 5000 µSiem leads to an equivalence 1 mgl7= 1.56 µSiem at 25<>c. An increase of l °C increases the conducti \'ity by about 2 percent (Todd, 1980). According to the relationshp, the equivalent resisti\1ity for 1000 mg/l TDS is 6.41 ohm-m at 25°C or 6.28 ohm-m at 26<'C.
This may be regarded as the. critical resistivity of saline water. Any water having a resistivity lower than the critical value is considered saline, otherwise it is considered fresh. A salty stratum is so-called because it's formation water is saline. The critical resistivities of salty strata are evaluated according to Archie's law and the parameters previously evaluated. They are 13.9 ohm-m for clay, 14.3 ohm-m for sandy mud, 16.1 ohm-m for fine sand, 18.9 ohm-m for medium sand, 22.6 ohm-m for coarse sand, and 30.7 ohm-m for pebble, as shown in the second column from the right in Table 2. They are evaluated at 26°C for which is the approxi mate average temperature of the formation water in the study area. Figures 4--7 show that, where the water well screens are in position, the resistivities of strata are higher than the critical resti\l·ities of salty strata. This confirms that the formation water is fresh if the resistivity of a stratum is higher than the critical value.

Seawater Intrusion
At some places, the A-formation contains a layer of low resistivity. This low resistivity layer is locally distributed according to the locations of channels. This low resistivity layer is salt)1 and is on or near to the ground surface with a depth of less than ten meters. The low resistivity of this layer is explained as a result of seawater intrusion. The seawater advanc . ed inland and infilt . rated the ground during typhoons, storms, and storm surges. The lowest resis tivity of this laye. r is 4 ohm-m, the corresponding salinity of formation water is 2.92 %0 which is equivalent to 8.35o/o seawater.
The lowest resistivity of the B-formation is 0.9 ohm-m, and the corresponding salinity is about 14%0 which is equivalent to about 41 o/o seawater. Though the B-formation is salty, the pattern of the resisti\lity distribution ( Figure 3) indicates that the central part of the B-forma tion does not connect with the sea. That implies that the saline water in the B-fom1ation does not come from the sea. Th� saline water does not come from the overlying or the underlying layer either, for both of them have a lower salinity than the B-formation, and also the flushing • Ping-Hl1 Che11g . � 329 action \JJould not render a thick layer to such high salinity. It is confi14med that the saline water in the . B-formation is the connate water in the stratum which sedime . nted in marine environ ments. The high salinit),. of water \�lhich is pumped from the aquit .. er of· the C-formation at some wells seems to contradict the inference that the groundwater in the C-formation is fresh. This phenomenon can be explained with a model of· migration. The saline "''ater in the B-t· ormation was migrating into the C-formation as the piezometric surf. ace of· the groundw·ater in the C formation was declining, so 1nore saline \:\later \\lOuld accumulate for a longe1� migration time. Therefore the salinity is higher at the beginning of a pumping and then decreases with time since the rate of migration of saline wate14 is slo,:ver than the rate of pumping. Extremely high values would appear at the beginning of a pumping at .. ter a long period without operation. A large amount of saline '�later \\'ould migrate into the C-formation during a pumping if the . grout seal around the well casing was destroyed .This would keep the . salinit)' of .. pumped water at a high level. The grout seal might fi ()t have been in place t'or many1 )'ears of .. operation of the wells. That \\1ould most probably happen when the area of the. piezometric surface v\las se\1erely reduced. This is consistent \.\i'ith the cases of v\lells WS and W6.

CONCLUSIONS
Several conclusions can be dra\\'n from this study.
( 1 )Three electrostratigraphic units can be specified corresponding to the strata less than one hundred meters in depth. From top to bottom, they are designated the A, B, and C formations. The A-formation is a combination of thin la)1ers with resisti\1it)' ra11ging from 16 to 60 ohm-m in most cases. At a fe . \\i places, a thin conductive layer with resistivity ranging from 4 to 12 ohm-m is included also. The A-t· ormation is be .
tween about 5 and 30 meters thick. Its correlated lithologic units are lay·ers of soil with fine sand on the top.
The B-formation has a resisti\'ity ranging from 0.9 to 12 ohm-m, and is between about 10 and 55 meters thick. The correlated lithologic units are mainly lay'ers of clay, mud, and fine sand.
The C-formation has a resistivity ranging mainl)' from 40 to 70 ohm-m and is greater than several tens of meters thick. The correlate . ct lithologic units are layers of .. pebble, sand, and clay. (2)The resistivity of a stratum interpreted from the YES data can be used for e\1aluating the parameters in Archie's equation, p == apv_.</J-'11 • Suitable values t"or a and 1n are obtained, and these are 0.858 an d 1. 367 res pee ti vely·. The t·ormation t .. actor (p Ip�·) ranges from 2.21 to 4.4 l and varies with the porosity of a stratum, which increases with decreasing porosity.
(3)The resistivity of ground\\later can be e\1aluated t·rom the 14esistivity interpreted t· rom VES data, if the porosity' and the parameters of stratum in Archie's equation are known. The resistivity of a str&tum offers a criterion for the salinity of ground�1ater. The ground water in the B-formation is saline, but that in the C-formation is t·14esh.
( 4 )There is seawater intrusion in the study area, but it is locally distributed and in strata less than 10 meters deep. The seawater intrusion \\1as caused by seawater floodings . which happened during typhoons, storms and storms surges. In these cases the seawa ter advanced inland and infiltrated the ground. (5)The saline '�later in the B-formation is the main saline groundwater body. It is not caused by seawate. r intrusion, but is the connate water in stratum sedimented in marine env·ironments. (6)The high salt content in the aquifer of the C-formation found at some wells was not a case of seawater intrusion, but was a case of migration of saline water from the B f ormation. This happened at the places where the piez. ometric . surface was severely diminished. It" the grout seal around the well casing was de. strayed, a large amount of saline water would migrate into the aquifer and keep the salinity of pumped water at a high le\1el during a pumping. •